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Lecture 4 Outline: Plate Tectonics – Mechanisms and Margins Learning Objectives: What are the types of plate boundaries? What processes occur at different types of plate boundaries? What are hotspots? How does tectonics build continents and ocean basins? What Happens at Plate Boundaries? Plate interiors stable - geologic activity limited to surface processes But interactions between plates at plate boundaries results in Magma and volcanism Faulting and earthquakes Mountain building Production of new crust Recycling of old crust What are the Types of Plate Boundaries? Divergent plates pulled apart Convergent plates collide Transform plates sheared Each plate surrounded by different types of boundaries What are the Types of Plate Boundaries? What are Divergent Plate Boundaries? Ridges Crust pulled apart Magma by decompression melting in asthenosphere Cools to make new oceanic crust Oceanic crust lithosphere asthenosphere magma central rift valley faults North Atlantic Ridge Mid-Atlantic Ridge East Pacific Ridge Indian Ridge Antarctic Ridge Where are Divergent Plate Boundaries Found? Ocean ridge above sea level in Iceland Where are Divergent Plate Boundaries Found? What are the Major Geologic Features of the Ocean Ridge? Shield Volcano Edge of North American Plate Fault Down-dropped fault block Central rift valley Filled by lava flows What are Convergent Plate Boundaries? Two plates collide with each other – two types Subduction zone Between two plates of different density - denser plate subducted melting in mantle by addition of water from subducted plate Trench and volcanic arc - chain of volcanoes on overriding plate Earthquakes What are Convergent Plate Boundaries? Collision zone between plates too buoyant to subduct Crust thickened and mountains raised instead Earthquakes but no volcanoes Indian Plate Eurasian Plate Younger and weaker Older and stronger deformed Which Plate gets Subducted? If both plates composed of oceanic crust older and denser crust subducted by younger and lighter crust Overriding plate Plate boundary Where Can We Find an Example of an Oceanic Plate Subducted by Another Oceanic Plate? Pacific Plate subducted by Philippine Plate at Mariana Trench Pacific Plate (older) Philippine Plate (younger) Japan Trench Mariana Trench Challenger Deep Eurasian Plate Which Plate gets Subducted? If one plate of continental crust and one of oceanic crust denser oceanic crust subducted by lighter continental crust Material too light to subduct added to continent as accreted terranes sediments, volcanic islands, fragments of continental crust Where Can We Find an Example of a Collision Zone? Indian and Eurasian Plates Collision began 45 mya when subduction completely closed ocean basin Himalaya and Tibetan Plateau Recent or continuing collisions produce Earth’s tallest mountains 50 mya today Closing Ocean Spreading Ocean 14 Oblique motion betw ...

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Lecture 4 Outline: Plate Tectonics – Mechanisms and Margins Learning Objectives: What are the types of plate boundaries? What processes occur at different types of plate boundaries? What are hotspots? How does tectonics build continents and ocean basins? What Happens at Plate Boundaries? Plate interiors stable - geologic activity limited to surface processes But interactions between plates at plate boundaries results in Magma and volcanism Faulting and earthquakes Mountain building Production of new crust Recycling of old crust What are the Types of Plate Boundaries? Divergent plates pulled apart Convergent plates collide
Transform plates sheared Each plate surrounded by different types of boundaries What are the Types of Plate Boundaries? What are Divergent Plate Boundaries? Ridges Crust pulled apart Magma by decompression melting in asthenosphere Cools to make new oceanic crust Oceanic crust lithosphere asthenosphere magma central rift valley faults North Atlantic Ridge Mid-Atlantic Ridge
East Pacific Ridge Indian Ridge Antarctic Ridge Where are Divergent Plate Boundaries Found? Ocean ridge above sea level in Iceland Where are Divergent Plate Boundaries Found? What are the Major Geologic Features of the Ocean Ridge? Shield Volcano Edge of North American Plate Fault Down-dropped fault block Central rift valley Filled by lava flows What are Convergent Plate Boundaries? Two plates collide with each other – two types Subduction zone Between two plates of different density - denser plate subducted melting in mantle by addition of water from subducted plate Trench and volcanic arc - chain of volcanoes on overriding plate Earthquakes
What are Convergent Plate Boundaries? Collision zone between plates too buoyant to subduct Crust thickened and mountains raised instead Earthquakes but no volcanoes Indian Plate Eurasian Plate Younger and weaker Older and stronger deformed Which Plate gets Subducted? If both plates composed of oceanic crust older and denser crust subducted by younger and lighter crust Overriding plate Plate boundary Where Can We Find an Example of an Oceanic Plate Subducted by Another Oceanic Plate?
Pacific Plate subducted by Philippine Plate at Mariana Trench Pacific Plate (older) Philippine Plate (younger) Japan Trench Mariana Trench Challenger Deep Eurasian Plate Which Plate gets Subducted? If one plate of continental crust and one of oceanic crust denser oceanic crust subducted by lighter continental crust Material too light to subduct added to continent as accreted terranes sediments, volcanic islands, fragments of continental crust Where Can We Find an Example of a Collision Zone? Indian and Eurasian Plates Collision began 45 mya when subduction completely closed ocean basin Himalaya and Tibetan Plateau
Recent or continuing collisions produce Earth’s tallest mountains 50 mya today Closing Ocean Spreading Ocean 14 Oblique motion between plates – without convergence or divergence Faulting and earthquakes - no volcanism What are Transform Plate Boundaries? Where Can We Find an Example of a Transform Plate Boundary? San Andreas Fault Transform boundary between Pacific and North American plates N. American plate Pacific plate
What are Hotspots? Volcanism normally at divergent or convergent plate boundaries Melting of mantle by decompression or addition of water But some volcanoes located in middle of plates! What’s the explanation? Hotspot: plume of hot rock rising from deep mantle What are Hotspots? Source of magma well below lithosphere Doesn’t move with plate – rather plate passes over magma source Results in age progression to volcanism - hotspot track Example: Hawaiian Islands What is a Tectonic Setting? Geologic environment of area relative to any nearby plate boundaries or hotspots Each setting associated with specific geologic processes types of volcanoes, earthquakes, etc Japan Hawaii
Oregon Nevada Divergent Plates pulled apart lithosphere created Convergent Plates move together lithosphere recycled What are the Different Tectonic Settings? Transform Plates slide past one another Lithosphere neither created nor recycled Hotspot Plate passes over deep mantle plume What are the Different Tectonic Settings? What is the Tectonic Setting of Oregon? Convergent - between North America and Juan de Fuca plates Subduction, terrane accretion, earthquakes, volcanic arc
22 Accretion buoyant material from subducting plate Coast Range Volcanism partial melting in mantle due to addition of water Cascades Coast Ranges and Cascades both result of subduction Specific processes are different What is the Tectonic Setting of Oregon? Continents assembled from pieces of crust too light to subduct How Does Plate Tectonics Build Continents? By accretion and collision Subduction removes intervening oceanic crust Accretion and volcanism adds buoyant material to overriding plate Intervening oceanic crust removed - continents collide How is Continental Crust Made? strongly enriched in silica relative to oceanic crust/mantle Subduction zones: recycling centers that sort out continental crust
Mantle (peridotite) 45% SiO2 Oceanic crust (basalt) 50-55% SiO2 Continental crust (granite) 60-75% SiO2 Degree of Silica Enrichment How do Subduction Zones Make Continental Crust? 1. Accretion of buoyant materials as ocean crust subducts Builds continent outward over time Buoyant materials often added as terranes - block of crust with different geologic origin and history from adjacent areas Volcanic island arcs, marine sediments, thick oceanic crust, fragments of continental crust Moved great distances on subducting plate How do Subduction Zones Make Continental Crust? 2. melting of subducted plate and mantle and fractional crystallization Makes magmas richer in silica than oceanic crust Erupted at surface volcanoes
Cascades Emplaced within crust intrusions Sierra Nevada Rifting of continent to create new divergent boundary Caused by mantle upwelling beneath continent Example: East African Rift Valley Active rifts grow over time to become new oceans How does Plate Tectonics Make Ocean Basins? African Rift Valley Red Sea advanced rift Rift Valley lakes rifts How do Rifts Start and Grow into Ocean Basins? Crust heated by upwelling mantle causing uplift Uplift collapses with continued stretching to form rift valley Rift floods to form narrow sea
Widens by seafloor spreading into new ocean basin Processes of continent assembly, breakup, and re-assembly Continental rifting, seafloor spreading, subduction and accretion, collision Rift breaks continent apart New divergent boundary/ridge forms Grows into mature ocean basin Subduction begins as oceanic crust becomes older, colder, and denser Terranes accreted to continent Continents collide when intervening oceanic crust completely subducted Stages in Tectonic Cycle with modern-day examples of each stage What is the Tectonic Cycle? 1. Continental rifting 2. Spreading center develops 3. Ocean basin 4. Subduction 5. Terrane Accretion 6. Continental collision What are the three types of plate boundaries? Describe the geologic processes occurring at each boundary. Provide a modern-day example of each type of plate boundary. How does plate tectonic activity build continents and make continental crust? How does plate tectonic activity break continents apart and make oceanic crust?
Describe the tectonic setting of the Pacific Northwest. Questions for Review Lecture 4: Plate Tectonics: Mechanisms and Margins Reading: Chapter 2, pages 42-61 Topics: · Divergent Plate Boundaries · Convergent Plate Boundaries · Transform Plate Boundaries · Hot Spots and Hot Spot Tracks · Plate Tectonics and the Building of Continents · Continental Rifting and the Development of Ocean Basins · Tectonic Settings · Significance of Plate Tectonics Next lecture: Mineral Composition and Properties 4.1 Plate Boundaries The interior of tectonic plates are usually geologically stable. The main geologic processes operating in the middle of plates are erosion and sedimentation. However, along plate boundaries dynamic interactions between plates are responsible for volcanic activity, faulting and earthquakes; the building of mountain ranges, the creation of new crust, and the recycling of old crust.
There are three types of plate boundaries: 1. Divergent plate boundaries occur where plates are being pulled apart. 2. Convergent plate boundaries occur where plates are moving together or colliding. 3. Transform plate boundaries occur where two plates are sliding past each other in different directions. 4.2. Divergent Plate Boundaries New oceanic crust is created at divergent plate boundaries as two plates pull away from each other. Divergent plate boundaries are associated with extensional stress, shallow earthquakes, and basaltic volcanism. Examples include the mid- ocean ridges such as the Mid-Atlantic Ridge. At divergent plate boundaries, extensional stresses pull two plates apart, allowing magma to rise from the asthenosphere towards the surface and create new oceanic crust. The extension is accommodated by normal faults in the crust, creating a rift valley. The Mid-Atlantic ridge rises above sea level in Iceland, allowing easy observation of its features. The central rift valley of the Mid-Atlantic Ridge in southwest Iceland. The photographer is standing at the edge of the North American Plate, looking across the rift valley towards the Eurasian Plate. The broad mountains in the distance are shield volcanoes, the characteristic style of volcanism in a divergent tectonic setting. Where a continent is being pulled apart a new divergent boundary called a continental rift valley is created. The East
African Rift Valley is an example of a continental rift. Continental rift valleys represent the earliest stage in the development of a new ocean basin as a single continental plate splits into two fragments as new oceanic crust grows between them. Development of a rift in continental crust by mantle upwelling Various stages of rifting are present in East Africa. The East African Rift Valley represents a very young rift not yet flooded by seawater. The Red Sea is a more advanced rift that is continuing to grow into a new ocean basin by seafloor spreading. 4.3 Convergent Plate Boundaries Convergent plate boundaries occur when two plates collide with each other. There are three types of convergent boundaries: 1. Ocean-Ocean: collision between two plates composed of oceanic crust; older denser oceanic crust is subducted and destroyed. 2. Ocean-Continent: collision between oceanic and continental crust; oceanic crust is subducted by the continent and destroyed. 3. Continent-Continent: collision between two plates composed of continental crust; both are too light to be subducted. 4.3.1. Ocean-Ocean Convergence In a collision between two plates of oceanic crust, the older and denser crust gets subducted beneath the younger and lighter crust. A deep-sea trench marks the subduction zone at the surface. As subducted slab descends into the mantle, it partially melts. The rising magma produces a chain of volcanoes on the overriding, younger oceanic plate called a volcanic island arc.
A convergent plate boundary between two oceanic plates. The older, denser plate is subducted by the younger, more buoyant plate. A trench marks the subduction zone at the surface. As the subducted plate descends into the mantle and melts, rising magma creates a volcanic island arc. The subduction of the Pacific Plate by the Philippine plate is an example of an ocean-ocean convergent boundary. The subduction zone is marked by the Mariana Trench. Challenger Deep, its deepest point, is 10,911 meters (35,798 ft) deep and is the deepest place in the ocean. 4.3.2. Ocean-Continent Convergence When a plate composed of oceanic crust collides with one made of continental crust, the denser oceanic crust is subducted beneath the lighter continental crust. As in the case of ocean- ocean subduction, there is a trench and volcanic arc, with the volcanic arc on the continent. Compression crumples the edge of the continent, producing mountains. Ocean sediments and anything else too light to subduct such as a volcanic island arc or fragment of continental crust is scraped off and added to edge of continent as accreted terranes. Both types of subduction zones (ocean-ocean and ocean-continent) are associated with compressional stress, deep earthquakes, and andesitic volcanism. At a convergent boundary between an oceanic plate and a continental plate, the denser oceanic plate is subducted by the lighter continental plate. A volcanic arc is built on the overriding continental plate as the subducted oceanic plate melts. 4.3.3. Continent-Continent Convergence Continental crust is too light to be subducted. So when two continents collide, thickening, thrust faulting, and uplift
produces mountain ranges and high plateaus. A suture zone marks the boundary where two formerly separate continents are joined. When two continents collide, neither is subducted. Instead, the compressional stress produces thickening and thrust faulting, which uplifts mountain ranges and high plateaus along and behind the collision zone. The collision of India and Asia is an example of an ongoing continental collision and is raising the Himalaya and Tibetan Plateau. The collision zone between India and Asia is marked by the abrupt Himalayan front and the world’s highest mountains. Behind the collision zone, an area of thickened continental crust supports the extensive Tibetan plateau, the largest high plateau in the world. The collision of India with Asia began about 20 million years ago when a subduction zone completely destroyed the ocean basin that formerly separated the two continents. This same process is closing the Mediterranean Sea and bringing Africa into a collision with Europe. The ancient collision between North America and Africa during the formation of Pangaea is marked today by the heavily eroded Appalachian Mountains. 4.4. Transform Plate Boundaries A transform plate boundary occurs where two plates are sliding past each other. The boundary is marked by a system of faults. Motion between the plates is not always smooth due to friction; instead the plates periodically lurch past each other, resulting in earthquakes. Shear stresses crumple the edges of the plates and rocks on either side of the faults. A transform plate boundary occurs where two plates slip past
each other in different directions. California’s San Andreas Fault is an excellent example of a transform plate boundary between the Pacific and North American plates. The fault marks the plate boundary and runs through the center of the photo from top to bottom. Note how the edges of the two plates are crumpled due to friction along the fault. 4.4.1 Evolution of the San Andreas Fault As we saw in the example of India and Asia, plate boundaries do not remain static, instead they evolve over time. The San Andreas Fault is another example. The San Andreas Fault originated when the subduction zone off the west coast of North America caught up to and swallowed the seafloor spreading center that separated two oceanic plates: the Pacific Plate and the Farallon Plate. The Farallon Plate and spreading center was split into two smaller pieces connected by a transform fault that we call the San Andreas Fault. The San Andreas Fault originated when the subduction zone off central and southern California caught up with and swallowed the offshore spreading center. This severed the oceanic Farallon Plate into two smaller oceanic plates; connected by a transform fault we call the San Andreas Fault. Seafloor spreading and subduction still occurs further north off the Pacific Northwest and further south off Central America. 4.5. Hot Spots Volcanism is usually associated with plate boundaries. However, some volcanic centers are located in the middle of a plate. Such centers are called hot spots and have a magma source in the deep mantle, perhaps extending all the way to the core boundary. Because the magma source is below the lithosphere, it does not move with the plate. Instead a chain of volcanoes called a hot spot track is produced as the plate moves
over the stationary hot spot. The Hawaiian Islands are an excellent example of a hot spot track. The big island of Hawaii is currently located over the hot spot. The Hawaiian Islands were produced as the Pacific Plate moved northwestwards over a fixed hot spot in the deep mantle, producing a chain of volcanic islands that get younger to the southeast. All of the Hawaiian Islands are volcanic, but the volcanoes become extinct as the plate carries the island off the hot spot. The big island of Hawaii is currently over the hot spot and is the only Hawaiian island with active volcanoes. 4.5.1. Hot Spot Tracks The hot spot responsible for the Hawaiian Islands has existed for more than 70 million years. A chain of extinct volcanic islands and underwater seamounts extends west and north from the currently active island of Hawaii all the way to the Aleutian trench. Southeast of the island of Hawaii a new volcanic island is growing beneath the sea as the big island moves away from the hot spot. Hot spot tracks are good indicators of plate motion. Note the bend in the Hawaiian hot spot track, where the Hawaiian Ridge meets the Emperor Seamounts. This bend indicates a change in the direction of the Pacific plate from northerly to northwesterly about 40 million years ago. The Hawaiian hot spot track extends from the big island of Hawaii, which is currently over the hot spot, all the way to the western Aleutian Trench. The bend in the track of extinct volcanic islands and submerged seamounts indicates a change in the direction of the Pacific Plate about 40 million years ago. 4.5.2. Continental Hot Spot Tracks The Yellowstone hotspot is the world’s only hot spot beneath a continent and is responsible for Yellowstone National park’s
famous geysers and hot springs as well as three enormous volcanic eruptions in the last two million years. The hot spot track is marked by a chain of progressively older volcanic centers that extends southwest across Idaho’s Snake River Plain to north-central Nevada. The origin of the hot spot is unknown, but is probably related to the extensive Columbia Flood Basalts of eastern Washington and Oregon that were erupted about 18 million years ago. The Columbia Plateau flood basalts were erupted about 18 million years ago. The flood basalts were followed by progression of volcanic calderas from the oldest in north central Nevada to the youngest in Yellowstone National Park. The flood basalts are believed to mark the first appearance of a mantle hot spot beneath the North American plate, which then produced a chain of volcanic calderas as the North American plate moves southwestwards over the hot spot. The hot spot is currently located beneath Yellowstone National Park. Note that the hot spot track does not line up with its presumed origin in the Columbia flood basalts; instead there is a pronounced offset to the south. This has not yet been satisfactorily explained, but may result from interactions in the mantle between the hot spot plume and the oceanic plate being subducted beneath North America. It is well established that the hot spot currently beneath Yellowstone has left a track of progressively older calderas across southern Idaho to an earliest location beneath north central Nevada (16 million years ago). This is a good match for the age of the Columbia plateau flood basalts (18-16 million years old), however the caldera track does not quite line up geographically with the eruption points of the Columbia basalts. Geologists continue to debate the exact relationship between the Yellowstone hot spot and the Columbia flood basalts.
