Earth's Interior

It is approximately 3,950 mi (6,370 km) from Earth's surface to its center. Geologists understand the structure and composition of the surface by direct observation and by analysis of rock samples raised by drilling projects; however, the depth of drill holes and, therefore, the depth limit of scientists' ability to directly observe Earth's interior is severely limited. Even the deepest drill holes (7.5 mi [12 km]) penetrate less than 0.2% of the distance to Earth's center. Thus, we know more about the layers near Earth's surface than about the depths, and can only investigate conditions deeper in the interior through indirect means.

Geologists collect indirect information about the deep interior from several different sources. Some rocks found at the surface, such as kimberlite, originate deep in Earth's crust and in the mantle. These rocks provide geologists with samples of the composition of Earth's interior; however, their depth limit is still on the order of a few tens of miles. Another source of information, because of its ability to probe Earth to its very core, is more important: seismic waves. When an earthquake occurs anywhere on the planet, seismic waves—mechanical vibrations transmitted by the solid or liquid rock of Earth's interior—travel outward from the earthquake center. The speed, motion, and direction of seismic waves changes dramatically at depth different levels within Earth, and these are known as seismic transition zones. From such data, scientists have concluded that Earth is composed of three basic parts: the crust, the mantle, and the core.