Teach Astronomy - Rocks of the Earth

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The rocks of the Earth that we are most familiar with are those right at the surface of the crust. The deepest mine we've every drilled on the Earth is only about 10 or 12 kilometers deep, doesn't even penetrate half the thickness of the crust. Occasionally rocks from deeper down are brought to the surface of the Earth by convection or uplifting processes. Most of the common low density minerals found in the crust are called feldspar, which is a form of molten lava that's floated to the surface. The rocks that include feldspar are basalt, which is the composition of a large part of the Earth and Moon's crust, and granite. Granite and basalt are actually low density materials, although you may not think this when you hold a lump of granite in your hand. The material deep below the Earth's crust is much denser because it's mostly metallic. The crust has mostly silicates with a composition that's nearly 50 percent oxygen and nearly 25 percent silicon and very small amounts of metals, eight percent aluminum, six percent iron, whereas the core, the iron-nickel core, both liquid and solid, occupy more than a half of the Earth's radius and almost purely metallic consisting of an alloy of iron plus nickel.

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