The very large volume of the Columbia basalts (several thousand cubic kilometers), their geologically short eruption time (less than 1.5 million years), and trace element compositions of the lavas themselves all suggest a deep mantle (hot spot) source. The Columbia basalts were erupted from three locations: the Chief Joseph dike swarm near Hells Canyon in Northeast Oregon; the Monument dike swarm near John Day, Oregon, and the Steens Mountain dikes south of Burns, Oregon. There is an age progression from north (older) to south (younger) within the Columbia basalts, so that also points to the hot spot (which next manifested itself as a caldera in north central Nevada). The geography of the hot spot eruptions is likely also complicated by interactions between the rising mantle plume and the subducted oceanic plate. Remember, the subducting plate is not instantaneously destroyed at the trench, but instead slowly sinks into the mantle, gradually re-assimilating into the mantle by melting. So the oceanic plate being subducted by the Cascadia Subduction Zone extends into the mantle beneath western North America. This means that a rising mantle plume will have to get past the subducted slab. The hot spot plume also appears to be inclined, rather than exactly vertical, and may even wiggle around a bit, instead of being fixed as portrayed in textbooks. So the connection between the Yellowstone hot spot and the Columbia flood basalts is quite complicated. What happens when a hot spot forms? Imagine a container filled with hot wax and oil. As the wax (deep mantle) is heated from below (Earth’s core), it becomes buoyant and begins to rise through the oil (mantle). The viscosity of the oil provides resistance, causing the rising wax to form a large glob, or head, followed by a narrow tail, or stem.
A mantle hot spot consists of leading glob, or head, followed by a narrower stem, or tail, because of the high viscosity of the mantle. The hot spot forms a large glob (head) followed by a narrow tail (stem) because of the resistance of the mantle to getting out of its way (viscosity). When the head reaches the lithosphere, it flattens out and erupts a huge volume of magma onto the surface. This type of eruption is known as a flood basalt. As the plate moves away from the hot spot, it is dragged over the narrower stem of the hot spot. This creates a chain of smaller, but still very large, eruptions called a hot spot track. A mantle hot spot plume has a wide head followed by a narrow stem. As the head reaches the surface, it erupts massive flood basalts. As the plate passes over the narrower stem, a hot spot track is produced leading away from the flood basalt. The flowing sequence of figures illustrates the events marking the passage of the North American plate over the Yellowstone hot spot. The head created the massive flood basalts that cover eastern Washington and Oregon. The tail, or stem, has produced a chain of progressively younger volcanic calderas from north-central Nevada to the present location of the hot spot beneath northwest Wyoming. As the calderas move away from the hot spot stem, they are buried by younger basalt lava flows fed by remnant pockets of magma in the lithosphere. This is typical of the style of volcanism on Idaho’s Snake River Plain.
The location of hotspots worldwide. The red dots mark the present location of a plate over a hot spot. The blue dots mark their earliest known location. The Hawaiian hot spot has no point of origin because its track disappears into the Aleutian subduction zone, meaning all earlier traces of the hot spot have been destroyed by subduction. Note that some hotspots first appeared at locations that have been divergent plate boundaries or mid ocean ridges, so the earliest evidence of these hotspots is now found on the coastlines of continents now separated by oceans, for example in the North Atlantic (Iceland) and South Atlantic. 4.6. Plate Tectonics and the Building of Continents Over hundreds of millions of years the modern continents have been assembled by plate tectonics from smaller fragments of material too light to be subducted. The pieces that make up a continent include: 1. Cratons: very ancient rock that make up the cores of continents. Exactly how cratons formed is still debated. 2. Orogens: belts of deformed rocks representing mountain building episodes during collisions between continental cratons. Only the youngest orogens still stand as mountain ranges. Most orogens are eroded roots of ancient mountain ranges. The collision of two cratons, for example India and Asia a. Subduction of oceanic crust crumples the edge of the overriding plate and creates a volcanic arc b. As collision begins, sediments along subducting plate are deformed and welded onto overriding plate c. As collision ends, subducted oceanic crust breaks off and
continues its descent into mantle The two continents are now joined along a high mountain range (the orogen) 4.6.1. Continental Shields Continental shields are assemblages of ancient cratons and their associated orogens sutured together by tectonic collisions. The North American continental shield is also called the Canadian Shield because glacial erosion has exposed the ancient rocks of the shield at the surface throughout much of central and eastern Canada, as well as the upper Midwest of the United States. Further south the shield is covered by younger sedimentary rocks. The shield is surrounded by five younger orogenic belts: the Grenville, Innuitian, Caledonian, Appalachian, and Cordilleran Orogens marking younger episodes of tectonic collision and uplift. The North American continental shield consists of several very ancient cratons joined by belts of deformed rocks called orogens that mark collisions between cratons. Note that the different parts of the shield are all older than 1.6 billion years. The shield is surrounded by younger orogens, the Grenville, Appalachian, and Cordilleran orogenies. 4.6.2. Accreted Terranes Accreted terranes are small pieces of crust too buoyant to be subducted that are carried along by oceanic plates and added to the edge of a continent. Accreted terranes may include old volcanic island arcs, small pieces of continental crust called micro continents, or margins of continents sliced off by transform faults. Accreted terranes can be rafted great distances by plate motions and get added to the edge of continent as the oceanic crust is subducted. The entire west coast of North America is complex jumble of more than 40
accreted terranes added to North America by 200 million years of oceanic subduction in this location. This has gradually shifted the plate boundary further west. Accreted terranes added to the western margin of the North American plate by more than 200 million years of subduction along the west coast. 4.7. Continental Rifting and the Development of New Ocean Basins In addition to building continents, plate motions also tear continents apart. This process is called continental rifting. 1. The process begins when crust is heated by rising magma from the mantle, causing uplift of a broad area 2. As rising magma continues to push the crust aside, the uplift collapses to form a rift valley 3. New oceanic crust is created as the rift spreads. The area is flooded to form a narrow sea such as the Red Sea 4. The narrow sea may grow to become a new ocean basin with a mid-ocean ridge and seafloor spreading center. Rifting can end at any point, not all rifts develop into a new ocean basin. The four stages of continental rifting and development of a new ocean basin, with modern day examples of each stage Two active spreading centers, the Red Sea and Gulf of Aden, have rifted the Arabian Plate from the African Plate. The East Africa Rift is less active and appears to be a failing rift that does not develop into an ocean basin. The more active part of
the rift extends through the Red Sea. At the north end of the Red Sea, the direction of rifting is changing again. Instead of continuing into the Mediterranean, the active rift turns northeast and is separating Sinai and Israel from Jordan. Only the southern portion is flooded by the sea, the remainder stands as much as 1000 feet below sea level and is partly filled by the Dead Sea, a salt lake. 4.7.1. Passive Continental Margins As a continental rift develops into a new ocean basin, the edges of the continent become a passive continental margin. A passive continental margin is a transition between oceanic and continental crust that does not coincide with an active plate boundary. Instead the plate boundary is at the mid-ocean ridge. Passive continental margins are characterized by cooling and subsidence of the crust as it moves away from rising magma at the mid-ocean ridge and the development of thick sequences of sediments eroded from the continent. The east coast of North America is an example of a passive continental margin Passive continental margins are transitions between continental and oceanic crust without an active plate boundary. The major features of a passive continental margin are a broad continental shelf followed by the abrupt continental slope which drops to the ocean floor. A thick sequence of sediments buries the ancient faults that mark the episode of rifting. The active plate boundary is out at the active spreading center at the mid-ocean ridge. 4.7.2. Tectonics and Ocean Basins The Atlantic Ocean is an example of a maturing ocean basin with passive continental margins. Its major topographic features such as the mid-ocean ridge, abyssal plain, and
continental shelf are explained by plate tectonics. The mid- ocean ridge is the site of seafloor spreading and the active plate boundary. The abyssal plain is underlain by older oceanic crust that has moved away from the ridge, cooled, and subsided. The abyssal plains end abruptly at the continental slope where the crust transitions to lighter and more buoyant continental crust. The continental shelf is the low-lying portion of the continent flooded by shallow seas. Tectonic features of a mature ocean basin, such as the Atlantic Ocean 4.7.3. The Wilson Cycle Eventually, a passive continental margin develops into a new subduction zone as the adjacent oceanic crust becomes older, colder, and denser. The ocean basin, while still spreading at the mid-ocean ridge, starts to grow smaller as oceanic crust is subducted. As the intervening ocean basin disappears the continents are drawn into a new round of collision, albeit in a new place. This cycle of continent formation, breakup, and re- assembly is called the Wilson Cycle. Stages in the Wilson Cycle with modern-day examples of each stage The Wilson Cycle and Drifting Continents Most of the continents were joined as the supercontinent Pangaea 200 million years ago (A). The continents became fragmented as first the Atlantic Ocean, between North America and Africa (B) and then the Indian Ocean, between Antarctica and India, (C) opened and grew (D). A new round of continental collision began as the Tethys Ocean separating India and Asia closed (E). The Atlantic and Indian continue to grow as the Pacific Ocean, ringed by subduction zones, continues to
close (F). Eventually a new supercontinent may be formed. 4.7.4. Active vs. Passive Continental Margins Active continental margins are found where subduction occurs between an oceanic and a continental plate. Active continental margins coincide with plate boundaries. Passive continental margins are found where there is a transition between oceanic and continental crust that is still moving as one plate. In this case the plate boundary is out at the mid-ocean ridge. Active continental margins are plate boundaries marked by subduction zones with earthquakes, volcanoes, and mountain building. Passive continental margins are transitions between oceanic and continental crust that do not coincide with a plate boundary. The western U.S. is an active continental margin with all four tectonic settings we have discussed: 1. Divergent: Gulf of California spreading center and extension in the Basin and Range and Rio Grande Rift (a failing continental rift) 2. Transform: the San Andreas and related fault systems 3. Convergent: the Cascadia Subduction Zone 4. Hot Spot: the Yellowstone hot spot 4.8. Tectonic Settings Tectonic setting describes the geologic environment of an area relative to any nearby plate boundaries or hot spots. This figure summarizes the different tectonic settings we have talked about. Each tectonic setting is associated with a particular style of volcanism as we will see in a future lecture.
Tectonic settings include divergent, convergent, and transform plate boundaries as well as hot spots. Each tectonic setting produces a characteristic style of volcanism. Summary of Tectonic Settings 4.8.1. Tectonic Setting of the Pacific Northwest The tectonic setting of the Pacific Northwest is an active plate boundary where small oceanic plates, the Gorda (south) Juan de Fuca (central), and Explorer (north) plates are colliding with and being subducted by the continental North American Plate. The active plate boundary lies just offshore and is called the Cascadia Subduction Zone. As the oceanic plates are subducted, materials too light to be subducted are scraped off the oceanic plate and accreted to the western margin of North America to form the Costal Ranges of Vancouver Island, Washington, Oregon, and Northern California. Further inland, a volcanic arc, the Cascade Range, is formed on the overriding plate above the depth where the subducted oceanic plates begin to melt and produce magma, which rises to the surface. The coastal and Cascade ranges are separated by a fore-arc basin, a topographic depression produced by down-warping of the continental plate like a rug due to the tremendous stresses from the subduction zone. This depression is occupied by Puget Sound and the Williamette Valley. SHAPE * MERGEFORMAT The tectonic setting of the Pacific Northwest in map view (left) and in cross section (right), showing the major tectonic features of western and central Oregon.
Cross section of the Cascadia subduction zone with major geologic processes and topographic features Although the Coastal Ranges and Cascades are both a product of subduction, the specific processes responsible for their formation are very different. Coastal Ranges like the Olympics of Washington and the Coast Range and Siskiyous of Oregon are a product of accretion and consist primarily of oceanic rocks added to the western margin of the continent. The Cascades, in contrast, are a volcanic range, and are a product of rising magma produced by heating and melting of the subducted plate. In Oregon, tectonic extension is occurring in the back arc basin (east of the Cascades). Why? Two reasons: 1. The Pacific Northwest is being rotated clockwise, creating extension in southeastern Oregon and compression in northwestern Washington. 2. The gradual ending of subduction with the destruction of the last remnants of the Farallon Plate is relieving compression on the western margin of North America, allowing the edge of the plate to “rebound” 4.9. Significance of Plate Tectonics Plate tectonics explains the global distribution of earthquakes, volcanoes, and mountain ranges. Plate tectonics is also responsible for creating and destroying ocean basins as well as building and rearranging the continents. Through the processes of subduction and volcanism, plate tectonics recycles earth materials. Our atmosphere is also maintained through volcanic degassing and climate regulated by controlling the atmospheric carbon dioxide content and the magnitude of the greenhouse effect. Plate Boundaries
Earthquakes Volcanoes SHAPE * MERGEFORMAT SHAPE * MERGEFORMAT Compare the map of plate boundaries to the global distribution of earthquakes and volcanoes. The locations of volcanoes do a pretty good job of outlining plate boundaries. Earthquakes do an even better job of outlining plate boundaries. Why? Not all types of plate boundaries are associated with volcanism (there are no volcanoes at transform plate boundaries), but all plate boundaries are associated with earthquakes! Terms to Know Divergent plate boundary
Hot spot track Continental rifting Continental rift valley Tectonic setting Passive margin Convergent plate boundary Craton Abyssal plain Transform plate boundary Orogen Continental slope Volcanic arc Continental shield Continental shelf Hot spot Accreted terrane Wilson Cycle Questions for Review 1. What are the three types of plate boundaries? Sketch simple diagrams showing the geologic processes occurring at each boundary. 2. Provide a modern-day example of each type of plate boundary. 3. What are hot spots and hot spot tracks? 4. Why is plate tectonics important? 5. Describe the different parts of a continent and how they are pieced together. 6. Contrast the processes and resulting features occurring along active and passive continental margins.
7. Describe the stages and features associated with continental rifting and continental collision. 8. Describe the tectonic setting of the Pacific Northwest and the major geologic features of this setting. rifts Iceland faults central rift valley magma extension asthenosphere lithosphere
Oceanic crust Rift Valley lakes The Red Sea is a more advanced rift African Rift Valley Ocean-Ocean Ocean-Continent Continent-Continent Overriding plate Today 20 million years ago Challenger Deep
Philippine Plate Pacific Plate 40 million years ago Divergent: Plates move away from one another. Lithosphere created. Transform: Plates slide past one another. Lithosphere neither created nor destroyed. Convergent: Plates move toward one another.
