Evidence on the structure and composition of the earth's interior comes from (1) observations of surface rocks, (2) geophysical data from earthquakes, flow of heat from the interior, the magnetic field, and gravity, (3) laboratory experiments on surface rocks and minerals, and (4) comparison of the earth with other planets, the sun, stars, and meteorites.
The major structural components in the earth that are separated by sharp discontinuities are the crust, the mantle, and the core. The crust forms a very thin surface skin, the mantle is a thick shell that extends half the radius down into the earth, and the core occupies the central part. The crust and upper mantle are known to vary in physical and chemical characteristics, both horizontally and vertically; the lower mantle and core are generally assumed to be uniform because their diagnostic geophysical phenomena are masked by the physical properties of the upper layers.
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USGS Numbered Series
The interior of the Earth, an elementary description