Plate Tectonics Test Key

Answer two questions from each section of questions.

  1. Explain the difference between the lithosphere and asthenosphere.

    The key difference is the physical characteristics of solid vs plastic. The lithosphere is solid - it does not flow when stressed but breaks (faults). It is important to note that the lithosphere includes the crust and the uppermost portion of the mantle. Thus the lithosphere is not defined by composition. The asthenosphere is mantle material than can flow due to a small amount of melted material. This ability to flow is sometimes called plastic-like. The asthenosphere begins below the lithosphere.

  2. Explain the difference between the crust and mantle.

    The key difference is the composition. The crust is the outermost portion of the Earth with primarily felsic (silicates) composition in the continents and a mafic rock in the oceanic basins. The mantle is completely mafic (Fe-Mg silicates) near the top but becomes ultramafic quickly. Note that some of the upper mantle is also solid.

  3. Explain the role of temperature-induced density differences in plate tectonics.

    As mafic mantle material is heated by the core(s) it expands and so becomes less dense. This less dense mantle material rises towards the surface. As it rises it cools particularly when in gets close to the lithosphere. Cooling increases the material's density and it would sink back into the lower mantle but is prevented by the upward flow of heated mantle. The cooler mantle material is pushed to the side along the top of the lithosphere where it finally starts the descent back into the lower mantle to repeat the process. The mantle materials move a few centimeters per year.

  1. Explain how the fossil record supported Wegener's theory of continental drift.

    Fossils supported continental drift theory because fossils assemblages spanned disconnected continents. These fossils were of organisms that would have a difficult time dispersing to the continents as they are positioned now. For example, freshwater mesosaurs and land reptiles that couldn't cross salty oceans. This implies that the continents were connected at one time.

  2. Explain how the glacial striations supported Wegener's theory of continental drift.

    Glacial striations on the continents today show odd patterns: equatorial glaciers, glaciers starting at sea level. Once the continents were reassembled in their Pangaea configuration the patterns indicated a continental glacier. This made sense.

  3. Explain how rock formations (strata) supported Wegener's theory of continental drift.

    Rock formations on widely separated continents show the same pattern or sequence of strata, i.e. the sequence of rock layers such as sandstone, shale, igneous flows, glacial deposits. How those unique sequences could exist across thousands of miles of ocean could not be explained. This puzzle could be solved if the strata was at one time contiguous (connected) and then separated.

  1. Explain why the sea floor has been described as a conveyor belt.
  2. Explain how radioactive dating of ocean rocks helped support the theory of sea floor spreading.
  3. Explain a complication with using ocean sediment thickness to estimate the ages of ocean crust.

Give an example on the earth then draw and label a cross-section of

Words to use: mantle, crust, lithosphere, asthenosphere, plate tectonics, ocean trench, subduction zones, plate boundaries, convergent boundary, convection currents, volcanoes, island arc, magma, earthquakes

  1. an oceanic-oceanic convergence boundary.
  2. an oceanic-continental convergence boundary.
  3. a continental-continental convergence boundary.