Lithosphere destroyed. Hotspot: Plate rides over plume of hot mantle. 4. 3. 2. 1. 6. Continental collision 5. Accreted terranes
4. Subduction zone 3. Ocean basin with passive margins 2. Spreading center develops 1. Continental rifting Cascade Range Volatiles from heating of subducted slab Coast Range Sediment and Basalt Scraped off Subducting Plate Lecture 3 Outline:
Plate Tectonics – from Hypothesis to Theory Learning Objectives: What is the evidence for continental drift? What is the evidence for seafloor spreading? What is the evidence for subduction? How does plate tectonics explain this evidence? How was Plate Tectonics Discovered? Using scientific method how researchers work collectively over time to develop accurate and reliable explanations By observing, hypothesizing, and testing What is the Scientific Method? Observe: gather information Hypothesize: attempt to explain observations Predict: use hypothesis to make testable predictions Test: gather additional evidence from observations or experiments to support or refute hypothesis Modify: modify hypothesis in light of new evidence Repeat steps 3-5 until hypothesis consistent with all available evidence and conceivable tests How does a Hypothesis become a Theory? Hypothesis gradually gains widespread acceptance by repeated testing and modification
Theory = hypothesis that withstands scrutiny over time predictions tested and shown to be accurate What is a Theory? Everyday “theory”: possible explanation or educated guess Scientific “theory”: explanation consistent with available evidence Example: Theory of Evolution supported by large body of scientific evidence succession of organisms in fossil record genetic relationships among modern organisms defined and observable mechanisms of variation and heredity How was Plate Tectonics Discovered? Plate tectonics too slow to directly observe So how do we know plates move? Three independent lines of evidence Continental Drift Seafloor Spreading Subduction What is Continental Drift? Early geologists knew continents moved vertically Fossil sea shells high above sea level
Uplift and subsidence following earthquakes Can continents also move laterally? Hypothesis proposed by Wegener (1920s) Continents once joined as one land mass Pangaea Broke apart and fragments - modern continents - “drifted” to current locations What was Wegener’s Evidence for Continental Drift? Fit of coastlines Distribution of fossils Similar types/ages of rocks on widely separated coastlines Wegener could not explain how continents move through solid rock of ocean floor Why was Wegener’s Hypothesis of Continental Drift Rejected? Discoveries about Earth’s magnetic field and mapping of ocean floor in 1950s provide mechanism for continental drift Seafloor spreading What was Discovered when the Seafloor was Mapped? Major topographic features Ridges
huge underwater mountain ranges through all ocean basins Trenches narrow but very deep, mostly encircle Pacific Ocean Mapping sea floor from ships trenches ridge ridge ridge ridge trench trench trench What was Discovered when the Seafloor was Mapped? Age of seafloor shows orderly distribution Rocks youngest along ridges Progressively older away from ridges No old oceanic crust Oldest rocks on continents (~4 billion yrs) nearly as old as Earth Nowhere on seafloor are rocks older than ~200 million years
Observations suggest ocean crust made at ridge Proven by pattern of magnetism recorded in ocean crust What was Discovered when the Seafloor was Mapped? Youngest rock in red - oldest in blue Symmetry in ages on either side of ridges What was Discovered when the Seafloor was Mapped? What is Earth’s Magnetic Field? Generated by Earth’s rotation and molten iron in outer core Two poles of opposite polarity Magnetic poles near rotational (geographic) poles because field generated by rotation What are Magnetic Reversals? Magnetic field flips polarity (direction) Normal: magnetic north near geographic North Pole Reverse: magnetic north near geographic South Pole
Importance of this discovery not realized until seafloor mapped How do Rocks Record Changes in Earth’s Magnetic Field? Iron minerals align parallel to ambient field as they crystallize Indicates polarity - direction to magnetic north Polarity locked in as rock cools Preserves record of magnetic field polarity at time rock formed High temperature: magnetic minerals randomly oriented During cooling: magnetic minerals align with field After cooling: magnetic orientation recorded in rock will not change as long as rock remains cool – even if magnetic field changes How are Magnetic Polarity Reversals Recorded on the Seafloor? Polarity along ridge south of Iceland Red: rocks formed during current period of normal polarity Other colors: rocks formed during earlier periods of normal polarity
White: rocks with reversed polarity Bands of alternating normal and reversed polarity Symmetric on either side of ridge Ridge axis How Does Seafloor Spreading Explain these Observations? Youngest rock at ridge formed during current period of normal polarity (N) Slightly older rock further from ridge formed during last time field had reverse polarity (R) Even older rock further from ridge formed during previous period of normal polarity (N) Produces magnetization pattern symmetric with respect to ridge How Does Seafloor Spreading Explain these Observations? Seafloor spreading results in Crust older with distance from ridge Stripes of normal and reverse magnetization symmetric to ridge
Provides mechanism for continental drift Seafloor acts as conveyor belt to move continents Amount of new oceanic crust produced at ridges must be balanced by equal amount destroyed elsewhere Where is oceanic crust destroyed? How Does Seafloor Spreading Explain these Observations? What is the Global Distribution of Earthquakes? Not randomly distributed Concentrated near trenches Trench Subduction = process that removes old oceanic crust Discovered by relationship of deep earthquakes to trenches Earthquakes deeper with distance from trench How was Subduction Discovered? Hot Cold How was Subduction Discovered? Earthquakes along and “inland” of trench - deeper with distance
Mark descent of old oceanic crust into mantle Process = subduction Places where it occurs = subduction zones Top left: Map view shows earthquakes become progressively deeper to west of Tonga trench Remaining panels: Vertical cross sections showing descending earthquake belts trench T T T What is the Global Distribution of Volcanoes? Volcanoes not randomly distributed either Concentrated in chains (arcs) inland of trenches Occur above depth where subducting crust causes melting in mantle Pacific “Ring of Fire” Earthquakes and volcanoes associated with subduction How Does Plate Tectonics Explain Earthquakes and Volcanoes? Compare location of plate boundaries to global distribution of earthquakes and volcanoes Earthquakes and volcanoes mostly at plate boundaries
convergence and divergence Earthquakes Plate Boundaries Volcanoes 25 Ridge new oceanic crust produced by upwelling of asthenosphere seafloor spreading What Happens Where Plates Diverge? Trench denser oceanic crust sinks beneath lighter continental crust subduction What Happens Where Plates Converge? Average about 5 cm per year Roughly rate fingernails grow
Not very fast, but over long periods of time plates move great distances 50 km per 1,000,000 yr Portland - Salem 5000 km per 100,000,000 yr Portland - Boston How Fast are Plate Motions? How Fast are Plate Motions? We can now directly measure plate motions using GPS What is the scientific method? What is the difference between a hypothesis and a scientific theory? Explain the geologic evidence that led to the discovery of a) continental drift, b) sea floor spreading, and c) subduction. How does plate tectonics explain this evidence? Why does Earth have a magnetic field? How do rocks provide a record of the magnetic field back through time? What evidence does this provide that shows seafloor spreading? What causes plates to move? Questions for Review Lecture 3: Plate Tectonics: Hypothesis to TheoryReading: Chapter 2, pages 30-41 Topics: · The scientific method · Evidence for continental drift · Evidence for seafloor spreading
· Evidence for subduction · Theory of Plate Tectonics Next lecture: Plate Tectonics: Mechanisms and Margins 3.1. Introduction The internal structure of the Earth is a result of two processes: accretion and differentiation. Planetary accretion left the earth in a near molten state as kinetic energy was converted to heat during collision. Once molten the earth segregated into layers of different densities and chemical composition. This left the earth with a light crust, an intermediate mantle, and a dense core. Continued additions of heat released by radioactive decay of unstable elements in the earth have kept the Earth’s interior hot. The distribution of pressure and temperature inside the earth (which together control a material’s melting point) has left the earth with layers of different physical properties. Layers below the melting point are rigid and strong (i.e., the lithosphere), while layers close to the melting point are plastic and weak (i.e., the asthenosphere). The rigid lithosphere (crust and uppermost mantle) is broken into pieces (plates) that float on the softer, deeper mantle asthenosphere. Today we will discuss the evidence that lithospheric plates move. This is called plate tectonics. 3.2. Plate Tectonics and the Scientific Method The development of plate tectonic theory is an excellent example of the scientific method in action. The scientific
method is the way researchers work collectively over time to develop an accurate, reliable, and unbiased explanation of the world around us. This is accomplished by repeated observation, testing, and modification. The scientific method consists of the following steps: 1. Observe: gather information about some phenomenon 2. Hypothesize: attempt to explain the observations 3. Predict: use the hypothesis to make testable predictions 4. Test: gather additional evidence from observations or experiments that may support or refute the hypothesis 5. Modify: modify the hypothesis in light of new evidence. Repeat steps 3-5 until a hypothesis has been developed that is consistent with all available evidence and conceivable tests. Once a hypothesis has been accepted by most researchers in a field, it is considered a scientific theory. Hypotheses gradually gain acceptance over many years through repeated testing and modification to become theories. A theory is a hypothesis that has withstood repeated scrutiny over time. However, it is still possible that a new observation will be made or a new experiment devised that produces evidence contradictory to theory. For example, future advances in technology might allow experiments that are inconceivable or impossible today; the results of which may or may not be consistent with established theory. If they are not consistent, a new round of modification and testing is required. Be aware that the meaning of theory in everyday conversation is vastly different from the meaning of theory in science. We often
use the word “theory” to refer to a possible explanation that may be nothing more than an educated guess. But to scientists, a theory is an explanation backed up by and consistent with a vast body of evidence. For example, the theory of evolution is a scientific explanation backed by evidence such as the succession of organisms in the fossil record and genetic relationships among modern organisms and includes defined and observable mechanisms of genetic variation and heredity. Despite the overwhelming evidence supporting evolution (essentially as Darwin originally envisioned it), whether or not evolution should even be taught in science classes is still debated by public school boards. 3.2.1 Discovery of Plate Tectonics The theory of plate tectonics states that Earth’s outermost layer (crust) is broken into a number of large and small plates. The plates move relative to one another because they rest on top of hotter, more mobile material. The theory of plate tectonics is the foundation for much of our understanding of how the earth works, yet the processes of plate tectonics generally operate too slowly for direct observation. So how do we know that lithospheric plates move? We will discuss three independent lines of evidence: 1. Evidence for Continental Drift 2. Evidence for Seafloor Spreading 3. Evidence for Subduction Plate tectonics ties these observations together and explains many related geologic phenomena, such as mid-ocean ridges, deep-sea trenches, volcanoes, and earthquakes. 3.3. Continental Drift
Fossil sea shells high above sea level and vertical land movements following earthquakes provided early scientists convincing evidence that the continents can move vertically. But could the continents move laterally as well? Alfred Wegener first proposed the hypothesis of continental drift in 1915. According to continental drift the modern continents were once joined as parts of preexisting larger supercontinents. Wegener’s hypothesis attempted to explain the jig-saw fit between continents on either side of the Atlantic Ocean. Wegner stated that the continents were once joined as one land mass he called Pangaea which broke apart and the fragments (today’s continents) slowly drifted to their present positions. This was not a new idea, but Wegner was the first to systematically describe the evidence that the continents were once joined. In addition to the fit of the coastlines, Wegener also noted the distribution of fossils across the southern continents. The same fossils of land animals (early reptiles) and plants (such as ferns) are found on continents that today are separated by vast oceans. This distribution could be explained if the continents were joined at the time these plants and animals lived. Fossil evidence for continental drift from the southern continents includes reptiles, freshwater crocodiles, and ferns which are found on continents now widely separated by oceans. Wegner was also aware that the ages and types of rocks along the African and South American coastlines, as well as the European and North American coastlines, were similar. When the
continents were pieced back together, the different rock types lined up nicely. This is illustrated by the distribution of 250 million-year old glacial deposits across the southern continents. The glacial deposits also indicate the direction the ice was moving, and suggest an ice cap centered in South Africa and radiating across the adjoining continents. Grooves carved by glaciers 250-300 million years ago (arrows) provide evidence for continental drift. The left diagram assumes the continents were in their present day locations. The distribution of the glaciers can best be explained if the continents were once joined as part of Pangaea. The supercontinent Pangaea (meaning “all lands” in Greek) started breaking up about 200 million years ago. When Pangaea began breaking apart the continents Gondwanaland and Laurasia formed. The Tethys Sea separated the two continents. By 65 million years ago the land masses looked like our modern-day continents. 3.3.1. Rejection of Continental Drift Wegener’s hypothesis of continental drift did not gain wide acceptance at the time because of an unresolved problem: he could not explain how the continents move through the solid rock of the ocean floor. He thought continents moved through Earth’s crust by tidal forces. Tidal forces are much too weak to move continents! Wegener was wrong on one big point: continents do not plow through ocean floor. Instead both continents and ocean floor are rigid plates that “float” on the asthenosphere. 3.4 Seafloor Spreading
Why was continental drift revisited? Discoveries about Earth’s magnetism in the 1950s and mapping of the ocean floor made possible by technologies developed during World War II eventually provided a mechanism for continental drift called seafloor spreading. Plate tectonics was widely accepted by the late 1960’s. Technology developed to track German U-boats allowed detailed mapping of the seafloor in peacetime. 3.4.1. Origin of Earth’s Magnetic Field The molten iron in the Earth’ outer core combined with the Earth’s rotation generates a magnetic field. Molten iron in the outer core flows around the solid inner core because of Earth’s rotation. The flow generates an electrical current, in turn creating a magnetic field. Earth’s magnetic field resembles that of a simple bar magnetic with two poles of opposite polarity. Because the magnetic field is generated by Earth’s rotation, the magnetic poles are within a few degrees of the rotational (geographic) poles. Earth’s magnetic field resembles a bar magnet with two poles of opposite polarity. The magnetic poles are inclined at 11.50 relative to the rotational (geographic) poles. Magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. The origin of the Earth's magnetic field is not completely understood, but is thought to be associated with electrical currents produced by rotation in the spinning liquid outer core
made of iron and nickel. The liquid iron in the outer core is also in motion due to convection, changes in convective currents may account for the observed variability in the strength and polarity of Earth’s magnetic field (Earth’s rotation changes only very slowly over geologic time). Note that Venus does not have a measurable magnetic field despite having a similar liquid iron core because the rotation of Venus (243 Earth days) is too slow to generate an electrical field. 3.4.2. Magnetic Inclination The magnetic field at any point on the Earth’s surface can be described in terms of its inclination. Inclination is the angle (dip) of the magnetic field lines with respect to the surface of the Earth and varies from 0o (horizontal) at the magnetic equator to 90o (vertical) at the magnetic poles. Magnetic field inclination varies as a function of magnetic latitude across the Earth’s surface. Iron-rich minerals align themselves parallel to the magnetic field as they crystallize from molten lava. At temperatures above 580°C the magnetic poles of individual atoms point in random directions. When a magnetic field is present and temperatures are below 580°C, the magnetic poles of individual atoms align with the magnetic field. Now the minerals are permanently magnetized. Therefore rocks containing iron minerals preserve a record of the ambient magnetic field at their time of formation (paleomagnetism) indicating both field polarity (direction to magnetic north) and inclination. At high temperatures magnetic mineral grains are randomly
oriented (left). During cooling the magnetic minerals align themselves with the ambient field (right). Once the temperature drops below the Curie temperature that orientation is locked in place and will not change even if the ambient field changes (so long as the rock is not re-heated). 3.4.3. Apparent Polar Wander When the magnetic direction and inclination of rocks of different ages from the same continent are plotted on the map, they appear to indicate that the magnetic poles have wandered great distances over time. Furthermore, rocks from different continents appear to show the magnetic poles taking different paths at the same time. This phenomenon is known as apparent polar wander. The apparent location of the magnetic pole follows vastly different tracks over time when reconstructed using rocks from different continents. Wandering Pole or Drifting Continents? The apparent polar wander curves for different continents appear to show a chaotic magnetic pole moving all over the place and even in different places at the same time! Although the magnetic pole does move, it remains within a few degrees of the geographic pole because the magnetic field is generated by the earth’s rotation. This is known as true polar wander. The apparent polar wander curves make more sense if the continents are moving in different directions with respect to the magnetic pole (and each other). This is another line of evidence for continental drift that Wegner was unaware of, but a mechanism for continental drift was still lacking.
3.4.4. Magnetic Reversals For reasons that are not understood, Earth’s magnetic field occasionally reverses polarity. In other words, the magnetic field flips direction (a magnetic reversal). The polarity or direction of the magnetic field is recorded in rocks in same way that inclination is. Rocks indicating a field direction the same as today’s (magnetic north pole near the geographic North Pole) are said to have normal polarity. Rocks indicating the opposite field (magnetic north pole near the geographic South Pole) have reversed polarity. Earth’s magnetic field during periods of normal polarity (left) and reversed polarity (right). The geologic record provides evidence of 171 field reversals in the last 71 million years. Time is divided into periods of predominately normal polarity or predominately reverse polarity. Periods are called magnetic chrons. Within each chron there can be subchrons (times of opposite polarity). Today we are in a period of normal polarity (the Brunhes chron). The period from 730,000 years ago to 2.45 million years ago was period of predominately reverse polarity called the Matuyama chron. Within the Matuyama chron are three short periods of normal polarity, these are subchrons. The importance of this discovery in understanding continental drift was not realized until the seafloor was mapped. Polarity reversals dating back 20 million years.
