Plate Tectonics
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Plate Tectonics

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  • The interior of the Earth is divided into layers based on chemical and physical properties. The Earth has an outer silica-rich, solid crust, a highly viscous mantle, and a core comprising a liquid outer core that is much less viscous than the mantle, and a solid inner core. Working from the centre of the Earth out we have: The inner core is a primarily solid sphere about 1220 km in radius situated at Earth's center. Based on the abundance of chemical elements in the solar system, their physical properties, and other chemical constraints regarding the remainder of Earth's volume, the inner core is believed to be composed primarily of a nickel-iron alloy, with small amounts of some unknown elements. The temperature is estimated at 5,000-6,000 degrees Celsius and the pressure to be about 330 to 360 GPa (which is over 3,000,000 times that of the atmosphere!) The liquid outer core is 2300 km thick and like the inner core composed of a nickel-iron alloy (but with less iron than the solid inner core). Iseismic and other geophysical evidence indicates that the outer core is so hot that the metals are in a liquid state. The mantle i s approximately 2,900 km thick and comprises 70% of Earth's volume. (the core makes up about 30% of Earth's volume, with the outer crust [where we live] <1%!!). The mantle is divided into sections based upon changes in its elastic properties with depth. In the mantle, temperatures range between 500-900 degrees Celsius at the upper boundary with the crust to over 4,000 degrees Celsius at the boundary with the core. Due to the temperature difference between the Earth's surface and outer core, and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle (mantle convection cells). Hot material rises up as mantle plumes (like a lava lamp!), while cooler (and heavier) material sinks downward to be reheated and rise up again. - We shall see that this process is very important for plate tectonic motion… The outer most layer is the crust - this is the most familiar to us as it is where we live. The distinction between crust and mantle is based on chemistry, rock types and seismic characteristics. Presenter: Ask the students to guess what the most abundant element in the earth’s crust is…..they may be surprised to learn that it is actually Oxygen ( 46.6% Oxygen; 27.7% Silica; 8.1% Aluminum; 5.0% Iron; 3.6% Calcium; 2.8% Sodium; 2.6% Potassium; 2.1% Magnesium; plus trace elements) Click to next slide for more on the Crust….
  • The Earth has two different types of crust: Continental crust and Oceanic crust. Each has different properties and therefore behaves in different ways. Continental crust: Continental crust forms the land (the continents, as the name suggests) that we see today. Continental crust averages about 35 km thick. Under some mountain chains, crustal thickness is approximately twice that thickness (about 70 km thick). - The mountains we see on earth have deep roots in the crust that we can’t see. The crust “floats” on the more dense mantle and, like how only the tip of an iceberg sticks up out of the water, we see only the tip of the continental crust - the mountain ranges. Continental crust is less dense and therefore more buoyant than oceanic crust Continental crust contains some of the oldest rocks on Earth. - Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) [Ga = billion years ago] and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley in the USA (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years). Oceanic crust: As the name already suggests, this crust is below the oceans. Compared to continental crust, Oceanic crust is thin (6-11 km). It is more dense than continental crust and therefore when the two types of crust meet, oceanic crust will sink underneath continental crust. The rocks of the oceanic crust are very young compared with most of the rocks of the continental crust. They are not older than 200 million years.
  • This diagram shows the major Tectonic Plates. Presenter: Point out the UK, sitting on the Eurasian Plate. Also the plate boundary between Africa and South America (note that it has the same shape as the coastlines in these countries).
  • How and Why do tectonic Plates move around? The question of how tectonic plates are moved around the globe is answered by understanding mantle convection cells. In the mantle hot material rises towards the lithosphere (like hot air rising out of an open oven - ever opened an oven door and felt the blast of hot air coming past your face?). The hot material reaches the base of the lithosphere where it cools and sinks back down through the mantle. The cool material is replaced by more hot material, and so on forming a large “convection cell” (as pictured in the diagram). This slow but incessant movement in the mantle causes the rigid tectonic plates to move (float) around the earth surface (at an equally slow rate).
  • To view this animation, click “View” and then “Slide Show” on the top navigation bar.
