Chapter 8 Notes

1. Chapter 8 Notes Place these notes into your Geology Notebooks.

2. The lithospheric plates of the Earth are rigid and move about on the asthenosphere. The shapes of South America and Africa’s coastlines led to early suggestions they had been joined at one time. Strong evidence that two continents were once joined might include similarities in similar coastal rock formations. Wegener’s continental drift hypothesis was not widely accepted because Wegener could not explain how the continents moved.

3. The theory of plate tectonics helps explain the locations of earthquakes and volcanoes. The lack of geologic activity is not a characteristic feature of at least one type of plate boundary. Plate boundaries are defined by: • Earthquake activity • Volcanic activity • High heat flow Click Here to See the Locations of these Activities.

4. Minerals in igneous rocks can show the direction of Earth’s magnetic field at the time the rocks were formed. With increasing distance from a mid-ocean ridge, sea-floor rocks become older. The rift valley along a mid-ocean ridge marks a boundary between two plates. The Mid-Atlantic Ridge is associated with hydrothermal vents, new rocks being formed there, and many earthquakes.

8. Both collision boundaries and subduction boundaries are types of convergent boundaries. When two ocean plates converge, the one that subducts is the one that is made of denser material. A chain of volcanoes commonly forms when an oceanic plate subducts. If two plates carrying continental crust share a convergent boundary, a likely result is that the continents will eventually become welded together.

11. The fracture zones that offset portions of mid-ocean ridges are examples of a transform boundary. Mantle convection, ridge push, and slab pull are all plausible causes for plate movements. The breakup of Pangea is a process that is continuing today. Only a portion of the North American craton is visible because the rest has been buried beneath other materials.

12. Deep-sea sediments most likely contribute to continent growth by being scraped off in a subduction zone and left at the surface. A terrane is completely bounded by major faults, has different fossils than those in the surrounding area, and the magnetic record of rocks in the terrane is different than the magnetic record of rocks in surrounding areas.

13. The diagrams show two types of plate boundaries. Draw an arrow above each plate to indicate its direction of relative motion. Circle the area(s) where molten rock is likely to erupt. (4 points)

14. Essay: How are the properties of the lithosphere and the asthenosphere different? How is this difference important to plate tectonics? The lithosphere is the outermost layer of Earth. It is rigid and brittle, and it is composed of Earth’s crust and outermost mantle. It is broken into plates. Because they are rigid, these plates move as blocks that interact primarily at their boundaries. The plates move over the asthenosphere.

15. The asthenosphere is a layer in the upper mantle. The rocks there are so hot they deform and convect slowly, though they remain solid. Convection in the mantle allows the overlying plates of the lithosphere to move over the asthenosphere.

16. Essay: Compare the elevation and the age of the sea floor at divergent boundaries with the elevation and the age of the sea floor at subduction boundaries. Explain any differences. At divergent boundaries the seafloor is being formed, so the rocks there are quite young. In this region there is high heat flow due to rising magma. This heat causes the rocks to expand and be elevated.

17. At subduction boundaries the seafloor has traveled far from its origin at a divergent boundary, and the rocks are old. The rocks are cold and dense. The greater density causes the seafloor to be deeper. As it plunges into a deep-sea trench, the seafloor reaches its greatest depths of anywhere in the ocean basin.