3.4.5 Evidence from the Seafloor When the seafloor was mapped in the 1950s, several discoveries were made that added to our understanding of continental drift and led to the development of the theory of plate tectonics: 1. The topography of the seafloor was found to be dominated by huge underwater mountain ranges called mid-ocean ridges running through the ocean basins like seams on a baseball. 2. Another curious feature of the ocean basins is the narrow but very deep trenches, most notably along the western margin of the Pacific Ocean. On this map, the shallower water over mid-ocean ridges is shown in yellow green, the deeper parts of the ocean basins in blue, and the very deep trenches in purple. The shallowest parts pf the oceans, show in red, are actually the flooded, low- lying parts of the continents called continental shelves. 3. The age of rocks on the seafloor shows an orderly distribution. Rocks were youngest along the mid-ocean ridges and progressively older away from the mid-ocean ridges. 4. The thickness of seafloor sediments also supports this age distribution. Sediments are thinner toward the ridge because they have had less time to accumulate. 5. Although the oldest rocks on the continents (4 billion years old) are nearly as old as the earth itself, nowhere on the seafloor are rocks found to be older than about 200 million years. In this map the youngest seafloor rocks are shown in red and the oldest in blue. Note the symmetry in ages on either side of the mid-ocean ridge system.
3.4.6. Paleomagnetism and Seafloor Spreading When the magnetic field direction recorded in the rocks of the seafloor was mapped in detail, a striking pattern emerged. Bands of rocks recording alternating periods of normal and reversed polarity are symmetric on either side of the mid-ocean ridge. This figure shows the magnetic orientation of rocks along a section of mid-ocean ridge south of Iceland. Rocks shown red are rocks that formed during the current period of normal polarity. Other colors show rocks formed during earlier periods of normal polarity. White areas indicate rocks formed during periods of reversed polarity. Note the symmetry with respect to the ridge axis. The Atlantic Ocean did not exist 250 million years ago! The continents that now border it were joined into a single vast continent that Alfred Wegener named Pangaea. About 200 million years ago, new spreading centers split the huge continent. The Atlantic continues to widen today at 2-4 cm/yr. If the spreading rate is fast, a larger amount of young warm oceanic lithosphere is produced and the ridge will be wider. A slow-spreading ridge will be narrower. The Atlantic Ocean spreads slowly, growing wider at 2-4 cm/yr. The Pacific spreading center is fast by comparison: 6-20 cm/yr. Note the differences in widths of the mid-Atlantic and East Pacific ridges (due to the differences in their spreading rates) on the maps above. Collectively, these observations of the age distribution and magnetic symmetry of the sea floor led to the idea of seafloor
spreading. Seafloor spreading is a result of rising magma at the mid-ocean ridges which cools to form new ocean crust. As new crust is formed the older crust is pushed away from the ridge. This explains both the age distribution of the seafloor and the symmetry with respect to the ridge. Rising magma creating new crust at the mid-ocean ridge during a period of normal polarity while older crust is pushed away from the ridge (a). Following a period of reversed polarity, crust is once again being formed with normal polarity (b). The resulting pattern of rocks formed during alternating periods of normal and reversed polarity is symmetric about the mid-ocean ridge (c). 3.5. Subduction Seafloor spreading also provides a mechanism for continental drift. The drifting continents do not plow through the seafloor, instead the seafloor acts as a giant conveyor belt in which the embedded continents are taken along for the ride. However, seafloor spreading creates a new problem: the amount of new ocean crust created by seafloor spreading must be balanced by equal amount being destroyed elsewhere; otherwise earth’s crust would swell like a balloon! Where is ocean crust being destroyed? The location of deep earthquakes near the deep-sea trenches provided the answer. Earthquakes are not randomly distributed globally. Adjacent to and inland from deep-sea trenches is one place where earthquakes are concentrated. Earthquakes are also concentrated along mid-ocean ridges. Earthquakes are not randomly distributed globally. Adjacent to
and inland from deep-sea trenches in one place where earthquakes are concentrated. The origin of earthquakes becomes deeper along one side of ocean trenches with increasing distance from the trench. This marks the descent of old oceanic crust into the mantle as it is destroyed. This process is known as subduction and the places where it occurs are called subduction zones. Top Left: Earthquakes get progressively deeper to the west of the Tonga trench. Remaining panels: Progressively deeper earthquakes away from the trench (arrow) mark the descent of subducted crust. 3.6. Plate Tectonics: Tying it all Together The theory of plate tectonics ties the observations of continental drift, seafloor spreading, and subduction together while also explaining related geologic phenomena (such as volcanoes and earthquakes). The earth’s lithosphere is broken into about a dozen major pieces and numerous smaller fragments called plates. The lithospheric plates float on the denser but weaker asthenosphere and move in different directions and speeds relative to one another. The major lithospheric plates Where plates are moving away from each other, such as at mid- ocean ridges, new oceanic crust is being created by magma rising from the asthenosphere. Where plates collide with each other, denser oceanic crust is forced under lighter continental crust and is destroyed by subduction. As the subducted plate melts, magma is generated, resulting in a chain of volcanoes on
the overriding plate. Through the processes of seafloor spreading and subduction, oceanic crust is continuously recycled. Continental crust is too buoyant to be subducted, so the oldest rocks are found on the continents because continental crust is not destroyed by subduction. New oceanic crust is created by rising magma at mid ocean ridges. As the crust moves away from the ridge and ages, it eventually is forced back down into the mantle and destroyed by subduction. The movement of the lithospheric plates is driven by convection currents in the mantle. This convection may overturn the entire mantle (as pictured below) or may be broken into different layers. Because each plate is a rigid body, two additional forces keep the plates in motion: 1. Ridge Push: as new crust is created at the ridge, it pushes the older crust aside. 2. Slab Pull: The weight of the subducted slab drags the rest of the plate into the trench, until it melts. Mechanisms of plate motion: mantle convection, ridge push, and slab pull. 3.6.1. Satellite Observation of Plate Motions Advancing technology continues to generate new evidence of plate motions. For example, Global Positioning System satellites are able to measure locations so accurately they are able to resolve the direction and velocity of plate motions that are no faster than about 8 cm per year (about the rate fingernails
grow). Plate motions as resolved by GPS. The arrows indicate direction, while length of the arrow indicates speed. Note that all locations within one plate are moving in the same direction and at the same speed. The Eurasian and North American plates are also rotating. Terms to Know Scientific method Magnetic field True polar wander Hypothesis Theory Continental drift Seafloor spreading Subduction Pangaea Polarity Inclination Magnetic reversal Normal polarity Reversed polarity Apparent polar wander Mid-ocean ridge Trench Subduction zone Plate Tectonics Plate Questions for Review 1. What is the scientific method? What is the difference between a hypothesis and a scientific theory? 2. Explain the geologic evidence for continental drift, sea floor spreading, and subduction. How does plate tectonics explain
this evidence? 3. Why does Earth have a magnetic field? How do rocks provide a record of the magnetic field back through time? What evidence does this provide that supports continental drift and seafloor spreading? 4. What causes plates to move? GEO 101 The Solid Earth Week 2 Tectonics Lab Answer Sheet Name:Learning OutcomeQuestionsPoints Comments Become familiar with Earth’s tectonic plates by identifying and locating plates (by name) and plate boundaries (by type) on a map 1, 2/3 Interpret the location and types of plate boundaries based on the location of surface features such as ridges, trenches, and volcanic arcs 4, 6, /5 Infer direction of plate motion based on distribution of plate boundaries and types 3, 5/3 Graph a dataset and use the graph to identify trends in the data 7, 8/3 Use hotspot tracks to infer velocity and direction of plate motion
9-17/6 Explain the age distribution of oceanic crust in relation to tectonic features on the seafloor such as ridges and trenches 18, 19/2 Compare the age of oceanic crust to the age of continental crust and the age of the Earth 20, 21, 22/3 TOTAL /25 Exercise 1 – Plates and Plate Boundaries 1. Plate Name A B C D E F G H I J K
L M N 2. Boundaries. Place an “x” in the appropriate column. # Convergent Divergent Transform # Convergent Divergent Transform 1 15 2 16 3
17 4 18 5 19 6 20 7
21 8 22 9 23 10 24 11
25 12 26 13 27 14 3. Place an x in the appropriate column
Closer Further No Change London (UK) & New York Honolulu, Hawaii & Tokyo, Japan Mecca, Saudi Arabia & Cairo, Egypt New York & Mexico City Rio de Janeiro, Brazil & Cape Town, South Africa Honolulu Hawaii & Los Angeles Cape Town, South Africa & Bombay, India Los Angeles & San Francisco, California Sydney, Australia & Bombay, India
4. 5. Place an x in the appropriate column. Boundary Divergent Convergent Transform 1 2 3 4 5 6 7
6. Exercise 2 – Hot Spots 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. Exercise 3 - The Age Distribution of Oceanic Crust 18. 19. 20. 21. 22.
Page 2 of 4 GEO 101 The Solid Earth Page 1 of 8 Week 2 Lab GEO 101 The Solid Earth Week 2 – Plate Tectonics Lab (25 points) Introduction Plate tectonics is a set of related concepts that explains how the Earth works – including where and why earthquakes and volcanoes occur and how the continents and oceans have changed over time. Learning Outcomes s tectonic plates by identifying and locating plates (by name) and plate boundaries (by type) on a map the location of surface features such as ridges, trenches, and volcanic arcs r direction of plate motion based on distribution of plate boundaries and types data motion ceanic crust in relation to
tectonic features on the seafloor such as ridges and trenches crust and the age of the Earth Exercise 1 – Plates and Plate Boundaries 1. Identify the tectonic plates on the map of plate boundaries (page 2). Refer to the information in the lecture slides or textbook. Record your answers in the answer sheet by identifying each lettered plate by its name. 2. Label each plate boundary as convergent, divergent, or transform (again, referring to the lecture to textbook) using arrows to indicate the relative motion of the plates. Use the following key. Record your answers on the answer sheet by identifying each numbered plate boundary as convergent (subduction zone or continental collision), divergent (mid-ocean ridge or continental rift), or transform by placing an “X” in the appropriate column of the table.
Please note that the ocean ridge is segmented – broken into sections where new ocean crust is made at different rates. These sections are connected by short sections of transform faults that allow the seafloor to spread at different rates. The ocean ridge is nonetheless divergent and not transform – crust is neither made nor recycled at a truly transform boundary. GEO 101 The Solid Earth Page 2 of 8 Week 2 Lab Map of plates (letters A-N) and plate boundaries (numbers 1- 27). GEO 101 The Solid Earth Page 3 of 8 Week 2 Lab 3. Based on the distribution of divergent and convergent plate boundaries, indicate which of the following locations are moving closer together, further apart or show no change by placing an x in the appropriate column. change (they will both be moving in the same direction at the same speed). plate boundary lies between them.
plates are moving away from each other as new crust is being produced, pulling the two cities further apart. are moving towards each other and old crust is being destroyed, bringing the two cities closer together. need to look at the relative directions of the plates to determine if the two cities are moving closer or further apart. Locations Closer Further No Change London (UK) & New York Honolulu, Hawaii & Tokyo, Japan Mecca, Saudi Arabia & Cairo, Egypt New York & Mexico City Rio de Janeiro, Brazil & Cape Town, S. Africa Honolulu, Hawaii & Los Angeles Cape Town, S. Africa & Bombay, India Los Angeles & San Francisco, California Sydney, Australia & Bombay, India Examine the figure and answer the questions that follow. The patterned areas labeled X, Y, and Z are continents; the rest of the map is ocean. 4. How many plates are present? Hint: first
identify what geologic features mark the boundaries between plates (separate different plates). 5. Draw arrows on the map showing the relative directions of plate motions. Record your answers in the answer sheet by indicating whether each numbered boundary is convergent, divergent, or transform. 6. Where is subduction taking place? Describe which plate(s) are being subducted and by which other plate by referring to the lettered continent it contains or the numbered boundaries that surround it. GEO 101 The Solid Earth Page 4 of 8 Week 2 Lab Exercise 2 – Hot Spots The volcanic rocks of the Hawaiian- Emperor volcanic chain are all younger than the surrounding oceanic crust. This volcanic chain also defines two linear trends (Figure 1). In 1963, a geologist named J. Tuzo Wilson suggested that this chain of volcanoes
formed as the Pacific Plate moved over a stationary plume or hot mantle rock, called a hot spot. Basalt magma from the mantle plume ascended through the oceanic crust forming the island chain. This is a testable hypothesis because if this hypothesis is correct, then: 1) the volcanoes should be progressively older farther away from the current location of the hot spot (the Kilauea volcano); 2) this age-distance relationship can be used to measure the rate of movement (velocity) of the Pacific Plate; 3) the trends of the Hawaiian-Emperor chain should define the direction of movement of the Pacific Plate. Procedure: 1. Plot the data in Table 1 (next page) on the graph paper provided. The x-axis (horizontal axis) will be age in millions of years and the y-axis (vertical axis) will be distance from Kilauea in kilometers. Choose a scale for the x and y axis so that the data fills most of the page. 2. Label the axes (x = age in millions of years; y = distance from Kilauea in kilometers). 3. Label the points for Kilauea, Brooks Bank, Gardner Pinnacles, and Suiko with their names. 4. Using a straight edge, draw a best fit straight line for the data between Kilauea and Brooks Bank. Remember, a best fit line means that not all the points will be on the line. Place your line so that an
approximately equal number of points fall on either side of the line. A straight piece of (uncooked) spaghetti works well for adjusting the fit of your line BEFORE you draw it. 5. Draw a second best fit line for the data between Gardner Pinnacles and Suiko. 6. Use your graph to answer the following questions on the answer sheet. If you prefer, you can graph the data using Microsoft Excel or a similar program; the data is provided as Excel and tab-delimited text files in the assignment materials in Blackboard. Step-by step directions for plotting the data in Excel are provided as separate documents in the assignment materials (Excel 2003 and earlier for Windows XP, or Excel 2007 for Windows 7/Vista). Map of the Hawaiian-Emperor chain. GEO 101 The Solid Earth Page 5 of 8 Week 2 Lab Table 1: Age and distance from Kilauea (along the chain) of selected volcanoes from the Hawaiian- Emperor Chain, obtained from Volcanism in Hawaii (Volume 1, 1987).
Volcano Name Age in millions of years (my) Distance from Kilauea (km) Kilauea 0 0 Kohala 0.43 100 West Maui 1.3 221 West Molokai 1.9 280 Oahu 3.7 374 Kauai 5.1 519 Nihoa 7.2 780 Unnamed 9.2 913 Necker 10.3 1058 La Perouse Pinnacle 12 1209 Brooks Bank 13 1256 Gardner Pinnacles 12.3 1435 Laysan 19.9 1818 Pearl and Hermes Reef 20.6 2281 Midway 27.7 2432 Abbott 38.7 3280 Daikakuji 42.2 3493 Yuryaku 43.4 3520 Koko 48.1 3758 Jingu 55.4 4175 Nintoku 56.2 4452 Suiko 64.7 4860
GEO 101 The Solid Earth Page 6 of 8 Week 2 Lab Graph Paper for Exercise 2 – Hot Spots GEO 101 The Solid Earth Page 7 of 8 Week 2 Lab Answer the following questions on the answer sheet. 7. What does this graph indicate about the general relationship between age and distance from Kilauea? In other words, how does age change with distance from the hot spot? 8. Describe (in words) how the two lines differ in terms of slope. What does this difference in slope tell you about the motion of the Pacific Plate? 9. Using the data in Table 1, calculate the velocity of the Pacific Plate in kilometers per million years (km/my) while the portion of the Hawaiian hotspot track between Kilauea and Brooks Bank was formed. Remember, velocity is distance divided by time (in this case, age expressed in millions of
years). First, determine the distance between Kilauea and Brooks Bank. Then divide by the difference in age between Kilauea and Brooks Bank. 10. Calculate the velocity of the Pacific Plate while Emperor Seamounts were formed. First, find the distance between Gardner Pinnacles and Suiko in Table 1. Then, divide by the difference in age between Gardner and Suiko. 11. Convert the plate velocity for each segment of the hotspot track from km/my to centimeters per year (cm/yr). To do this multiply your results by 0.1. Kilauea-Brooks Bank (Q9): Gardner- Suiko (Q10): 12. Compare the plate velocity between Kilauea and Brooks Bank to the plate velocity between Gardner and Suiko. Are the velocities the same? Or did the plate speed up or slow down? 13. In map view, the Hawaiian-Emperor chain bends at Daikakuji seamount where the Hawaiian and Emperor chains meet. What does this bend represent? 14. What direction was the plate moving while the Emperor Seamounts formed?
15. What direction is the plate moving after the bend (and currently), while the Hawaiian Ridge was forming? 16. How long ago did the bend, or change in the direction of motion of the Pacific Plate, occur? Hint: Find Daikakuji in the table. 17. Compare the timing of the bend with the timing of the velocity change. Which happened first, the change in velocity or the change in plate motion? GEO 101 The Solid Earth Page 8 of 8 Week 2 Lab Exercise 3 - The Age Distribution of Oceanic Crust Use this week’s lectures to answer the following questions. 18. Near what major geologic feature of the Earth’s surface is older oceanic crust always located?
