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The study of the Earth's surface and interior is the domain of geology. We know little directly about the interior of the Earth. Most of our information in that regard has come from seismic waves, which are vibrations in the body of the Earth.Seismic WavesThere are two general categories of seismic waves.P-waves, which are longitudinal pressure waves and can propagate in both solids and liquids.S-waves, which are transverse waves that can propagate in solids but not in liquidsHere is an illustration of the difference between P-waves and S-waves. These seismic waves, which are generated naturally by earthquakes, by volcanoes, and by impacts, and may be produced artificially by explosions and mechanical devices, tell us about the interior in several general ways. The figure on the right illustrates for a planet with varying interior density and a liquid core. 
First, seismic waves have their direction of motion changed (refracted) by variations in the interior density. Thus, by studying the way such waves propagate in the Earth we can learn something about density variations. Second, the fact that P-waves propagate in liquids but S-waves do not allows us to determine if portions of the interior are liquid.Structure of the InteriorAccumulated and detailed seismic studies, coupled with theoretical speculation, suggests the interior structure shown schematically on the left (the figure is not to scale). The Earth is believed to have a solid inner core, made mostly of iron and nickel. This is surrounded by aliquid outer core, also mostly iron and nickel. The diameter of the core is estimated to be 7000 km, compared with a 12,700 km diameter for the entire planet. The crust is only a few tens of kilometers thick. The region between the core and the crust is called the mantle. The upper part of the mantle and the crust together are called the lithosphere. Sitting just below the lithosphere is a region of plastic consistency called the aesthenosphere. We shall have more to say about the lithosphere and aesthenosphere shortly. 
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