Take It To The Next Level

Layering by Rock Strength

As you have already learned, Earth’s interior is separated into three major layers by composition and density: the core, the mantle, and the crust. However, for some purposes — and especially for understanding plate motions — it is more useful to think about Earth’s upper layers in terms of the strength of their rock.

As we discussed briefly in Section 3.4.3, even solid rock can in some cases slowly deform and flow. However, if rock is deformed too much or too rapidly, it may simply break instead of flowing. In fact, the behavior of rock is actually a lot like that of the toy called Silly Putty, which breaks when you pull it sharply but deforms and stretches when you pull it slowly (Figure 1).

 

Figure 1 – These photos show a child playing with Silly Putty. When she pulls on it slowly (left), it deforms and stretches. But if she pulls on it rapidly, it breaks. Credit: Richard Megna for The Cosmic Perspective.

The composition, temperature, and pressure all affect how rock flows, making some rock more brittle (meaning it breaks more easily) than other rock. Geologists have discovered that rock in Earth’s crust and upper mantle is fairly brittle, while rock below that is much softer even though it is still solid. Together, the brittle rock of the crust and upper mantle are said to make up what geologists call the lithosphere (Figure 2), which gets its name from the Greek root lithos, meaning “stone.”

Earth’s lithosphere is about 100 kilometers thick. Because the crust ranges in thickness from about 8 to 50 kilometers, this means that the lithosphere extends well below the crust, into the mantle. Note, however, that it only goes a short way into the mantle (which extends downward for almost 3,000 kilometers).

Figure 2 – This is the same core-mantle-crust diagram you saw in Figure 4.17, but this time with an added dashed circle showing the lithosphere, which encompasses the crust and the uppermost portion of the mantle. Note: the relative thicknesses of the crust and lithosphere are exaggerated in this figure, because they would be difficult to see if drawn correctly to scale. Credit: Adapted from The Cosmic Perspective.

In the main text, we noted that the crust is broken into plates. More technically, it is the lithosphere that is broken into the plates. In essence, the brittle rock of the lithosphere broke to make the plates, and these plates essentially “float” on the warmer, softer rock below. (In case you are wondering, the warmer, softer rock layer also has a name: it is called the asthenosphere.)

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