19. What is the age of the oldest oceanic crust found on Earth? 20. How does this age compare with the oldest continental rocks? 21. Assuming that the Earth is 4.6 billion years old, what percentage of Earth history is recorded by the rocks of the ocean basins? Hint: Determine the age of the oldest rock in the ocean basins by referring to the second figure in section 3.4.5 of the lecture. Divide the age of the oldest oceanic crust by the age of the earth to determine what fraction of earth's history is recorded by the rocks of the seafloor. Multiply the result by 100 to express your answer as a percent. 22. Why are there no really “old” rocks found on the ocean floor? Why are really old rocks only found on the continents?

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Lecture 4 Outline Plate Tectonics – Mechanisms and MarginsL.docx

  • 1. Lecture 4 Outline: Plate Tectonics – Mechanisms and Margins Learning Objectives: What are the types of plate boundaries? What processes occur at different types of plate boundaries? What are hotspots? How does tectonics build continents and ocean basins? What Happens at Plate Boundaries? Plate interiors stable - geologic activity limited to surface processes But interactions between plates at plate boundaries results in Magma and volcanism Faulting and earthquakes Mountain building Production of new crust Recycling of old crust What are the Types of Plate Boundaries? Divergent plates pulled apart Convergent plates collide
  • 2. Transform plates sheared Each plate surrounded by different types of boundaries What are the Types of Plate Boundaries? What are Divergent Plate Boundaries? Ridges Crust pulled apart Magma by decompression melting in asthenosphere Cools to make new oceanic crust Oceanic crust lithosphere asthenosphere magma central rift valley faults North Atlantic Ridge Mid-Atlantic Ridge
  • 3. East Pacific Ridge Indian Ridge Antarctic Ridge Where are Divergent Plate Boundaries Found? Ocean ridge above sea level in Iceland Where are Divergent Plate Boundaries Found? What are the Major Geologic Features of the Ocean Ridge? Shield Volcano Edge of North American Plate Fault Down-dropped fault block Central rift valley Filled by lava flows What are Convergent Plate Boundaries? Two plates collide with each other – two types Subduction zone Between two plates of different density - denser plate subducted melting in mantle by addition of water from subducted plate Trench and volcanic arc - chain of volcanoes on overriding plate Earthquakes
  • 4. What are Convergent Plate Boundaries? Collision zone between plates too buoyant to subduct Crust thickened and mountains raised instead Earthquakes but no volcanoes Indian Plate Eurasian Plate Younger and weaker Older and stronger deformed Which Plate gets Subducted? If both plates composed of oceanic crust older and denser crust subducted by younger and lighter crust Overriding plate Plate boundary Where Can We Find an Example of an Oceanic Plate Subducted by Another Oceanic Plate?
  • 5. Pacific Plate subducted by Philippine Plate at Mariana Trench Pacific Plate (older) Philippine Plate (younger) Japan Trench Mariana Trench Challenger Deep Eurasian Plate Which Plate gets Subducted? If one plate of continental crust and one of oceanic crust denser oceanic crust subducted by lighter continental crust Material too light to subduct added to continent as accreted terranes sediments, volcanic islands, fragments of continental crust Where Can We Find an Example of a Collision Zone? Indian and Eurasian Plates Collision began 45 mya when subduction completely closed ocean basin Himalaya and Tibetan Plateau
  • 6. Recent or continuing collisions produce Earth’s tallest mountains 50 mya today Closing Ocean Spreading Ocean 14 Oblique motion between plates – without convergence or divergence Faulting and earthquakes - no volcanism What are Transform Plate Boundaries? Where Can We Find an Example of a Transform Plate Boundary? San Andreas Fault Transform boundary between Pacific and North American plates N. American plate Pacific plate
  • 7. What are Hotspots? Volcanism normally at divergent or convergent plate boundaries Melting of mantle by decompression or addition of water But some volcanoes located in middle of plates! What’s the explanation? Hotspot: plume of hot rock rising from deep mantle What are Hotspots? Source of magma well below lithosphere Doesn’t move with plate – rather plate passes over magma source Results in age progression to volcanism - hotspot track Example: Hawaiian Islands What is a Tectonic Setting? Geologic environment of area relative to any nearby plate boundaries or hotspots Each setting associated with specific geologic processes types of volcanoes, earthquakes, etc Japan Hawaii
  • 8. Oregon Nevada Divergent Plates pulled apart lithosphere created Convergent Plates move together lithosphere recycled What are the Different Tectonic Settings? Transform Plates slide past one another Lithosphere neither created nor recycled Hotspot Plate passes over deep mantle plume What are the Different Tectonic Settings? What is the Tectonic Setting of Oregon? Convergent - between North America and Juan de Fuca plates Subduction, terrane accretion, earthquakes, volcanic arc
  • 9. 22 Accretion buoyant material from subducting plate Coast Range Volcanism partial melting in mantle due to addition of water Cascades Coast Ranges and Cascades both result of subduction Specific processes are different What is the Tectonic Setting of Oregon? Continents assembled from pieces of crust too light to subduct How Does Plate Tectonics Build Continents? By accretion and collision Subduction removes intervening oceanic crust Accretion and volcanism adds buoyant material to overriding plate Intervening oceanic crust removed - continents collide How is Continental Crust Made? strongly enriched in silica relative to oceanic crust/mantle Subduction zones: recycling centers that sort out continental crust
  • 10. Mantle (peridotite) 45% SiO2 Oceanic crust (basalt) 50-55% SiO2 Continental crust (granite) 60-75% SiO2 Degree of Silica Enrichment How do Subduction Zones Make Continental Crust? 1. Accretion of buoyant materials as ocean crust subducts Builds continent outward over time Buoyant materials often added as terranes - block of crust with different geologic origin and history from adjacent areas Volcanic island arcs, marine sediments, thick oceanic crust, fragments of continental crust Moved great distances on subducting plate How do Subduction Zones Make Continental Crust? 2. melting of subducted plate and mantle and fractional crystallization Makes magmas richer in silica than oceanic crust Erupted at surface volcanoes
  • 11. Cascades Emplaced within crust intrusions Sierra Nevada Rifting of continent to create new divergent boundary Caused by mantle upwelling beneath continent Example: East African Rift Valley Active rifts grow over time to become new oceans How does Plate Tectonics Make Ocean Basins? African Rift Valley Red Sea advanced rift Rift Valley lakes rifts How do Rifts Start and Grow into Ocean Basins? Crust heated by upwelling mantle causing uplift Uplift collapses with continued stretching to form rift valley Rift floods to form narrow sea
  • 12. Widens by seafloor spreading into new ocean basin Processes of continent assembly, breakup, and re-assembly Continental rifting, seafloor spreading, subduction and accretion, collision Rift breaks continent apart New divergent boundary/ridge forms Grows into mature ocean basin Subduction begins as oceanic crust becomes older, colder, and denser Terranes accreted to continent Continents collide when intervening oceanic crust completely subducted Stages in Tectonic Cycle with modern-day examples of each stage What is the Tectonic Cycle? 1. Continental rifting 2. Spreading center develops 3. Ocean basin 4. Subduction 5. Terrane Accretion 6. Continental collision What are the three types of plate boundaries? Describe the geologic processes occurring at each boundary. Provide a modern-day example of each type of plate boundary. How does plate tectonic activity build continents and make continental crust? How does plate tectonic activity break continents apart and make oceanic crust?
  • 13. Describe the tectonic setting of the Pacific Northwest. Questions for Review Lecture 4: Plate Tectonics: Mechanisms and Margins Reading: Chapter 2, pages 42-61 Topics: · Divergent Plate Boundaries · Convergent Plate Boundaries · Transform Plate Boundaries · Hot Spots and Hot Spot Tracks · Plate Tectonics and the Building of Continents · Continental Rifting and the Development of Ocean Basins · Tectonic Settings · Significance of Plate Tectonics Next lecture: Mineral Composition and Properties 4.1 Plate Boundaries The interior of tectonic plates are usually geologically stable. The main geologic processes operating in the middle of plates are erosion and sedimentation. However, along plate boundaries dynamic interactions between plates are responsible for volcanic activity, faulting and earthquakes; the building of mountain ranges, the creation of new crust, and the recycling of old crust.
  • 14. There are three types of plate boundaries: 1. Divergent plate boundaries occur where plates are being pulled apart. 2. Convergent plate boundaries occur where plates are moving together or colliding. 3. Transform plate boundaries occur where two plates are sliding past each other in different directions. 4.2. Divergent Plate Boundaries New oceanic crust is created at divergent plate boundaries as two plates pull away from each other. Divergent plate boundaries are associated with extensional stress, shallow earthquakes, and basaltic volcanism. Examples include the mid- ocean ridges such as the Mid-Atlantic Ridge. At divergent plate boundaries, extensional stresses pull two plates apart, allowing magma to rise from the asthenosphere towards the surface and create new oceanic crust. The extension is accommodated by normal faults in the crust, creating a rift valley. The Mid-Atlantic ridge rises above sea level in Iceland, allowing easy observation of its features. The central rift valley of the Mid-Atlantic Ridge in southwest Iceland. The photographer is standing at the edge of the North American Plate, looking across the rift valley towards the Eurasian Plate. The broad mountains in the distance are shield volcanoes, the characteristic style of volcanism in a divergent tectonic setting. Where a continent is being pulled apart a new divergent boundary called a continental rift valley is created. The East
  • 15. African Rift Valley is an example of a continental rift. Continental rift valleys represent the earliest stage in the development of a new ocean basin as a single continental plate splits into two fragments as new oceanic crust grows between them. Development of a rift in continental crust by mantle upwelling Various stages of rifting are present in East Africa. The East African Rift Valley represents a very young rift not yet flooded by seawater. The Red Sea is a more advanced rift that is continuing to grow into a new ocean basin by seafloor spreading. 4.3 Convergent Plate Boundaries Convergent plate boundaries occur when two plates collide with each other. There are three types of convergent boundaries: 1. Ocean-Ocean: collision between two plates composed of oceanic crust; older denser oceanic crust is subducted and destroyed. 2. Ocean-Continent: collision between oceanic and continental crust; oceanic crust is subducted by the continent and destroyed. 3. Continent-Continent: collision between two plates composed of continental crust; both are too light to be subducted. 4.3.1. Ocean-Ocean Convergence In a collision between two plates of oceanic crust, the older and denser crust gets subducted beneath the younger and lighter crust. A deep-sea trench marks the subduction zone at the surface. As subducted slab descends into the mantle, it partially melts. The rising magma produces a chain of volcanoes on the overriding, younger oceanic plate called a volcanic island arc.
  • 16. A convergent plate boundary between two oceanic plates. The older, denser plate is subducted by the younger, more buoyant plate. A trench marks the subduction zone at the surface. As the subducted plate descends into the mantle and melts, rising magma creates a volcanic island arc. The subduction of the Pacific Plate by the Philippine plate is an example of an ocean-ocean convergent boundary. The subduction zone is marked by the Mariana Trench. Challenger Deep, its deepest point, is 10,911 meters (35,798 ft) deep and is the deepest place in the ocean. 4.3.2. Ocean-Continent Convergence When a plate composed of oceanic crust collides with one made of continental crust, the denser oceanic crust is subducted beneath the lighter continental crust. As in the case of ocean- ocean subduction, there is a trench and volcanic arc, with the volcanic arc on the continent. Compression crumples the edge of the continent, producing mountains. Ocean sediments and anything else too light to subduct such as a volcanic island arc or fragment of continental crust is scraped off and added to edge of continent as accreted terranes. Both types of subduction zones (ocean-ocean and ocean-continent) are associated with compressional stress, deep earthquakes, and andesitic volcanism. At a convergent boundary between an oceanic plate and a continental plate, the denser oceanic plate is subducted by the lighter continental plate. A volcanic arc is built on the overriding continental plate as the subducted oceanic plate melts. 4.3.3. Continent-Continent Convergence Continental crust is too light to be subducted. So when two continents collide, thickening, thrust faulting, and uplift
  • 17. produces mountain ranges and high plateaus. A suture zone marks the boundary where two formerly separate continents are joined. When two continents collide, neither is subducted. Instead, the compressional stress produces thickening and thrust faulting, which uplifts mountain ranges and high plateaus along and behind the collision zone. The collision of India and Asia is an example of an ongoing continental collision and is raising the Himalaya and Tibetan Plateau. The collision zone between India and Asia is marked by the abrupt Himalayan front and the world’s highest mountains. Behind the collision zone, an area of thickened continental crust supports the extensive Tibetan plateau, the largest high plateau in the world. The collision of India with Asia began about 20 million years ago when a subduction zone completely destroyed the ocean basin that formerly separated the two continents. This same process is closing the Mediterranean Sea and bringing Africa into a collision with Europe. The ancient collision between North America and Africa during the formation of Pangaea is marked today by the heavily eroded Appalachian Mountains. 4.4. Transform Plate Boundaries A transform plate boundary occurs where two plates are sliding past each other. The boundary is marked by a system of faults. Motion between the plates is not always smooth due to friction; instead the plates periodically lurch past each other, resulting in earthquakes. Shear stresses crumple the edges of the plates and rocks on either side of the faults. A transform plate boundary occurs where two plates slip past
  • 18. each other in different directions. California’s San Andreas Fault is an excellent example of a transform plate boundary between the Pacific and North American plates. The fault marks the plate boundary and runs through the center of the photo from top to bottom. Note how the edges of the two plates are crumpled due to friction along the fault. 4.4.1 Evolution of the San Andreas Fault As we saw in the example of India and Asia, plate boundaries do not remain static, instead they evolve over time. The San Andreas Fault is another example. The San Andreas Fault originated when the subduction zone off the west coast of North America caught up to and swallowed the seafloor spreading center that separated two oceanic plates: the Pacific Plate and the Farallon Plate. The Farallon Plate and spreading center was split into two smaller pieces connected by a transform fault that we call the San Andreas Fault. The San Andreas Fault originated when the subduction zone off central and southern California caught up with and swallowed the offshore spreading center. This severed the oceanic Farallon Plate into two smaller oceanic plates; connected by a transform fault we call the San Andreas Fault. Seafloor spreading and subduction still occurs further north off the Pacific Northwest and further south off Central America. 4.5. Hot Spots Volcanism is usually associated with plate boundaries. However, some volcanic centers are located in the middle of a plate. Such centers are called hot spots and have a magma source in the deep mantle, perhaps extending all the way to the core boundary. Because the magma source is below the lithosphere, it does not move with the plate. Instead a chain of volcanoes called a hot spot track is produced as the plate moves
  • 19. over the stationary hot spot. The Hawaiian Islands are an excellent example of a hot spot track. The big island of Hawaii is currently located over the hot spot. The Hawaiian Islands were produced as the Pacific Plate moved northwestwards over a fixed hot spot in the deep mantle, producing a chain of volcanic islands that get younger to the southeast. All of the Hawaiian Islands are volcanic, but the volcanoes become extinct as the plate carries the island off the hot spot. The big island of Hawaii is currently over the hot spot and is the only Hawaiian island with active volcanoes. 4.5.1. Hot Spot Tracks The hot spot responsible for the Hawaiian Islands has existed for more than 70 million years. A chain of extinct volcanic islands and underwater seamounts extends west and north from the currently active island of Hawaii all the way to the Aleutian trench. Southeast of the island of Hawaii a new volcanic island is growing beneath the sea as the big island moves away from the hot spot. Hot spot tracks are good indicators of plate motion. Note the bend in the Hawaiian hot spot track, where the Hawaiian Ridge meets the Emperor Seamounts. This bend indicates a change in the direction of the Pacific plate from northerly to northwesterly about 40 million years ago. The Hawaiian hot spot track extends from the big island of Hawaii, which is currently over the hot spot, all the way to the western Aleutian Trench. The bend in the track of extinct volcanic islands and submerged seamounts indicates a change in the direction of the Pacific Plate about 40 million years ago. 4.5.2. Continental Hot Spot Tracks The Yellowstone hotspot is the world’s only hot spot beneath a continent and is responsible for Yellowstone National park’s
  • 20. famous geysers and hot springs as well as three enormous volcanic eruptions in the last two million years. The hot spot track is marked by a chain of progressively older volcanic centers that extends southwest across Idaho’s Snake River Plain to north-central Nevada. The origin of the hot spot is unknown, but is probably related to the extensive Columbia Flood Basalts of eastern Washington and Oregon that were erupted about 18 million years ago. The Columbia Plateau flood basalts were erupted about 18 million years ago. The flood basalts were followed by progression of volcanic calderas from the oldest in north central Nevada to the youngest in Yellowstone National Park. The flood basalts are believed to mark the first appearance of a mantle hot spot beneath the North American plate, which then produced a chain of volcanic calderas as the North American plate moves southwestwards over the hot spot. The hot spot is currently located beneath Yellowstone National Park. Note that the hot spot track does not line up with its presumed origin in the Columbia flood basalts; instead there is a pronounced offset to the south. This has not yet been satisfactorily explained, but may result from interactions in the mantle between the hot spot plume and the oceanic plate being subducted beneath North America. It is well established that the hot spot currently beneath Yellowstone has left a track of progressively older calderas across southern Idaho to an earliest location beneath north central Nevada (16 million years ago). This is a good match for the age of the Columbia plateau flood basalts (18-16 million years old), however the caldera track does not quite line up geographically with the eruption points of the Columbia basalts. Geologists continue to debate the exact relationship between the Yellowstone hot spot and the Columbia flood basalts.
