The Interior of the Earth

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 Waves

There are two general categories of seismic waves.

1. P-waves, which are longitudinal pressure waves and can propagate in both solids and liquids.

2. S-waves, which are transverse waves that can propagate in solids but not in liquids

Here 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.