The outer most layer of Earth is often referred to as the biosphere. The biosphere is the layer of living organisms that cover the surface of the planet and it readily distinguishes the planet from all others in the solar system ("Biosphere", 2003). Due to its flowing water and oxygen, Earth is the only planet in the galaxy that is naturally habitable by humans. It is estimated that life has existed on Earth for 3.5 to 4 billion years.

The Earth's surface developed much like the other planets in the galaxy, through four major geological processes. The first process is known as Impact Cratering. Impact craters are the result of comets or asteroids impacting with the surface of a planet. Second, is the process known as volcanism. Volcanism is the result of molten rock and lava spewing from the Earth's interior to its surface. Third, is the process of tectonics. Tectonics is the disruption of a planet's surface by internal stresses. Fourth and finally is the process of erosion. Erosion is the result of wearing down or building up geological features by wind, water, and other planetary weather .

Between the crust and mantle of the Earth, a layer of rock known as the lithosphere exists. The lithosphere is a hard layer of rock that is composed of mostly crust but does include a small portion of the mantle. The lithosphere is broken into plates and allows the softer rock, known as the asthenosphere, to move and flow. This process results in the lithosphere gradually moving and "creating the phenomenon known as continental drift" (Bennet, J., et al., p. 230).

The study of the Earth's interior is aided by measuring the seismic waves that are generated from earthquakes. The two types