  • 21. The very large volume of the Columbia basalts (several thousand cubic kilometers), their geologically short eruption time (less than 1.5 million years), and trace element compositions of the lavas themselves all suggest a deep mantle (hot spot) source. The Columbia basalts were erupted from three locations: the Chief Joseph dike swarm near Hells Canyon in Northeast Oregon; the Monument dike swarm near John Day, Oregon, and the Steens Mountain dikes south of Burns, Oregon. There is an age progression from north (older) to south (younger) within the Columbia basalts, so that also points to the hot spot (which next manifested itself as a caldera in north central Nevada). The geography of the hot spot eruptions is likely also complicated by interactions between the rising mantle plume and the subducted oceanic plate. Remember, the subducting plate is not instantaneously destroyed at the trench, but instead slowly sinks into the mantle, gradually re-assimilating into the mantle by melting. So the oceanic plate being subducted by the Cascadia Subduction Zone extends into the mantle beneath western North America. This means that a rising mantle plume will have to get past the subducted slab. The hot spot plume also appears to be inclined, rather than exactly vertical, and may even wiggle around a bit, instead of being fixed as portrayed in textbooks. So the connection between the Yellowstone hot spot and the Columbia flood basalts is quite complicated. What happens when a hot spot forms? Imagine a container filled with hot wax and oil. As the wax (deep mantle) is heated from below (Earth’s core), it becomes buoyant and begins to rise through the oil (mantle). The viscosity of the oil provides resistance, causing the rising wax to form a large glob, or head, followed by a narrow tail, or stem.
  • 22. A mantle hot spot consists of leading glob, or head, followed by a narrower stem, or tail, because of the high viscosity of the mantle. The hot spot forms a large glob (head) followed by a narrow tail (stem) because of the resistance of the mantle to getting out of its way (viscosity). When the head reaches the lithosphere, it flattens out and erupts a huge volume of magma onto the surface. This type of eruption is known as a flood basalt. As the plate moves away from the hot spot, it is dragged over the narrower stem of the hot spot. This creates a chain of smaller, but still very large, eruptions called a hot spot track. A mantle hot spot plume has a wide head followed by a narrow stem. As the head reaches the surface, it erupts massive flood basalts. As the plate passes over the narrower stem, a hot spot track is produced leading away from the flood basalt. The flowing sequence of figures illustrates the events marking the passage of the North American plate over the Yellowstone hot spot. The head created the massive flood basalts that cover eastern Washington and Oregon. The tail, or stem, has produced a chain of progressively younger volcanic calderas from north-central Nevada to the present location of the hot spot beneath northwest Wyoming. As the calderas move away from the hot spot stem, they are buried by younger basalt lava flows fed by remnant pockets of magma in the lithosphere. This is typical of the style of volcanism on Idaho’s Snake River Plain.
  • 23. The location of hotspots worldwide. The red dots mark the present location of a plate over a hot spot. The blue dots mark their earliest known location. The Hawaiian hot spot has no point of origin because its track disappears into the Aleutian subduction zone, meaning all earlier traces of the hot spot have been destroyed by subduction. Note that some hotspots first appeared at locations that have been divergent plate boundaries or mid ocean ridges, so the earliest evidence of these hotspots is now found on the coastlines of continents now separated by oceans, for example in the North Atlantic (Iceland) and South Atlantic. 4.6. Plate Tectonics and the Building of Continents Over hundreds of millions of years the modern continents have been assembled by plate tectonics from smaller fragments of material too light to be subducted. The pieces that make up a continent include: 1. Cratons: very ancient rock that make up the cores of continents. Exactly how cratons formed is still debated. 2. Orogens: belts of deformed rocks representing mountain building episodes during collisions between continental cratons. Only the youngest orogens still stand as mountain ranges. Most orogens are eroded roots of ancient mountain ranges. The collision of two cratons, for example India and Asia a. Subduction of oceanic crust crumples the edge of the overriding plate and creates a volcanic arc b. As collision begins, sediments along subducting plate are deformed and welded onto overriding plate c. As collision ends, subducted oceanic crust breaks off and
  • 24. continues its descent into mantle The two continents are now joined along a high mountain range (the orogen) 4.6.1. Continental Shields Continental shields are assemblages of ancient cratons and their associated orogens sutured together by tectonic collisions. The North American continental shield is also called the Canadian Shield because glacial erosion has exposed the ancient rocks of the shield at the surface throughout much of central and eastern Canada, as well as the upper Midwest of the United States. Further south the shield is covered by younger sedimentary rocks. The shield is surrounded by five younger orogenic belts: the Grenville, Innuitian, Caledonian, Appalachian, and Cordilleran Orogens marking younger episodes of tectonic collision and uplift. The North American continental shield consists of several very ancient cratons joined by belts of deformed rocks called orogens that mark collisions between cratons. Note that the different parts of the shield are all older than 1.6 billion years. The shield is surrounded by younger orogens, the Grenville, Appalachian, and Cordilleran orogenies. 4.6.2. Accreted Terranes Accreted terranes are small pieces of crust too buoyant to be subducted that are carried along by oceanic plates and added to the edge of a continent. Accreted terranes may include old volcanic island arcs, small pieces of continental crust called micro continents, or margins of continents sliced off by transform faults. Accreted terranes can be rafted great distances by plate motions and get added to the edge of continent as the oceanic crust is subducted. The entire west coast of North America is complex jumble of more than 40
  • 25. accreted terranes added to North America by 200 million years of oceanic subduction in this location. This has gradually shifted the plate boundary further west. Accreted terranes added to the western margin of the North American plate by more than 200 million years of subduction along the west coast. 4.7. Continental Rifting and the Development of New Ocean Basins In addition to building continents, plate motions also tear continents apart. This process is called continental rifting. 1. The process begins when crust is heated by rising magma from the mantle, causing uplift of a broad area 2. As rising magma continues to push the crust aside, the uplift collapses to form a rift valley 3. New oceanic crust is created as the rift spreads. The area is flooded to form a narrow sea such as the Red Sea 4. The narrow sea may grow to become a new ocean basin with a mid-ocean ridge and seafloor spreading center. Rifting can end at any point, not all rifts develop into a new ocean basin. The four stages of continental rifting and development of a new ocean basin, with modern day examples of each stage Two active spreading centers, the Red Sea and Gulf of Aden, have rifted the Arabian Plate from the African Plate. The East Africa Rift is less active and appears to be a failing rift that does not develop into an ocean basin. The more active part of
  • 26. the rift extends through the Red Sea. At the north end of the Red Sea, the direction of rifting is changing again. Instead of continuing into the Mediterranean, the active rift turns northeast and is separating Sinai and Israel from Jordan. Only the southern portion is flooded by the sea, the remainder stands as much as 1000 feet below sea level and is partly filled by the Dead Sea, a salt lake. 4.7.1. Passive Continental Margins As a continental rift develops into a new ocean basin, the edges of the continent become a passive continental margin. A passive continental margin is a transition between oceanic and continental crust that does not coincide with an active plate boundary. Instead the plate boundary is at the mid-ocean ridge. Passive continental margins are characterized by cooling and subsidence of the crust as it moves away from rising magma at the mid-ocean ridge and the development of thick sequences of sediments eroded from the continent. The east coast of North America is an example of a passive continental margin Passive continental margins are transitions between continental and oceanic crust without an active plate boundary. The major features of a passive continental margin are a broad continental shelf followed by the abrupt continental slope which drops to the ocean floor. A thick sequence of sediments buries the ancient faults that mark the episode of rifting. The active plate boundary is out at the active spreading center at the mid-ocean ridge. 4.7.2. Tectonics and Ocean Basins The Atlantic Ocean is an example of a maturing ocean basin with passive continental margins. Its major topographic features such as the mid-ocean ridge, abyssal plain, and
  • 27. continental shelf are explained by plate tectonics. The mid- ocean ridge is the site of seafloor spreading and the active plate boundary. The abyssal plain is underlain by older oceanic crust that has moved away from the ridge, cooled, and subsided. The abyssal plains end abruptly at the continental slope where the crust transitions to lighter and more buoyant continental crust. The continental shelf is the low-lying portion of the continent flooded by shallow seas. Tectonic features of a mature ocean basin, such as the Atlantic Ocean 4.7.3. The Wilson Cycle Eventually, a passive continental margin develops into a new subduction zone as the adjacent oceanic crust becomes older, colder, and denser. The ocean basin, while still spreading at the mid-ocean ridge, starts to grow smaller as oceanic crust is subducted. As the intervening ocean basin disappears the continents are drawn into a new round of collision, albeit in a new place. This cycle of continent formation, breakup, and re- assembly is called the Wilson Cycle. Stages in the Wilson Cycle with modern-day examples of each stage The Wilson Cycle and Drifting Continents Most of the continents were joined as the supercontinent Pangaea 200 million years ago (A). The continents became fragmented as first the Atlantic Ocean, between North America and Africa (B) and then the Indian Ocean, between Antarctica and India, (C) opened and grew (D). A new round of continental collision began as the Tethys Ocean separating India and Asia closed (E). The Atlantic and Indian continue to grow as the Pacific Ocean, ringed by subduction zones, continues to
  • 28. close (F). Eventually a new supercontinent may be formed. 4.7.4. Active vs. Passive Continental Margins Active continental margins are found where subduction occurs between an oceanic and a continental plate. Active continental margins coincide with plate boundaries. Passive continental margins are found where there is a transition between oceanic and continental crust that is still moving as one plate. In this case the plate boundary is out at the mid-ocean ridge. Active continental margins are plate boundaries marked by subduction zones with earthquakes, volcanoes, and mountain building. Passive continental margins are transitions between oceanic and continental crust that do not coincide with a plate boundary. The western U.S. is an active continental margin with all four tectonic settings we have discussed: 1. Divergent: Gulf of California spreading center and extension in the Basin and Range and Rio Grande Rift (a failing continental rift) 2. Transform: the San Andreas and related fault systems 3. Convergent: the Cascadia Subduction Zone 4. Hot Spot: the Yellowstone hot spot 4.8. Tectonic Settings Tectonic setting describes the geologic environment of an area relative to any nearby plate boundaries or hot spots. This figure summarizes the different tectonic settings we have talked about. Each tectonic setting is associated with a particular style of volcanism as we will see in a future lecture.
  • 29. Tectonic settings include divergent, convergent, and transform plate boundaries as well as hot spots. Each tectonic setting produces a characteristic style of volcanism. Summary of Tectonic Settings 4.8.1. Tectonic Setting of the Pacific Northwest The tectonic setting of the Pacific Northwest is an active plate boundary where small oceanic plates, the Gorda (south) Juan de Fuca (central), and Explorer (north) plates are colliding with and being subducted by the continental North American Plate. The active plate boundary lies just offshore and is called the Cascadia Subduction Zone. As the oceanic plates are subducted, materials too light to be subducted are scraped off the oceanic plate and accreted to the western margin of North America to form the Costal Ranges of Vancouver Island, Washington, Oregon, and Northern California. Further inland, a volcanic arc, the Cascade Range, is formed on the overriding plate above the depth where the subducted oceanic plates begin to melt and produce magma, which rises to the surface. The coastal and Cascade ranges are separated by a fore-arc basin, a topographic depression produced by down-warping of the continental plate like a rug due to the tremendous stresses from the subduction zone. This depression is occupied by Puget Sound and the Williamette Valley. SHAPE * MERGEFORMAT The tectonic setting of the Pacific Northwest in map view (left) and in cross section (right), showing the major tectonic features of western and central Oregon.
  • 30. Cross section of the Cascadia subduction zone with major geologic processes and topographic features Although the Coastal Ranges and Cascades are both a product of subduction, the specific processes responsible for their formation are very different. Coastal Ranges like the Olympics of Washington and the Coast Range and Siskiyous of Oregon are a product of accretion and consist primarily of oceanic rocks added to the western margin of the continent. The Cascades, in contrast, are a volcanic range, and are a product of rising magma produced by heating and melting of the subducted plate. In Oregon, tectonic extension is occurring in the back arc basin (east of the Cascades). Why? Two reasons: 1. The Pacific Northwest is being rotated clockwise, creating extension in southeastern Oregon and compression in northwestern Washington. 2. The gradual ending of subduction with the destruction of the last remnants of the Farallon Plate is relieving compression on the western margin of North America, allowing the edge of the plate to “rebound” 4.9. Significance of Plate Tectonics Plate tectonics explains the global distribution of earthquakes, volcanoes, and mountain ranges. Plate tectonics is also responsible for creating and destroying ocean basins as well as building and rearranging the continents. Through the processes of subduction and volcanism, plate tectonics recycles earth materials. Our atmosphere is also maintained through volcanic degassing and climate regulated by controlling the atmospheric carbon dioxide content and the magnitude of the greenhouse effect. Plate Boundaries
  • 31. Earthquakes Volcanoes SHAPE * MERGEFORMAT SHAPE * MERGEFORMAT Compare the map of plate boundaries to the global distribution of earthquakes and volcanoes. The locations of volcanoes do a pretty good job of outlining plate boundaries. Earthquakes do an even better job of outlining plate boundaries. Why? Not all types of plate boundaries are associated with volcanism (there are no volcanoes at transform plate boundaries), but all plate boundaries are associated with earthquakes! Terms to Know Divergent plate boundary
  • 32. Hot spot track Continental rifting Continental rift valley Tectonic setting Passive margin Convergent plate boundary Craton Abyssal plain Transform plate boundary Orogen Continental slope Volcanic arc Continental shield Continental shelf Hot spot Accreted terrane Wilson Cycle Questions for Review 1. What are the three types of plate boundaries? Sketch simple diagrams showing the geologic processes occurring at each boundary. 2. Provide a modern-day example of each type of plate boundary. 3. What are hot spots and hot spot tracks? 4. Why is plate tectonics important? 5. Describe the different parts of a continent and how they are pieced together. 6. Contrast the processes and resulting features occurring along active and passive continental margins.
  • 33. 7. Describe the stages and features associated with continental rifting and continental collision. 8. Describe the tectonic setting of the Pacific Northwest and the major geologic features of this setting. rifts Iceland faults central rift valley magma extension asthenosphere lithosphere
  • 34. Oceanic crust Rift Valley lakes The Red Sea is a more advanced rift African Rift Valley Ocean-Ocean Ocean-Continent Continent-Continent Overriding plate Today 20 million years ago Challenger Deep
  • 35. Philippine Plate Pacific Plate 40 million years ago Divergent: Plates move away from one another. Lithosphere created. Transform: Plates slide past one another. Lithosphere neither created nor destroyed. Convergent: Plates move toward one another.
