Plate Tectonics

Slide 1: Lithospheric Environments SAC IB

Slide 2: The Earth’s Crust is Part of a Dynamic System

Slide 3: INTRODUCTION TO LITHOSPHERIC ENVIRONMENTS LESSON 1

Slide 10: You need to know the characteristics of the core, mantle and crust. Cutaway views showing the internal structure of the Earth. Below: This view drawn to scale demonstrates that the Earth's crust literally is only skin deep. Below is a view not drawn to scale to show the Earth's three main layers (crust, mantle, and core) in more detail

Slide 11: The Size and Structure of the Earth The Earth is about 12,750 kilometres (km) in diameter This was known by the ancient Greeks, but it was not until the turn of the 20th century that scientists determined that our planet is made up of three main layers: crust, mantle, and core. This layered structure can be compared to that of a boiled egg. The crust, the outermost layer, is rigid and very thin compared with the other two. Beneath the oceans, the crust varies little in thickness, generally extending only to about 5 km. The thickness of the crust beneath continents is much more variable but averages about 30 km; under large mountain ranges, such as the Alps or the Sierra Nevada, however, the base of the crust can be as deep as 100 km. Like the shell of an egg, the Earth's crust is brittle and can break.

Slide 12: Below the crust is the mantle, a dense, hot layer of semi-solid rock approximately 2,900 km thick. The mantle, which contains more iron, magnesium, and calcium than the crust, is hotter and denser because temperature and pressure inside the Earth increase with depth. At the centre of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron-nickel alloy) rather than stony. The core is actually made up of two distinct parts: a 2,200 km- thick liquid outer core and a 1,250 km-thick solid inner core. As the Earth rotates, the liquid outer core spins, creating the Earth's magnetic field. Not surprisingly, the Earth's internal structure influences plate tectonics. The upper part of the mantle is cooler and more rigid than the deep mantle; in many ways, it behaves like the overlying crust. Together they form a rigid layer of rock called the lithosphere (from lithos, Greek for stone). The lithosphere tends to be thinnest under the oceans and in volcanically active continental areas, such as the Western United States. Averaging at least 80 km in thickness over much of the Earth, the lithosphere has been broken up into the moving plates that contain the world's continents and oceans. Scientists believe that below the lithosphere is a relatively narrow, mobile zone in the mantle called the asthenosphere (from asthenes, Greek for weak). This zone is composed of hot, semi-solid material, which can soften and flow after being subjected to high temperature and pressure over geologic time. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere.

Slide 18: Earth Quakes and Volcanoes

Slide 21: Continental Drift v Plate Tectonics Alfred Wegener (1880-1930),

Slide 22: The changing positions of the continents as described by Wegener.

Slide 24: Evidence for continental drift and plate tectonics Evidence includes: Geological and structural evidence Fossil evidence Evidence from climate change Palaeomagmetism The age of basalt

Slide 26: Evidence Supporting Continental Drift The Earth's crust is constantly moving, both vertically and horizontally, at rates of up to several centimetres a year. A widely-held theory that explains these movements is called "plate tectonics." It was developed in the mid 1960s by geophysicists. The term "plate" refers to large rigid blocks of the Earth's surface which appear to move as a unit. These plates may include both oceans and continents. When the plates move, the continents and ocean floor above them move as well. Continental Drift occurs when the continents change position in relation to each other. While plate tectonics is a relatively new idea, scientists have been gathering data in support of the Continental Drift theory for a very long time. In 1912, Alfred Wegener and Frank Taylor first proposed the theory that 200 million years ago the Earth had only one giant continent, from which today's continents broke apart and drifted into their current locations. Wegener used the fit of the continents, the distribution of fossils, a similar sequence of rocks at numerous locations, ancient climates, and the apparent wandering of the Earth's polar regions to support his idea.

Slide 27: The Shapes Match The continents look as if they were pieces of a giant jigsaw puzzle that could fit together to make one giant super-continent. The bulge of Africa fits the shape of the coast of North America while Brazil fits along the coast of Africa beneath the bulge. The Plants and Animals Match Wegener noted that plant fossils of late Paleozoic age found on several different continents were quite similar. This suggests that they evolved together on a single large land mass. He was intrigued by the occurrences of plant and animal fossils found on the matching coastlines of South America and Africa, which are now widely separated by the Atlantic Ocean. He reasoned that it was physically impossible for most of these organisms to have travelled or have been transported across the vast ocean. To him, the presence of identical fossil species along the coastal parts of Africa and South America was the most compelling evidence that the two continents were once joined.

