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 is broken into tectonic plates. The next layer, the asthenosphere, allows the plates to move because it deforms by flowing.
3.2 Imaging Earth’s Interior
Seismic waves that travel through Earth are either P-waves or S-waves. P-waves are faster than S-waves, and can pass through fluids. Earth’s layers can be identified by looking at changes in the velocity of seismic waves. Seismic wave shadow zones contributed to knowledge of the depth of the core-mantle boundary, and the knowledge that the outer core is liquid. Plate tectonic structures within Earth can also be mapped using the seismic waves generated by earthquakes.
3.3 Earth’s Interior Heat
Earth’s temperature increases with depth (to around 6000°C at the centre), but the rate of increase is not the same everywhere. In the lithosphere, thickness and plate tectonic setting are are factors. Deeper within the mantle, convection currents are more important.
3.4 Earth’s Magnetic Field
Earth’s magnetic field is generated by convection of the liquid outer core. The magnetic field is similar to that of a bar magnet, and has force directions that vary with latitude. The polarity of the field is not constant, meaning that the positions of the north and south magnetic poles have flipped from “normal” (as it is now) to reversed and back many times in Earth’s history.
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 and heavier because of mountain building, it pushes farther down into the mantle. Oceanic crust, being denser than continental crust, floats lower on the mantle than continental crust.
Questions for Review
- What parts of Earth are most closely represented by typical stony meteorites and typical iron meteorites?
- Draw a simple diagram of Earth’s layers, and label the approximate locations of the following boundaries: crust/mantle, mantle/core, outer core/inner core.
- How do P-waves and S-waves differ?
- Why does P-wave velocity decrease dramatically at the core-mantle boundary?
- Why do both P-waves and S-waves gradually bend as they move through the mantle?
- What is the evidence for mantle convection, and what causes mantle convection?
- How is Earth’s magnetic field generated?
- When were the last two reversals of Earth’s magnetic field?
- What property of the mantle is essential for the isostatic relationship between the crust and the mantle?
- 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?
- British Columbia is still experiencing weak post-glacial isostatic uplift, especially in the interior, but also along the coast (see Figure 3.21). Meanwhile, offshore areas are experiencing weak isostatic subsidence. Why?