  • 36. Lithosphere destroyed. Hotspot: Plate rides over plume of hot mantle. 4. 3. 2. 1. 6. Continental collision 5. Accreted terranes
  • 37. 4. Subduction zone 3. Ocean basin with passive margins 2. Spreading center develops 1. Continental rifting Cascade Range Volatiles from heating of subducted slab Coast Range Sediment and Basalt Scraped off Subducting Plate Lecture 3 Outline:
  • 38. Plate Tectonics – from Hypothesis to Theory Learning Objectives: What is the evidence for continental drift? What is the evidence for seafloor spreading? What is the evidence for subduction? How does plate tectonics explain this evidence? How was Plate Tectonics Discovered? Using scientific method how researchers work collectively over time to develop accurate and reliable explanations By observing, hypothesizing, and testing What is the Scientific Method? Observe: gather information Hypothesize: attempt to explain observations Predict: use hypothesis to make testable predictions Test: gather additional evidence from observations or experiments to support or refute hypothesis Modify: modify hypothesis in light of new evidence Repeat steps 3-5 until hypothesis consistent with all available evidence and conceivable tests How does a Hypothesis become a Theory? Hypothesis gradually gains widespread acceptance by repeated testing and modification
  • 39. Theory = hypothesis that withstands scrutiny over time predictions tested and shown to be accurate What is a Theory? Everyday “theory”: possible explanation or educated guess Scientific “theory”: explanation consistent with available evidence Example: Theory of Evolution supported by large body of scientific evidence succession of organisms in fossil record genetic relationships among modern organisms defined and observable mechanisms of variation and heredity How was Plate Tectonics Discovered? Plate tectonics too slow to directly observe So how do we know plates move? Three independent lines of evidence Continental Drift Seafloor Spreading Subduction What is Continental Drift? Early geologists knew continents moved vertically Fossil sea shells high above sea level
  • 40. Uplift and subsidence following earthquakes Can continents also move laterally? Hypothesis proposed by Wegener (1920s) Continents once joined as one land mass Pangaea Broke apart and fragments - modern continents - “drifted” to current locations What was Wegener’s Evidence for Continental Drift? Fit of coastlines Distribution of fossils Similar types/ages of rocks on widely separated coastlines Wegener could not explain how continents move through solid rock of ocean floor Why was Wegener’s Hypothesis of Continental Drift Rejected? Discoveries about Earth’s magnetic field and mapping of ocean floor in 1950s provide mechanism for continental drift Seafloor spreading What was Discovered when the Seafloor was Mapped? Major topographic features Ridges
  • 41. huge underwater mountain ranges through all ocean basins Trenches narrow but very deep, mostly encircle Pacific Ocean Mapping sea floor from ships trenches ridge ridge ridge ridge trench trench trench What was Discovered when the Seafloor was Mapped? Age of seafloor shows orderly distribution Rocks youngest along ridges Progressively older away from ridges No old oceanic crust Oldest rocks on continents (~4 billion yrs) nearly as old as Earth Nowhere on seafloor are rocks older than ~200 million years
  • 42. Observations suggest ocean crust made at ridge Proven by pattern of magnetism recorded in ocean crust What was Discovered when the Seafloor was Mapped? Youngest rock in red - oldest in blue Symmetry in ages on either side of ridges What was Discovered when the Seafloor was Mapped? What is Earth’s Magnetic Field? Generated by Earth’s rotation and molten iron in outer core Two poles of opposite polarity Magnetic poles near rotational (geographic) poles because field generated by rotation What are Magnetic Reversals? Magnetic field flips polarity (direction) Normal: magnetic north near geographic North Pole Reverse: magnetic north near geographic South Pole
  • 43. Importance of this discovery not realized until seafloor mapped How do Rocks Record Changes in Earth’s Magnetic Field? Iron minerals align parallel to ambient field as they crystallize Indicates polarity - direction to magnetic north Polarity locked in as rock cools Preserves record of magnetic field polarity at time rock formed High temperature: magnetic minerals randomly oriented During cooling: magnetic minerals align with field After cooling: magnetic orientation recorded in rock will not change as long as rock remains cool – even if magnetic field changes How are Magnetic Polarity Reversals Recorded on the Seafloor? Polarity along ridge south of Iceland Red: rocks formed during current period of normal polarity Other colors: rocks formed during earlier periods of normal polarity
  • 44. White: rocks with reversed polarity Bands of alternating normal and reversed polarity Symmetric on either side of ridge Ridge axis How Does Seafloor Spreading Explain these Observations? Youngest rock at ridge formed during current period of normal polarity (N) Slightly older rock further from ridge formed during last time field had reverse polarity (R) Even older rock further from ridge formed during previous period of normal polarity (N) Produces magnetization pattern symmetric with respect to ridge How Does Seafloor Spreading Explain these Observations? Seafloor spreading results in Crust older with distance from ridge Stripes of normal and reverse magnetization symmetric to ridge
  • 45. Provides mechanism for continental drift Seafloor acts as conveyor belt to move continents Amount of new oceanic crust produced at ridges must be balanced by equal amount destroyed elsewhere Where is oceanic crust destroyed? How Does Seafloor Spreading Explain these Observations? What is the Global Distribution of Earthquakes? Not randomly distributed Concentrated near trenches Trench Subduction = process that removes old oceanic crust Discovered by relationship of deep earthquakes to trenches Earthquakes deeper with distance from trench How was Subduction Discovered? Hot Cold How was Subduction Discovered? Earthquakes along and “inland” of trench - deeper with distance
  • 46. Mark descent of old oceanic crust into mantle Process = subduction Places where it occurs = subduction zones Top left: Map view shows earthquakes become progressively deeper to west of Tonga trench Remaining panels: Vertical cross sections showing descending earthquake belts trench T T T What is the Global Distribution of Volcanoes? Volcanoes not randomly distributed either Concentrated in chains (arcs) inland of trenches Occur above depth where subducting crust causes melting in mantle Pacific “Ring of Fire” Earthquakes and volcanoes associated with subduction How Does Plate Tectonics Explain Earthquakes and Volcanoes? Compare location of plate boundaries to global distribution of earthquakes and volcanoes Earthquakes and volcanoes mostly at plate boundaries
  • 47. convergence and divergence Earthquakes Plate Boundaries Volcanoes 25 Ridge new oceanic crust produced by upwelling of asthenosphere seafloor spreading What Happens Where Plates Diverge? Trench denser oceanic crust sinks beneath lighter continental crust subduction What Happens Where Plates Converge? Average about 5 cm per year Roughly rate fingernails grow
  • 48. Not very fast, but over long periods of time plates move great distances 50 km per 1,000,000 yr Portland - Salem 5000 km per 100,000,000 yr Portland - Boston How Fast are Plate Motions? How Fast are Plate Motions? We can now directly measure plate motions using GPS What is the scientific method? What is the difference between a hypothesis and a scientific theory? Explain the geologic evidence that led to the discovery of a) continental drift, b) sea floor spreading, and c) subduction. How does plate tectonics explain this evidence? Why does Earth have a magnetic field? How do rocks provide a record of the magnetic field back through time? What evidence does this provide that shows seafloor spreading? What causes plates to move? Questions for Review Lecture 3: Plate Tectonics: Hypothesis to TheoryReading: Chapter 2, pages 30-41 Topics: · The scientific method · Evidence for continental drift · Evidence for seafloor spreading
  • 49. · Evidence for subduction · Theory of Plate Tectonics Next lecture: Plate Tectonics: Mechanisms and Margins 3.1. Introduction The internal structure of the Earth is a result of two processes: accretion and differentiation. Planetary accretion left the earth in a near molten state as kinetic energy was converted to heat during collision. Once molten the earth segregated into layers of different densities and chemical composition. This left the earth with a light crust, an intermediate mantle, and a dense core. Continued additions of heat released by radioactive decay of unstable elements in the earth have kept the Earth’s interior hot. The distribution of pressure and temperature inside the earth (which together control a material’s melting point) has left the earth with layers of different physical properties. Layers below the melting point are rigid and strong (i.e., the lithosphere), while layers close to the melting point are plastic and weak (i.e., the asthenosphere). The rigid lithosphere (crust and uppermost mantle) is broken into pieces (plates) that float on the softer, deeper mantle asthenosphere. Today we will discuss the evidence that lithospheric plates move. This is called plate tectonics. 3.2. Plate Tectonics and the Scientific Method The development of plate tectonic theory is an excellent example of the scientific method in action. The scientific
  • 50. method is the way researchers work collectively over time to develop an accurate, reliable, and unbiased explanation of the world around us. This is accomplished by repeated observation, testing, and modification. The scientific method consists of the following steps: 1. Observe: gather information about some phenomenon 2. Hypothesize: attempt to explain the observations 3. Predict: use the hypothesis to make testable predictions 4. Test: gather additional evidence from observations or experiments that may support or refute the hypothesis 5. Modify: modify the hypothesis in light of new evidence. Repeat steps 3-5 until a hypothesis has been developed that is consistent with all available evidence and conceivable tests. Once a hypothesis has been accepted by most researchers in a field, it is considered a scientific theory. Hypotheses gradually gain acceptance over many years through repeated testing and modification to become theories. A theory is a hypothesis that has withstood repeated scrutiny over time. However, it is still possible that a new observation will be made or a new experiment devised that produces evidence contradictory to theory. For example, future advances in technology might allow experiments that are inconceivable or impossible today; the results of which may or may not be consistent with established theory. If they are not consistent, a new round of modification and testing is required. Be aware that the meaning of theory in everyday conversation is vastly different from the meaning of theory in science. We often
  • 51. use the word “theory” to refer to a possible explanation that may be nothing more than an educated guess. But to scientists, a theory is an explanation backed up by and consistent with a vast body of evidence. For example, the theory of evolution is a scientific explanation backed by evidence such as the succession of organisms in the fossil record and genetic relationships among modern organisms and includes defined and observable mechanisms of genetic variation and heredity. Despite the overwhelming evidence supporting evolution (essentially as Darwin originally envisioned it), whether or not evolution should even be taught in science classes is still debated by public school boards. 3.2.1 Discovery of Plate Tectonics The theory of plate tectonics states that Earth’s outermost layer (crust) is broken into a number of large and small plates. The plates move relative to one another because they rest on top of hotter, more mobile material. The theory of plate tectonics is the foundation for much of our understanding of how the earth works, yet the processes of plate tectonics generally operate too slowly for direct observation. So how do we know that lithospheric plates move? We will discuss three independent lines of evidence: 1. Evidence for Continental Drift 2. Evidence for Seafloor Spreading 3. Evidence for Subduction Plate tectonics ties these observations together and explains many related geologic phenomena, such as mid-ocean ridges, deep-sea trenches, volcanoes, and earthquakes. 3.3. Continental Drift
  • 52. Fossil sea shells high above sea level and vertical land movements following earthquakes provided early scientists convincing evidence that the continents can move vertically. But could the continents move laterally as well? Alfred Wegener first proposed the hypothesis of continental drift in 1915. According to continental drift the modern continents were once joined as parts of preexisting larger supercontinents. Wegener’s hypothesis attempted to explain the jig-saw fit between continents on either side of the Atlantic Ocean. Wegner stated that the continents were once joined as one land mass he called Pangaea which broke apart and the fragments (today’s continents) slowly drifted to their present positions. This was not a new idea, but Wegner was the first to systematically describe the evidence that the continents were once joined. In addition to the fit of the coastlines, Wegener also noted the distribution of fossils across the southern continents. The same fossils of land animals (early reptiles) and plants (such as ferns) are found on continents that today are separated by vast oceans. This distribution could be explained if the continents were joined at the time these plants and animals lived. Fossil evidence for continental drift from the southern continents includes reptiles, freshwater crocodiles, and ferns which are found on continents now widely separated by oceans. Wegner was also aware that the ages and types of rocks along the African and South American coastlines, as well as the European and North American coastlines, were similar. When the
  • 53. continents were pieced back together, the different rock types lined up nicely. This is illustrated by the distribution of 250 million-year old glacial deposits across the southern continents. The glacial deposits also indicate the direction the ice was moving, and suggest an ice cap centered in South Africa and radiating across the adjoining continents. Grooves carved by glaciers 250-300 million years ago (arrows) provide evidence for continental drift. The left diagram assumes the continents were in their present day locations. The distribution of the glaciers can best be explained if the continents were once joined as part of Pangaea. The supercontinent Pangaea (meaning “all lands” in Greek) started breaking up about 200 million years ago. When Pangaea began breaking apart the continents Gondwanaland and Laurasia formed. The Tethys Sea separated the two continents. By 65 million years ago the land masses looked like our modern-day continents. 3.3.1. Rejection of Continental Drift Wegener’s hypothesis of continental drift did not gain wide acceptance at the time because of an unresolved problem: he could not explain how the continents move through the solid rock of the ocean floor. He thought continents moved through Earth’s crust by tidal forces. Tidal forces are much too weak to move continents! Wegener was wrong on one big point: continents do not plow through ocean floor. Instead both continents and ocean floor are rigid plates that “float” on the asthenosphere. 3.4 Seafloor Spreading
  • 54. Why was continental drift revisited? Discoveries about Earth’s magnetism in the 1950s and mapping of the ocean floor made possible by technologies developed during World War II eventually provided a mechanism for continental drift called seafloor spreading. Plate tectonics was widely accepted by the late 1960’s. Technology developed to track German U-boats allowed detailed mapping of the seafloor in peacetime. 3.4.1. Origin of Earth’s Magnetic Field The molten iron in the Earth’ outer core combined with the Earth’s rotation generates a magnetic field. Molten iron in the outer core flows around the solid inner core because of Earth’s rotation. The flow generates an electrical current, in turn creating a magnetic field. Earth’s magnetic field resembles that of a simple bar magnetic with two poles of opposite polarity. Because the magnetic field is generated by Earth’s rotation, the magnetic poles are within a few degrees of the rotational (geographic) poles. Earth’s magnetic field resembles a bar magnet with two poles of opposite polarity. The magnetic poles are inclined at 11.50 relative to the rotational (geographic) poles. Magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. The origin of the Earth's magnetic field is not completely understood, but is thought to be associated with electrical currents produced by rotation in the spinning liquid outer core
  • 55. made of iron and nickel. The liquid iron in the outer core is also in motion due to convection, changes in convective currents may account for the observed variability in the strength and polarity of Earth’s magnetic field (Earth’s rotation changes only very slowly over geologic time). Note that Venus does not have a measurable magnetic field despite having a similar liquid iron core because the rotation of Venus (243 Earth days) is too slow to generate an electrical field. 3.4.2. Magnetic Inclination The magnetic field at any point on the Earth’s surface can be described in terms of its inclination. Inclination is the angle (dip) of the magnetic field lines with respect to the surface of the Earth and varies from 0o (horizontal) at the magnetic equator to 90o (vertical) at the magnetic poles. Magnetic field inclination varies as a function of magnetic latitude across the Earth’s surface. Iron-rich minerals align themselves parallel to the magnetic field as they crystallize from molten lava. At temperatures above 580°C the magnetic poles of individual atoms point in random directions. When a magnetic field is present and temperatures are below 580°C, the magnetic poles of individual atoms align with the magnetic field. Now the minerals are permanently magnetized. Therefore rocks containing iron minerals preserve a record of the ambient magnetic field at their time of formation (paleomagnetism) indicating both field polarity (direction to magnetic north) and inclination. At high temperatures magnetic mineral grains are randomly
  • 56. oriented (left). During cooling the magnetic minerals align themselves with the ambient field (right). Once the temperature drops below the Curie temperature that orientation is locked in place and will not change even if the ambient field changes (so long as the rock is not re-heated). 3.4.3. Apparent Polar Wander When the magnetic direction and inclination of rocks of different ages from the same continent are plotted on the map, they appear to indicate that the magnetic poles have wandered great distances over time. Furthermore, rocks from different continents appear to show the magnetic poles taking different paths at the same time. This phenomenon is known as apparent polar wander. The apparent location of the magnetic pole follows vastly different tracks over time when reconstructed using rocks from different continents. Wandering Pole or Drifting Continents? The apparent polar wander curves for different continents appear to show a chaotic magnetic pole moving all over the place and even in different places at the same time! Although the magnetic pole does move, it remains within a few degrees of the geographic pole because the magnetic field is generated by the earth’s rotation. This is known as true polar wander. The apparent polar wander curves make more sense if the continents are moving in different directions with respect to the magnetic pole (and each other). This is another line of evidence for continental drift that Wegner was unaware of, but a mechanism for continental drift was still lacking.
  • 57. 3.4.4. Magnetic Reversals For reasons that are not understood, Earth’s magnetic field occasionally reverses polarity. In other words, the magnetic field flips direction (a magnetic reversal). The polarity or direction of the magnetic field is recorded in rocks in same way that inclination is. Rocks indicating a field direction the same as today’s (magnetic north pole near the geographic North Pole) are said to have normal polarity. Rocks indicating the opposite field (magnetic north pole near the geographic South Pole) have reversed polarity. Earth’s magnetic field during periods of normal polarity (left) and reversed polarity (right). The geologic record provides evidence of 171 field reversals in the last 71 million years. Time is divided into periods of predominately normal polarity or predominately reverse polarity. Periods are called magnetic chrons. Within each chron there can be subchrons (times of opposite polarity). Today we are in a period of normal polarity (the Brunhes chron). The period from 730,000 years ago to 2.45 million years ago was period of predominately reverse polarity called the Matuyama chron. Within the Matuyama chron are three short periods of normal polarity, these are subchrons. The importance of this discovery in understanding continental drift was not realized until the seafloor was mapped. Polarity reversals dating back 20 million years.
  • 58. 3.4.5 Evidence from the Seafloor When the seafloor was mapped in the 1950s, several discoveries were made that added to our understanding of continental drift and led to the development of the theory of plate tectonics: 1. The topography of the seafloor was found to be dominated by huge underwater mountain ranges called mid-ocean ridges running through the ocean basins like seams on a baseball. 2. Another curious feature of the ocean basins is the narrow but very deep trenches, most notably along the western margin of the Pacific Ocean. On this map, the shallower water over mid-ocean ridges is shown in yellow green, the deeper parts of the ocean basins in blue, and the very deep trenches in purple. The shallowest parts pf the oceans, show in red, are actually the flooded, low- lying parts of the continents called continental shelves. 3. The age of rocks on the seafloor shows an orderly distribution. Rocks were youngest along the mid-ocean ridges and progressively older away from the mid-ocean ridges. 4. The thickness of seafloor sediments also supports this age distribution. Sediments are thinner toward the ridge because they have had less time to accumulate. 5. Although the oldest rocks on the continents (4 billion years old) are nearly as old as the earth itself, nowhere on the seafloor are rocks found to be older than about 200 million years. In this map the youngest seafloor rocks are shown in red and the oldest in blue. Note the symmetry in ages on either side of the mid-ocean ridge system.