Slide 28: What is some Evidence to support his Theory Good Fit

Slide 30: Rocks & Mountains

Slide 31: Fossils

Slide 32: The Rocks Match Broad belts of rocks in Africa and South America are the same type. These broad belts then match when the end of the continents are joined. The Ice Matches Wegener was aware that a continental ice sheet covered parts of South America, southern Africa, India, and southern Australia about 300 million years ago. Glacial striations on rocks show that glaciers moved from Africa toward the Atlantic Ocean and from the Atlantic Ocean onto South America. Such a glaciation is most likely if the Atlantic Ocean were missing and the continents joined. The Positions Don't Match If the continents were cold enough so that ice covered the southern continents, why is no evidence found for ice in the northern continents? The explanation is that the present northern continents were at the equator at 300 million years ago. The discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow.

Slide 33: Why Few People Believed Wegener Wegener's Continental Drift theory was not readily accepted by the science community of his day. It was difficult to conceive of large continents ploughing through the sea floor to move to new locations. What kind of forces could be strong enough to move such large masses of solid rock over such great distances? Wegener suggested that the continents simply ploughed through the ocean floor, but Harold Jeffreys, a noted English geophysicist, argued correctly that it was physically impossible for a large mass of solid rock to plough through the ocean floor without breaking up. It was the work of Hess, a British geologist, who identified evidence from ocean floor exploration and other studies, that rekindled interest in Wegener's theory, and lead to the development of the theory of plate tectonics.

Slide 34: Evidence for Plate tectonics Paeleomagmetism The age of basalt

Slide 36: The Pattern of Paeleomagnetic Reversal Either Side of a Mid-Ocean Ridge.

Slide 37: Computer generated image of a mid-ocean ridge. Brighter colours are higher.

Slide 38: Age of the Sea Floor

Slide 39: Sea Floor Spreading

Slide 40: How do Plates Move?

Slide 41: You need to understand that convection currents are generated by the heat produced from radioactive decay in the core. These convection currents are linked to the movement of plates leading to the creation of new oceanic crust from the mantle. A simplified model of the currents of semi-molten magma within the aesthenosphere.

Slide 42: The locations of the main constructive plate margins marked by ocean ridges.

Slide 43: You need to know the names of the major plates, their direction of movement and their approximate rates of movement (cm/year). You need to know examples of each type of plate margin in order to illustrate the processes involved. You should be able to draw annotated diagrams of each type of plate margin.

Slide 44: The Main Lithosheric / Crustal plates

Slide 46: Plate movements – direction and speed

Slide 47: You need to know the processes of constructive, destructive and conservative plate margins include: convection  divergence  subduction  earthquake activity  hot spot activity 

Slide 49: Mid-Ocean Ridge – A Mid-Ocean Ridge Constructive or Divergent Plate with underwater As the semi-molten Boundary volcanic activity rock spreads, the and shallow crustal plates are Lighter, thicker Denser, earthquake activity slowly wrenched apart and older “raft” thinner and caused as the of less dense younger plates are continental crust oceanic wrenched apart. crust Atlantic Ocean South African American Plate Plate Lithosphere – solid mantle As it rises, the Asthenosphere – semi-molten rock Semi-molten mantle spreads out and cools The cooling mantle A Hot-Spot descends again. deep in the earth heats the semi-molten mantle which The earthquakes are rises towards the surface The lava is basic or basaltic. This caused by the release (some escapes between is very fluid leading to gently in tension as the the plates to the ocean sloping volcanoes if the lava plates are forced floor forming underwater breaks the sea surface. volcanoes) apart. Underwater,“pillow” lava forms.

Slide 50: Iceland is one of the few places where volcanic activity along a mid-ocean ridge has breached the sea surface leading to basic lava volcanicity.

