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Lecture

a cross-sectional view of Earth showing the layers classified by chemical composition.
(Click to view larger.) A cross-sectional view of Earth showing Earth's layers classified by chemical composition along the left side of the diagram. For comparison, Earth's layers classified by physical properties are shown along the right side of the diagram. (Source: Pearson Prentice Hall)

Putting it all together – Earth’s layers

We now have the information we need to examine each of Earth’s layers in detail. The list below summarizes the major compositional and physical properties of Earth’s layers. Remember that at the center of the Earth we find our highest pressures and temperatures, and these decrease toward the surface. Working from the middle of the Earth outward we find:

Inner Core. About 800 miles in diameter and composed of more than 90% iron with a maximum density of 13 g/cm3. This iron in this layer, although very very hot, is solid due to the immense pressure at the center of the Earth.

Outer Core. About 1240 miles thick, the outer core is also made mostly of iron with a density of about 10 g/cm3. However, a reduction in pressure makes this hot layer a liquid.

Mantle. About 1860 miles thick, the mantle is made of iron and magnesium-rich silicate rocks and has a density of about 4.5 g/cm3. The mantle is hot, but mostly solid because the minerals are under pressure.

Asthenosphere. The upper reaches of the mantle are not solid; they are considered plastic and flow very slowly. This is due to the reduction in pressure as we approach the top of the mantle- the rocks are more likely to begin to melt.

Lithosphere. This layer varies in thickness from 1 to 250 miles. The lithosphere is a cool, strong and rigid layer. Its uppermost part is called the crust and is divided into oceanic and continental-type crusts discussed in the next section.

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