  • 59. 3.4.6. Paleomagnetism and Seafloor Spreading When the magnetic field direction recorded in the rocks of the seafloor was mapped in detail, a striking pattern emerged. Bands of rocks recording alternating periods of normal and reversed polarity are symmetric on either side of the mid-ocean ridge. This figure shows the magnetic orientation of rocks along a section of mid-ocean ridge south of Iceland. Rocks shown red are rocks that formed during the current period of normal polarity. Other colors show rocks formed during earlier periods of normal polarity. White areas indicate rocks formed during periods of reversed polarity. Note the symmetry with respect to the ridge axis. The Atlantic Ocean did not exist 250 million years ago! The continents that now border it were joined into a single vast continent that Alfred Wegener named Pangaea. About 200 million years ago, new spreading centers split the huge continent. The Atlantic continues to widen today at 2-4 cm/yr. If the spreading rate is fast, a larger amount of young warm oceanic lithosphere is produced and the ridge will be wider. A slow-spreading ridge will be narrower. The Atlantic Ocean spreads slowly, growing wider at 2-4 cm/yr. The Pacific spreading center is fast by comparison: 6-20 cm/yr. Note the differences in widths of the mid-Atlantic and East Pacific ridges (due to the differences in their spreading rates) on the maps above. Collectively, these observations of the age distribution and magnetic symmetry of the sea floor led to the idea of seafloor
  • 60. spreading. Seafloor spreading is a result of rising magma at the mid-ocean ridges which cools to form new ocean crust. As new crust is formed the older crust is pushed away from the ridge. This explains both the age distribution of the seafloor and the symmetry with respect to the ridge. Rising magma creating new crust at the mid-ocean ridge during a period of normal polarity while older crust is pushed away from the ridge (a). Following a period of reversed polarity, crust is once again being formed with normal polarity (b). The resulting pattern of rocks formed during alternating periods of normal and reversed polarity is symmetric about the mid-ocean ridge (c). 3.5. Subduction Seafloor spreading also provides a mechanism for continental drift. The drifting continents do not plow through the seafloor, instead the seafloor acts as a giant conveyor belt in which the embedded continents are taken along for the ride. However, seafloor spreading creates a new problem: the amount of new ocean crust created by seafloor spreading must be balanced by equal amount being destroyed elsewhere; otherwise earth’s crust would swell like a balloon! Where is ocean crust being destroyed? The location of deep earthquakes near the deep-sea trenches provided the answer. Earthquakes are not randomly distributed globally. Adjacent to and inland from deep-sea trenches is one place where earthquakes are concentrated. Earthquakes are also concentrated along mid-ocean ridges. Earthquakes are not randomly distributed globally. Adjacent to
  • 61. and inland from deep-sea trenches in one place where earthquakes are concentrated. The origin of earthquakes becomes deeper along one side of ocean trenches with increasing distance from the trench. This marks the descent of old oceanic crust into the mantle as it is destroyed. This process is known as subduction and the places where it occurs are called subduction zones. Top Left: Earthquakes get progressively deeper to the west of the Tonga trench. Remaining panels: Progressively deeper earthquakes away from the trench (arrow) mark the descent of subducted crust. 3.6. Plate Tectonics: Tying it all Together The theory of plate tectonics ties the observations of continental drift, seafloor spreading, and subduction together while also explaining related geologic phenomena (such as volcanoes and earthquakes). The earth’s lithosphere is broken into about a dozen major pieces and numerous smaller fragments called plates. The lithospheric plates float on the denser but weaker asthenosphere and move in different directions and speeds relative to one another. The major lithospheric plates Where plates are moving away from each other, such as at mid- ocean ridges, new oceanic crust is being created by magma rising from the asthenosphere. Where plates collide with each other, denser oceanic crust is forced under lighter continental crust and is destroyed by subduction. As the subducted plate melts, magma is generated, resulting in a chain of volcanoes on
  • 62. the overriding plate. Through the processes of seafloor spreading and subduction, oceanic crust is continuously recycled. Continental crust is too buoyant to be subducted, so the oldest rocks are found on the continents because continental crust is not destroyed by subduction. New oceanic crust is created by rising magma at mid ocean ridges. As the crust moves away from the ridge and ages, it eventually is forced back down into the mantle and destroyed by subduction. The movement of the lithospheric plates is driven by convection currents in the mantle. This convection may overturn the entire mantle (as pictured below) or may be broken into different layers. Because each plate is a rigid body, two additional forces keep the plates in motion: 1. Ridge Push: as new crust is created at the ridge, it pushes the older crust aside. 2. Slab Pull: The weight of the subducted slab drags the rest of the plate into the trench, until it melts. Mechanisms of plate motion: mantle convection, ridge push, and slab pull. 3.6.1. Satellite Observation of Plate Motions Advancing technology continues to generate new evidence of plate motions. For example, Global Positioning System satellites are able to measure locations so accurately they are able to resolve the direction and velocity of plate motions that are no faster than about 8 cm per year (about the rate fingernails
  • 63. grow). Plate motions as resolved by GPS. The arrows indicate direction, while length of the arrow indicates speed. Note that all locations within one plate are moving in the same direction and at the same speed. The Eurasian and North American plates are also rotating. Terms to Know Scientific method Magnetic field True polar wander Hypothesis Theory Continental drift Seafloor spreading Subduction Pangaea Polarity Inclination Magnetic reversal Normal polarity Reversed polarity Apparent polar wander Mid-ocean ridge Trench Subduction zone Plate Tectonics Plate Questions for Review 1. What is the scientific method? What is the difference between a hypothesis and a scientific theory? 2. Explain the geologic evidence for continental drift, sea floor spreading, and subduction. How does plate tectonics explain
  • 64. this evidence? 3. Why does Earth have a magnetic field? How do rocks provide a record of the magnetic field back through time? What evidence does this provide that supports continental drift and seafloor spreading? 4. What causes plates to move? GEO 101 The Solid Earth Week 2 Tectonics Lab Answer Sheet Name:Learning OutcomeQuestionsPoints Comments Become familiar with Earth’s tectonic plates by identifying and locating plates (by name) and plate boundaries (by type) on a map 1, 2/3 Interpret the location and types of plate boundaries based on the location of surface features such as ridges, trenches, and volcanic arcs 4, 6, /5 Infer direction of plate motion based on distribution of plate boundaries and types 3, 5/3 Graph a dataset and use the graph to identify trends in the data 7, 8/3 Use hotspot tracks to infer velocity and direction of plate motion
  • 65. 9-17/6 Explain the age distribution of oceanic crust in relation to tectonic features on the seafloor such as ridges and trenches 18, 19/2 Compare the age of oceanic crust to the age of continental crust and the age of the Earth 20, 21, 22/3 TOTAL /25 Exercise 1 – Plates and Plate Boundaries 1. Plate Name A B C D E F G H I J K
  • 66. L M N 2. Boundaries. Place an “x” in the appropriate column. # Convergent Divergent Transform # Convergent Divergent Transform 1 15 2 16 3
  • 69. 25 12 26 13 27 14 3. Place an x in the appropriate column
  • 70. Closer Further No Change London (UK) & New York Honolulu, Hawaii & Tokyo, Japan Mecca, Saudi Arabia & Cairo, Egypt New York & Mexico City Rio de Janeiro, Brazil & Cape Town, South Africa Honolulu Hawaii & Los Angeles Cape Town, South Africa & Bombay, India Los Angeles & San Francisco, California Sydney, Australia & Bombay, India
  • 71. 4. 5. Place an x in the appropriate column. Boundary Divergent Convergent Transform 1 2 3 4 5 6 7
  • 72. 6. Exercise 2 – Hot Spots 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. Exercise 3 - The Age Distribution of Oceanic Crust 18. 19. 20. 21. 22.
  • 73. Page 2 of 4 GEO 101 The Solid Earth Page 1 of 8 Week 2 Lab GEO 101 The Solid Earth Week 2 – Plate Tectonics Lab (25 points) Introduction Plate tectonics is a set of related concepts that explains how the Earth works – including where and why earthquakes and volcanoes occur and how the continents and oceans have changed over time. Learning Outcomes s tectonic plates by identifying and locating plates (by name) and plate boundaries (by type) on a map the location of surface features such as ridges, trenches, and volcanic arcs r direction of plate motion based on distribution of plate boundaries and types data motion ceanic crust in relation to
  • 74. tectonic features on the seafloor such as ridges and trenches crust and the age of the Earth Exercise 1 – Plates and Plate Boundaries 1. Identify the tectonic plates on the map of plate boundaries (page 2). Refer to the information in the lecture slides or textbook. Record your answers in the answer sheet by identifying each lettered plate by its name. 2. Label each plate boundary as convergent, divergent, or transform (again, referring to the lecture to textbook) using arrows to indicate the relative motion of the plates. Use the following key. Record your answers on the answer sheet by identifying each numbered plate boundary as convergent (subduction zone or continental collision), divergent (mid-ocean ridge or continental rift), or transform by placing an “X” in the appropriate column of the table.
  • 75. Please note that the ocean ridge is segmented – broken into sections where new ocean crust is made at different rates. These sections are connected by short sections of transform faults that allow the seafloor to spread at different rates. The ocean ridge is nonetheless divergent and not transform – crust is neither made nor recycled at a truly transform boundary. GEO 101 The Solid Earth Page 2 of 8 Week 2 Lab Map of plates (letters A-N) and plate boundaries (numbers 1- 27). GEO 101 The Solid Earth Page 3 of 8 Week 2 Lab 3. Based on the distribution of divergent and convergent plate boundaries, indicate which of the following locations are moving closer together, further apart or show no change by placing an x in the appropriate column. change (they will both be moving in the same direction at the same speed). plate boundary lies between them.
  • 76. plates are moving away from each other as new crust is being produced, pulling the two cities further apart. are moving towards each other and old crust is being destroyed, bringing the two cities closer together. need to look at the relative directions of the plates to determine if the two cities are moving closer or further apart. Locations Closer Further No Change London (UK) & New York Honolulu, Hawaii & Tokyo, Japan Mecca, Saudi Arabia & Cairo, Egypt New York & Mexico City Rio de Janeiro, Brazil & Cape Town, S. Africa Honolulu, Hawaii & Los Angeles Cape Town, S. Africa & Bombay, India Los Angeles & San Francisco, California Sydney, Australia & Bombay, India Examine the figure and answer the questions that follow. The patterned areas labeled X, Y, and Z are continents; the rest of the map is ocean. 4. How many plates are present? Hint: first
  • 77. identify what geologic features mark the boundaries between plates (separate different plates). 5. Draw arrows on the map showing the relative directions of plate motions. Record your answers in the answer sheet by indicating whether each numbered boundary is convergent, divergent, or transform. 6. Where is subduction taking place? Describe which plate(s) are being subducted and by which other plate by referring to the lettered continent it contains or the numbered boundaries that surround it. GEO 101 The Solid Earth Page 4 of 8 Week 2 Lab Exercise 2 – Hot Spots The volcanic rocks of the Hawaiian- Emperor volcanic chain are all younger than the surrounding oceanic crust. This volcanic chain also defines two linear trends (Figure 1). In 1963, a geologist named J. Tuzo Wilson suggested that this chain of volcanoes
  • 78. formed as the Pacific Plate moved over a stationary plume or hot mantle rock, called a hot spot. Basalt magma from the mantle plume ascended through the oceanic crust forming the island chain. This is a testable hypothesis because if this hypothesis is correct, then: 1) the volcanoes should be progressively older farther away from the current location of the hot spot (the Kilauea volcano); 2) this age-distance relationship can be used to measure the rate of movement (velocity) of the Pacific Plate; 3) the trends of the Hawaiian-Emperor chain should define the direction of movement of the Pacific Plate. Procedure: 1. Plot the data in Table 1 (next page) on the graph paper provided. The x-axis (horizontal axis) will be age in millions of years and the y-axis (vertical axis) will be distance from Kilauea in kilometers. Choose a scale for the x and y axis so that the data fills most of the page. 2. Label the axes (x = age in millions of years; y = distance from Kilauea in kilometers). 3. Label the points for Kilauea, Brooks Bank, Gardner Pinnacles, and Suiko with their names. 4. Using a straight edge, draw a best fit straight line for the data between Kilauea and Brooks Bank. Remember, a best fit line means that not all the points will be on the line. Place your line so that an
  • 79. approximately equal number of points fall on either side of the line. A straight piece of (uncooked) spaghetti works well for adjusting the fit of your line BEFORE you draw it. 5. Draw a second best fit line for the data between Gardner Pinnacles and Suiko. 6. Use your graph to answer the following questions on the answer sheet. If you prefer, you can graph the data using Microsoft Excel or a similar program; the data is provided as Excel and tab-delimited text files in the assignment materials in Blackboard. Step-by step directions for plotting the data in Excel are provided as separate documents in the assignment materials (Excel 2003 and earlier for Windows XP, or Excel 2007 for Windows 7/Vista). Map of the Hawaiian-Emperor chain. GEO 101 The Solid Earth Page 5 of 8 Week 2 Lab Table 1: Age and distance from Kilauea (along the chain) of selected volcanoes from the Hawaiian- Emperor Chain, obtained from Volcanism in Hawaii (Volume 1, 1987).
  • 80. Volcano Name Age in millions of years (my) Distance from Kilauea (km) Kilauea 0 0 Kohala 0.43 100 West Maui 1.3 221 West Molokai 1.9 280 Oahu 3.7 374 Kauai 5.1 519 Nihoa 7.2 780 Unnamed 9.2 913 Necker 10.3 1058 La Perouse Pinnacle 12 1209 Brooks Bank 13 1256 Gardner Pinnacles 12.3 1435 Laysan 19.9 1818 Pearl and Hermes Reef 20.6 2281 Midway 27.7 2432 Abbott 38.7 3280 Daikakuji 42.2 3493 Yuryaku 43.4 3520 Koko 48.1 3758 Jingu 55.4 4175 Nintoku 56.2 4452 Suiko 64.7 4860
  • 81. GEO 101 The Solid Earth Page 6 of 8 Week 2 Lab Graph Paper for Exercise 2 – Hot Spots GEO 101 The Solid Earth Page 7 of 8 Week 2 Lab Answer the following questions on the answer sheet. 7. What does this graph indicate about the general relationship between age and distance from Kilauea? In other words, how does age change with distance from the hot spot? 8. Describe (in words) how the two lines differ in terms of slope. What does this difference in slope tell you about the motion of the Pacific Plate? 9. Using the data in Table 1, calculate the velocity of the Pacific Plate in kilometers per million years (km/my) while the portion of the Hawaiian hotspot track between Kilauea and Brooks Bank was formed. Remember, velocity is distance divided by time (in this case, age expressed in millions of
  • 82. years). First, determine the distance between Kilauea and Brooks Bank. Then divide by the difference in age between Kilauea and Brooks Bank. 10. Calculate the velocity of the Pacific Plate while Emperor Seamounts were formed. First, find the distance between Gardner Pinnacles and Suiko in Table 1. Then, divide by the difference in age between Gardner and Suiko. 11. Convert the plate velocity for each segment of the hotspot track from km/my to centimeters per year (cm/yr). To do this multiply your results by 0.1. Kilauea-Brooks Bank (Q9): Gardner- Suiko (Q10): 12. Compare the plate velocity between Kilauea and Brooks Bank to the plate velocity between Gardner and Suiko. Are the velocities the same? Or did the plate speed up or slow down? 13. In map view, the Hawaiian-Emperor chain bends at Daikakuji seamount where the Hawaiian and Emperor chains meet. What does this bend represent? 14. What direction was the plate moving while the Emperor Seamounts formed?
  • 83. 15. What direction is the plate moving after the bend (and currently), while the Hawaiian Ridge was forming? 16. How long ago did the bend, or change in the direction of motion of the Pacific Plate, occur? Hint: Find Daikakuji in the table. 17. Compare the timing of the bend with the timing of the velocity change. Which happened first, the change in velocity or the change in plate motion? GEO 101 The Solid Earth Page 8 of 8 Week 2 Lab Exercise 3 - The Age Distribution of Oceanic Crust Use this week’s lectures to answer the following questions. 18. Near what major geologic feature of the Earth’s surface is older oceanic crust always located?
  • 84. 19. What is the age of the oldest oceanic crust found on Earth? 20. How does this age compare with the oldest continental rocks? 21. Assuming that the Earth is 4.6 billion years old, what percentage of Earth history is recorded by the rocks of the ocean basins? Hint: Determine the age of the oldest rock in the ocean basins by referring to the second figure in section 3.4.5 of the lecture. Divide the age of the oldest oceanic crust by the age of the earth to determine what fraction of earth's history is recorded by the rocks of the seafloor. Multiply the result by 100 to express your answer as a percent. 22. Why are there no really “old” rocks found on the ocean floor? Why are really old rocks only found on the continents?