Overview
The planet Earth was once part of the cloud surrounding the sun. Scientists believe that as it condensed and cooled, it formed into layers. The crust is a rocky shell that surrounds the mantle, and the mantle surrounds a molten iron-nickel core.
Formation of the Solar System
Before the solar system was formed, it was a cloud of interstellar gas and dust at about -270°C, not much above absolute zero. It rotated very slowly, and as it rotated, the cloud became a disk. Friction and energy from gravitational attraction moved most of the matter into the center of the disk. Eventually it became hot enough to start nuclear reactions, and form a star. Although most of the matter collapsed to the center, the rest of the disk cooled, and small gravitational eddies coalesced into larger bodies. Many of those bodies collided to form planets. The inner planets were composed of heavier rocks and minerals, while the outer planets were composed of lighter liquids and gases.
A Molten Earth
As the third planet from the sun formed, heat was generated from its gravitational collapse, as well as energy released when meteorites struck the surface. More heat resulted from radioactive decay from small amounts of minerals throughout its structure. After hundreds of millions of years, the molten planet separated into layers. Most of the heavy iron and nickel settled at the core, the lightest rocks migrated to the surface, and the rest of the material was in between to form the mantle. Even as most of the material cooled and became more solid, it stayed in the same relative place.
The Lithosphere
The crust and the upper mantle are cool, brittle, and rocky. They form the portion of the earth known as the lithosphere. The crust is from about 7 km under the surface at the thinnest point to about 70 km under mountain ranges. The upper mantle extends to about 100 km. The lithosphere contains the tectonic plates. Most geologic activity occurs in the lithosphere.
The Asthenosphere and Core
The asthenosphere is the layer of the mantle between 100 km to 350 km below the surface of the earth. It is so hot that some of the rock is melted, but it is still solid. The mantle below the asthenosphere is under great pressure, so even though the temperature is very high, it doesn’t melt. The temperature within the iron-nickel core is even higher, because it is insulated by the rest of the planet. The outer part is molten, and the inner part is solid. The behavior of seismic waves traveling throughout the earth is different when traveling through liquid and solid rock.
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