earth's crust
Although more than 80 elements occur naturally on the earth, the bulk of it is made of iron (35%), oxygen (28%), magnesium (17%), and silicon (13%). By taking rock samples from the earth's interior, geologists have been able to understand the earth's chemical makeup. The earth's outer layer of rock (crust) varies in thickness. Beneath the oceans, it is 4-7 miles thick, but it stretches up to 43 miles under mountain ranges.
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