Below the surface, the Earth's interior consists of three main parts: the crust, the mantle and the core. The crust, which is most accessible for study, includes the Earth's surface and the tectonic plates, which are in constant motion, creating earthquakes and new geological features. Below the crust lies the mantle, composed of silicate rocks which are so hot they can liquefy while moving toward the crust and drive seismic activity. At the center of the planet, the core consists of two parts, a liquid outer core and a hot, solid inner core composed primarily of iron.
Much information about the Earth's interior comes from the study of seismic activity. Sophisticated instruments placed deep in the earth track seismic waves, which vary in speed and structure in different parts of the Earth's mantle and core. For example, the outer core transmits few shear waves, a type of seismic movement, suggesting that it is liquid rather than solid. Recent advances in seismic tomography of the Earth allows researchers to create computer generated 3D images of the Earth's interior.
Over 300 years ago, Sir Isaac Newton calculated the Earth's core density by observing the movement of planets and the force of gravity. His observations remain largely correct today, and monitoring of the Earth in space, as well as the behavior of the moon and planets, continues to provide information about the relative density of the Earth's core and its composition. The study of neighboring planets such as Mars also provides clues about the nature of Earth's formation and the behavior of its deep inner parts.
Laboratory analyses of rock and mineral samples from the Earth's crust and surface, as well as those obtained from deep probes, provide important information about the temperature and composition of the deeper layers of the Earth's interior. Experiments on rocks at high temperatures and pressures provide clues about the behavior of rocks and minerals in the mantle and core. Analyses of lava samples and volcanic rock reveal information about the composition and behavior of various types of rock, liquid or solid, at various depths in the Earth's interior.