Chapter 3 Main Ideas

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.

3.5 Isostasy

The plastic nature of the mantle, which allows for mantle convection, also determines the nature of the relationship between the lithosphere and the mantle. The lithosphere floats on the mantle in an isostatic relationship. Where the lithosphere 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.

Key Term Check

What key term from Chapter 3 is each card describing? Turn the card to check your answer.