  • Ocean Ridges: This map shows the age of the oceanic crust. The red colouring shows the youngest ages, whilst the dark blue shows the oldest ages (around 200 million years old). Presenter ask: Where are the Ocean Ridges located? I.e. where are the divergent boundaries? Answer: The divergent boundaries are where the plates are pulling apart and new material is being produced. Therefore the Ocean ridges are in the middle of the red areas (the boundaries are in fact shown on the map). We can see a progression of the oceanic crust getting older away from the ocean ridges (like a conveyer belt). Presenter: Before moving on to the next slide, point out Iceland. The divergent boundary runs straight through Iceland….
  • In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. These areas can form in the middle of continents or on the ocean floor. As the plates pull apart, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity. Presenter: Reiterate the process by going through the diagram, including the presence of mantle convection cells causing the plates to break apart and also as a source for new molten material. Where a divergent boundary forms on a continent it is called a RIFT or CONTINENTAL RIFT, e.g. African Rift Valley. Where a divergent boundary forms under the ocean it is called an OCEAN RIDGE.
  • Iceland is located right on top of a divergent boundary. In fact, the island exists because of this feature. As the North American and Eurasian plates were pulled apart (see map) volcanic activity occurred along the cracks and fissures (see photographs). With many eruptions over time the island grew out of the sea! Question: Why don’t we have islands like Iceland where ever we get an Ocean Ridge? Answer: Scientists believe that there is a large mantle plume (an upwelling of hot mantle material) located right underneath where Iceland has formed. This would mean that more material would be erupted in the Iceland area compared with if there was just the divergent boundary without the plume underneath it.
  • The Andes mountain range along the western edge of the South American continent is an example of a mountain belt formed by subduction. The continental crust of the South American plate has buckled under the compressional strain of converging with the Nasca and Antarctic plates. Additionally there are many volcanoes, the result of melting of the subducting slab and the production of new material that has risen through the crust to the surface.
  • At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust. This is called a Subduction Zone. The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the “Subducting Slab” (see diagram). When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge (Presenter: explain all of this using the diagram). The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes. Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.
  • When continental crust pushes against continental crust both sides of the convergent boundary have the same properties (think back to the description of continental crust: thick and buoyant). Neither side of the boundary wants to sink beneath the other side, and as a result the two plates push against each other and the crust buckles and cracks, pushing up (and down into the mantle) high mountain ranges. For example, the European Alps and Himalayas formed this way.
  • Example: India used to be an island, but about 15 million years ago it crashed into Asia (see map). As continental crust was pushing against continental crust the Himalayan mountain belt was pushed up. “ Mountains” were also pushed down into the mantle as the normally 35 km thick crust is approximately 70 km thick in this region. Mt Everest is the highest altitude mountain on our planet standing 8,840 metres high. This means that below the surface at the foot of the mountain the crust is a further 61 km deep!!
  • To view this animation, click “View” and then “Slide Show” on the top navigation bar.
  • To view this animation, click “View” and then “Slide Show” on the top navigation bar.
  • From here is ESCP only since I am combining plates and mountains chapters for them

Plate Tectonics Plate Tectonics Presentation Transcript

  • Plate Tectonics
  • Wegener, Continental Drift and Pangaea
  • Pangaea
  • Pangaea
  • Evidence for Continental Drift
    • Jigsaw Puzzle fit of continents
    Alfred Wegener during Greenland expedition
  • More evidence
    • Matching fossils on continents now located thousands of miles apart. Example = Mesosaurus, a freshwater reptile
    • Many others
  • Matching mountain ranges
  • More evidence
    • Matching geologic structures including:
      • Mountain chains
      • Ore deposits
      • Same rocks of same age
  • More evidence
    • Climate change evidence
      • Glacial deposits at current equator
      • Fossilized palm trees in Greenland
    • Map shows why according to the placements of current continents within Pangaea
  •  
  • Wegener not believed
    • Why? -
      • What could possibly force the continents to move across the ocean floor in this way. They would be crushed.
      • He was a meteorologist,
      • not a geologist
  • Developments 1950s and 1960s
    • World War II submarines using sonar found mountains under the oceans – the mid-ocean ridges
    • Sea floor drilling showed rocks younger than expected and youngest towards the center of the mid-ocean ridge
    • Theory of seafloor spreading suggested by Princeton professor Dr. Harry Hess
  • Seafloor spreading First look at the earth’s layers as shown here.
