BASIC CONCEPTS OF GEOLOGY
Astronomers and physicists think today that the earth was formed at least 4.55 billion years ago out of a cloud of cosmic dust. As gravity pulled the planet together, the heat of compression and of its radioactive elements caused it to become molten. The heaviest components, mostly iron and nickel, sank to the earth's center and became the core. Geologists believe that the core has two parts: an inner, solid core and an outer liquid core (fig. 1.1). Both are very hot, dense, and under tremendous pressure. Lighter minerals formed a thick, probably solid mantle around the outer core. Certain minerals rich in aluminum, silicon, magnesium, and other light elements solidified into a thin, rocky crust above the mantle.
Figure 1.1 A cross section of the earth shows its inner and outer cores, the mantle, and the crust.
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