Chapter 4 Summary

The topics covered in this chapter can be summarized as follows:

4.1 Alfred Wegener’s Arguments for Plate Tectonics

The evidence for continental drift in the early 20th century included the matching of continental shapes on either side of the Atlantic, and the geological and fossil matchups between continents that are now thousands of kilometres apart.

4.2 Global Geological Models of the Early 20th Century

The established theories of global geology were permanentism and contractionism, but neither of these theories was able to explain some of the evidence that supported the idea of continental drift.

4.3 Geological Renaissance of the Mid-20th Century

Giant strides were made in understanding Earth during the middle decades of the 20th century, including discovering magnetic evidence of continental drift, mapping the topography of the ocean floor, describing the depth relationships of earthquakes along ocean trenches, measuring heat flow differences in various parts of the ocean floor, and mapping magnetic reversals on the sea floor. By the mid-1960s, the fundamentals of the theory of plate tectonics were in place.

4.4 Plates, Plate Motions, and Plate-Boundary Processes

Earth’s lithosphere is made up of over 20 plates that are moving in different directions at rates of between 1 cm/y to greater than 10 cm/y. The three types of plate boundaries are divergent (plates moving apart and new crust forming), convergent (plates moving together and one possibly being subducted), and transform (plates moving side by side). Divergent boundaries form where existing plates are rifted apart, and it is hypothesized that this is caused by a series of mantle plumes. Subduction zones can form where accumulation of sediment at a passive margin leads to separation of oceanic and continental lithosphere. Supercontinents form and break up through these processes.

4.5 Mechanisms for Plate Motion

It is widely believed that ridge-push and slab-pull are the main mechanisms for plate motion, as opposed to traction by mantle convection. Mantle convection is a key factor for producing the conditions necessary for ridge-push and slab-pull.

Review Questions

1. List some of the evidence used by Wegener to support his idea of moving continents.
2. What was the primary weakness in Wegener’s continental drift hypothesis?
3. How were mountains thought to be formed (a) by contractionists and (b) by permanentists?
4. How were the trans-Atlantic paleontological matchups explained in the late 19th century?
5. How did we learn about the topography of the sea floor in the early part of the 20th century?
6. What evidence from paleomagnetic studies provided support for continental drift?
7. Which parts of the oceans are the deepest?
8. Why is there less sediment along ocean ridges than on other parts of the sea floor?
9. How were the oceanic heat-flow data related to mantle convection?
10. Describe the spatial distribution of earthquakes at ocean ridges and ocean trenches.
11. In the model for ocean basins developed by Harold Hess, what happens at oceanic ridges and what happens at oceanic trenches?
12. What aspect of plate tectonics was not included in the Hess model?
13. What is a mantle plume and what is its expected lifespan?
14. Describe the nature of movement at an ocean ridge transform fault (a) between the ridge segments, and (b) outside of ridge segments.
15. How is it possible for a plate to include both oceanic and continental crust?
16. What is the likely relationship between mantle plumes and the development of a continental rift?
17. Why does subduction not take place at a continent-continent convergence zone?
18. Where are Earth’s most recent sites of continental rifting and creation of new ocean floor?
19. What geological situation might eventually lead to the generation of a subduction zone at a passive ocean-continent boundary such as the eastern coast of North America?