  • Structure of the Earth
    • The Earth is made up of 3 main layers:
      • Core
      • Mantle
      • Crust
    Inner core Outer core Mantle Crust
  • The Crust
    • This is where we live!
    • The Earth’s crust is made of:
    • Continental Crust
    • thick (10-70km) - buoyant (less dense than oceanic crust) - mostly old
    Oceanic Crust - thin (~7 km) - dense (sinks under continental crust) - young
  • Evidence for sea floor spreading
    • Alvin and other submersibles found cracks and lava showing spreading and volcanism at mid ocean ridges and odd life forms
    • Matthews and Vine’s survey of the Indian Ocean sea floor showed matching stripes of reversing polarities on either side of ridge – what caused these?
      • Lava spreading during alternating magnetic periods
  • The Earth’s magnetic field
    • It is produced by the outer core of the earth which is made of liquid iron and nickel
    • This moving magnetic material produces a moving magnetic field, which in turn produces a moving electric field. It is a dynamo!
    • Earth’s magnetic field varies over time and it protects us from cosmic radiation
  • Seafloor spreading and paleomagnetism
  • Pangaea revisited
    • By piecing together this information, we can see how the continents have moved over the past 200 million years, due to seafloor spreading
  • Plate tectonics
    • Sea floor spreading provides the driving mechanism for movement
    • However, it is not the continents that are moving, but the “plates” of lithosphere “floating” in effect on the asthenosphere
    • The lithosphere is made up of about 20 plates which move relative to each other in several ways
    • Let’s look at a generalized sketch
  • Confirmation of the Theory of Plate Tectonics Evidence supporting the theory of plate tectonics: Apparent Polar wandering: plate movement causes the apparent position of the magnetic poles to have shifted. The paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart.
  • Confirmation of Plate Tectonics Theory Paleomagnetism: strips of alternating magnetic polarity at spreading regions. The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes.
  • The Plates
  •  
  • “ Plates” of lithosphere are moved around by the underlying hot mantle convection cells
  • Plate Tectonics An overview of the tectonic system.
  • Sea-Floor Spreading and Plate Boundaries
  • 3 types of boundaries
    • Convergent – where plates come together. See example next slide
  • Age of Oceanic Crust Courtesy of www.ngdc.noaa.gov
  • Mid Atlantic Ridge Mid Atlantic Ridge
    • Spreading ridges
      • As plates move apart new material is erupted to fill the gap
    Divergent Boundaries
  •  
    • Iceland has a divergent plate boundary running through its middle
    • North American Plate and Eurasia Plate diverging
    Iceland: An example of continental rifting
  • East Africa Rift Valley
  • East Africa Rift Valley
    • Oceanic lithosphere subducts underneath the continental lithosphere
    • Oceanic lithosphere heats and dehydrates as it subsides
    • The melt rises forming volcanism
    • E.g. The Andes
    Subduction
    • Called SUBDUCTION
    Continent-Oceanic Crust Collision
  • Deep Ocean Trench
  •  
  •  
  • Andes Mountains in South America
  • Andes Mountains formed along the coast of South America
    • Nazca Plate dives under the South American Plate
    • Subduction
  • Island Formation
  • Japan Volcanoes in Japan
  •  
  • Japan formed as an island chain as the Pacific Plate dives beneath the Eurasian Plate Mt Fuji
  • Aleutian Islands Chain off Alaska
  • Aleutian Islands formed as an island chain as Pacific Plate dives beneath North American Plate
    • Forms mountains, e.g. European Alps, Himalayas
    Continent-Continent Collision
  •  
  • Himalayas
  • Converging Margins: India-Asia Collision
  • Alps in Europe
    • Collision of Africa and Eurasia
  • Interesting plate collision
    • This picture shows a place in Newfoundland where a massive collision actually forced mantle rock on top of the crust, during the collision that formed Pangaea and the Appalachian mountains. This looks down the old plate boundary.
  • Mantle rocks are toxic
    • These rocks have very different compositions than crustal rocks.
    • They contain heavy metals, which do not support life forms on the earth’s surface, so few organisms live there.
    • However, in some places their heavy metal concentrations produce rich metal deposits and are mined
  • Divergent Boundaries
    • Found at spreading centers – either mid ocean ridges or mid continental rift zones
  • Transform fault boundary
    • This shows the San Andreas Fault.
