Chapter 3 Summary

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

3.1 Earth’s Layers

Earth is divided into a rocky crust and mantle, and a core consisting largely of iron. The crust and the uppermost mantle form the lithosphere, which comprises tectonic plates.  The next layer, the asthenosphere, allows the plates to move because it deforms by flowing.

3.2 Understanding the Earth through Seismology

Seismic waves that travel through Earth are either P-waves (compression, or “push” waves) or S-waves (shear waves). P-waves are faster than S-waves, and can pass through fluids. By studying seismic waves, we can discover the nature and temperature characteristics of the various parts of Earth’s interior.

3.3 The Temperature of Earth’s Interior

Earth’s temperature increases with depth (to around 6000°C at the centre), but there are significant variations in the rate of temperature increase. These variations are related to differences in composition and the existence of convection in the mantle and liquid part of the core.

3.4 Earth’s Magnetic Field

Because of outer-core convection, Earth has a magnetic field. The magnetic force directions are different at different latitudes. The polarity of the field is not constant, and has flipped from “normal” (as it is now) to reversed and back to normal hundreds of times in the past.

3.5 Isostasy

The plastic nature of the mantle, which allows for mantle convection, also determines the nature of the relationship between the crust and the mantle. The crust floats on the mantle in an isostatic relationship. Where the crust becomes thicker because of mountain building, it pushes farther down into the mantle. Oceanic crust, being denser than continental crust, floats lower on the mantle.

Questions for Review

  1. What parts of Earth are most closely represented by typical stony meteorites and typical iron meteorites?
  2. On the diagram shown here, draw (from memory) and label the approximate locations of the following boundaries: crust/mantle, mantle/core, outer core/inner core.
  3. Describe the important differences between P-waves and S-waves.
  4. Why does P-wave velocity decrease dramatically at the core-mantle boundary?
  5. Why do both P-waves and S-waves gradually bend as they move through the mantle?
  6. What is the evidence for mantle convection, and what is the mechanism that causes it?
  7. Where and how is Earth’s magnetic field generated?
  8. When were the last two reversals of Earth’s magnetic field?
  9. What property of the mantle is essential for the isostatic relationship between the crust and the mantle?
  10. How would you expect the depth to the crust-mantle boundary in the area of the Rocky Mountains to differ from that in central Saskatchewan?
  11. As you can see in Figure 3.19, British Columbia is still experiencing weak post-glacial isostatic uplift, especially in the interior, but also along the coast. Meanwhile offshore areas are experiencing weak isostatic subsidence. Why?

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