Plate Tectonics

Plate Tectonics Wegener, Continental Drift and Pangaea Video shows a fast motion animation over the past 740 million years 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  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 50s and 60s war 2 submarines found mountains under the oceans – the midocean 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  World Seafloor spreading First look at the earth’s layers as shown here. Let’s draw them. How do we know about them? Animation and Video 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  Discussion of electromagnetism, paleomagnet ism and earth’s core 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 VIDEO protects us from cosmic radiation TRUE or FALSE? 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 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  Sea The Plates Sea-Floor Spreading and Plate Boundaries 3 types of boundaries  Convergent – where plates come together. See example next slide Converging Margins: India-Asia Collision 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 Faults and Seafloor Spreading Review of different boundaries Divergent –mid ocean ridge like Iceland or continental rift zone like the African Rift Valley  Convergent  Transform fault like the San Andreas fault  Hot spots are not at plate boundaries, but give us information about plate motion  Activity  – Ocean/ocean like Japanese Islands – Continent/ocean like Andes and Cascades – Continent/continent like the Himalayas 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 What will the future bring?  Link to animation Volcanoes and Earthquakes and Plate Boundaries – GIS activity Structural Geology  Isostasy  Mountains  Stress  Folds  Faults and strain 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 mountainsform by subduction and melting of plates  Volcanic mountains form over hot spots  Volcanic mountains form at rift zones  Volcanoes 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 a good example of Newton’s 3rd law. For every force, there is an equal and opposing force. In this case the force of gravity is opposed by the buoyant force. •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? 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  Stress and strain video anticline syncline  A fold above and a fault below  Economic value of folds/faults 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. 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  1 2 3 Matching  These form at different plate boundaries. Can you figure out which forms where? Match them A. Convergent B. Divergent C. Transform Fault 3 1 2 1 2 3 Can you match the stress and strain? 1 2 3 A is 1st, B is 3rd, C is 2nd