Slide 51: The East African Rift Valleys are thought to be embryonic constructive plate margins. The Valleys are zones of active volcanic and earthquake activity

Slide 53: A Subduction zone – A Deep Andes – Range Extrusive Volcanic Destructive or Peru- of Fold Activity at the Convergent Chile Mountains Surface Ocean Plate Margin Trench Earthquake Epicentre Solid Lithosphere Oceanic Crustal Plate Pacific Ocean South American Earthquake (continental) Plate Focus moving westward Nazca (oceanic) Su ere Plate moving osph bd uc Lith tio eastwards nZ on Intrusive Volcanic e Activity creating solid Asthenosphere – batholiths of granite. Semi-molten mantle The oceanic crust melts back Friction between the two into the mantle. Earthquakes result from the plates creates heat release in pressure as the which melts the crust huge plates try to move creating molten magma. The volcanic lava is acidic or past each other. andesitic. This is viscous and Earthquake shock waves gassy leading to steep sided, radiate away from the explosive volcanoes. focus.

Slide 54: Destructive Plate Margins destroy oceanic crust and can lead to large scale landforms including: •Fold mountains •Ocean trenches •Volcanic Chains •Island Arcs The are also active earthquake zones characterised by both shallow and deep earthquakes down to a depth of about 70km.

Slide 55: Where two oceanic plates converge, island arcs are generated.

Slide 56: Ocean trenches

Slide 57: Last Thirty Days of Earthquake Activity

Slide 58: The Pacific Ring of Fire

Slide 60: A Collision Zone leads to fold mountains.

Slide 61: Where two continental plates converge at a collision zone, high mountain ranges and plateaus are formed.

Slide 62: Everest, and the rest of the Himalayas, are a result of a collision zone.

Slide 63: Half Dome in Yosemite National Park, rises more than 1km above the valley floor. The granitic rocks that form Half Dome and other spectacular Park features represent unerupted magma later exposed by deep erosion (Photograph by Carroll Ann Hodges, USGS.)

Slide 65: The San Andreas Fault and its sister faults represent a conservative plate margin. Landforms are low key (ranges of hills and shallow trenches) but earthquakes are common and potentially devastating.

Slide 66: Impact of earthquake activity on the San Andreas Fault

Slide 68: Some volcanic activity occurs away from plate boundaries at Hot-Spots.

Slide 69: The Hawaiian Island Chain is a result of Hot-Spot activity.

Slide 71: The range of plate boundaries: transform (conservative), divergent (constructive) and convergent (destructive).

Slide 72: Ocean trenches, island arcs and island chains in the Pacific.

Slide 73: These four diagrams illustrate the shrinking of the formerly very large Farallon Plate, as it was progressively consumed beneath the North American and Caribbean Plates, leaving only the present-day Juan de Fuca, Rivera, and Cocos Plates as small remnants. Large solid arrows show the present-day sense of relative movement between the Pacific and North American Plates.

Slide 74: You need to know the resulting global patterns of landforms, including:  fold mountains  ocean trenches  island arcs  ocean ridges

Slide 75: Unusual life- forms found in ocean trenches.

Slide 76: The Hawaiian Islands are especially vulnerable to destructive tsunamis generated by major earthquakes in the circum-Pacific Ring of Fire. Travel times (in hours) are shown for the tsunamis produced by the 1960 Concepción, Chile, earthquake (purple curves) and by the 1964 Good Friday, Valdez (Anchorage), Alaska earthquake (red curves). The 1960 tsunamis killed 61 people and caused about $24 million in damage

Slide 77: A giant wave engulfs the pier at Hilo, Hawaii, during the 1946 tsunami, which killed 159 people. The arrow points to a man who was swept away seconds later. (Retouched photograph courtesy of NOAA/EDIS.)

Slide 78: Aerial view of the city of Armero, Colombia, devastated by mudflows triggered by the eruption of Nevado del Ruiz in November 1985. The mudflows destroyed everything in their paths and killed about 25,000 people. (Photograph by Darrell G. Herd, USGS.)

Slide 79: Wahaula Visitor Center, Hawaii Volcanoes National Park, was one of more than 200 structures overrun by lava flows (foreground) from the 1983-present eruption at Kilauea Volcano. (Photograph by J.D. Griggs, USGS.)