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click on diagram to enlarge |
The "skin" of Earth, called the
crust, is very thin like the skin of a peach. It is so thin, in fact, that
we have to magnify a small piece of our sketch of Earth to see any detail
at all. There are two types of crust, oceanic and continental. Oceanic
crust is usually found under the oceans, and continental crust is found
on the continents. Oceanic crust is made primarily
of a rock called basalt. It contains almost as much iron and magnesium
as the mantle rock. Oceanic crust also contains thin layers of limestone,
serpentine, and clay. Limestone is made of calcium and carbon dioxide.
Serpentine and clay are rocks like basalt that contain parts of water molecules.
An important fact about these rocks is that when they are heated, the carbon
dioxide and water "boil" out as gases.
Continental crust is made mostly of granite, a rock that contains smaller amounts of iron and larger amounts of quartz. As you may know from just looking around, continental crust also contains some limestone, sandstone, and basalt--a little bit of just about everything. An important fact about continental crust is that it is less dense than basalt or mantle rocks. Thus it "floats" on the mantle rocks like a piece of wood in water. In fact, you and I live on dry land because the continents "float" much higher in the mantle "ocean" than oceanic crust does. [ Locations
of Volcanoes ]
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