    • It is a transform fault boundary, where the plates move sideways past each other, rather than away from each other (at divergent boundaries), or towards each other (at convergent boundaries)
  • Transform Fault Boundary
  •  
  • Transform Faults and Seafloor Spreading
  • Review of different boundaries
    • Divergent –mid ocean ridge like Iceland or continental rift zone like the African Rift Valley
    • Convergent
      • Ocean/ocean like Japanese Islands
      • Continent/ocean like Andes and Cascades
      • Continent/continent like the Himalayas
    • Transform fault like the San Andreas fault
    • Hot spots are not at plate boundaries, but give us information about plate motion
    • Activity
  • What causes plate tectonics?
    • Convection in the mantle, as the plastic asthenosphere flows, carrying the plates with it.
    • This is probably aided by slab pull at subduction zones and ridge push at mid ocean ridges and rising plumes in the mantle
    • This diagram shows several different model hypotheses
  • Plate tectonics causes volcanic activity
  • and earthquakes
  • It causes rocks to be tilted
  • Or even to fold or break
  • Volcanoes and Earthquakes and Plate Boundaries – GIS activity
  • Structural Geology
    • Isostasy
    • Mountains
    • Stress and strain
    • Folds
    • Faults
  • Formation of mountains
    • Two forces are constantly at work on the earth.
      • Weathering and erosion tear structures down while
      • Plate tectonics builds them up
  • So we have mountains! But they will not last forever.
  • Mountains form in different ways -Volcanic mountains-
    • Volcanoes form by subduction and melting of plates
    • Volcanic mountains form over hot spots
    • Volcanic mountains form at rift zones
  • Other mountain types
    • Folded mountains form From converging continents like the Himalayas
    • How do these look? Direction?
    • Fault block mountains form where blocks of rock drop at faults – mostly near plate boundaries, but not always
    • Uplifted mountains – where large sections of the crust are pushed up, perhaps by magma, or other forces
  • Isostasy
    • Just as a boat sinks or rises with changes in weight, so does the crust sink or rise with changes in weight. Plate tectonics builds mountains and the extra weight causes the crust to sink. As erosion occurs the weight of the mountains decreases and the crust rises again. This process is called isostasy or isostatic change
  • Isostasy Isostasy is balance. A floating object is balanced in the water, like an iceberg. If some of the top melts, the iceberg rises in the water to stay in balance. If you get into a boat, the boat sinks to maintain balance. The same thing happens with mountains. As plate motions push it higher the mountain sinks into the mantle to stay in balance. On the other hand, when the mountain erodes, it will rise in the mantle as the top erodes. So, a mountain may lose 1 meter from erosion, but regain 0.8 meters from rising due to isostasy. It maintains isostatic equilibrium.
    • This is also a great example of feedback within a system. Is it positive or negative? Figure it out. Remember that maintaining equilibrium is __ feedback because it keeps the system in balance. So, what is it, positive or negative?
    • This is a good example of Newton’s 3 rd law. For every force, there is an equal and opposing force. In this case the force of gravity is opposed by the buoyant force.
    Right! It’s negative.
  • Stress Due mostly to plate movements, the earth’s crust is under a lot of stress. There are 3 types, shown at the right A occurs where plates pull apart, divergent boundaries, and is called tension B occurs where plates converge, and is called compression C occurs where plates move past each other, at transform fault boundaries and is called shearing
  • Strain
    • This stress leads to strain on the crust which bends it. –
    • If it is warm, underground, it can bend. This called ductile deformation. Features are called folds. Upturned folds are anticlines while downturned folds are synclines.
    • Or, the rock may break, if it is brittle. This causes faults –breaks of the earth.
    A fold above and a fault below anticline syncline Stress and strain video Economic value of folds/faults
  • Different faults
    • Faults move in different ways, depending on the type of stress on them. Here are 3 types that form.
    • 1 is a normal fault
    • 2 is a strike slip fault
    • 3 is a reverse fault
    2 3 1
  • Matching
    • These form at different plate boundaries. Can you figure out which forms where? Match them
    A. Convergent C. Transform Fault B. Divergent 2 3 3 2 1 1
  • Can you match the stress and strain? A is 1 st , B is 3 rd , C is 2nd 2 3 1