Earth passed through four distinct phases from its formation to the present. They are fairly typical phases for all terrestrial planets to have gone through and the better we understand Earth the more we can infer to other planets such as Mars.

     The second stage was one of heavy cratering. As Earth solidified, impacts from objects left the typical cratering marks we can still see on the Moon. During the early history of the solar system, there was a great deal of debris left over from planet making which floated all around the planets. At one time the Earth's surface much have resembled the Moon's as we see it today, heavily cratered with craters on top of craters. As the debris began to clear, cratering slowed down.

     Flooding typified the third stage of Earth's history. As radioactive decay of some elements heated up the Earth's interior, lava began upwelling through fissures in the Earth's crust. Lava flooded crater impacts and other basins and then as the atmosphere cooled, rain began to condense from the sky. The first flooding rains began to form the early oceans and lakes.

     Earth is still undergoing the fourth stage, surface evolution. This process began about 3.5 billion years ago soon after rain began to fall. In this stage, there is crustal movement, uplifting mountains in some are as and sections sliding against each other and causing faults in others. Wind and moving water erode and change the surface on a continual basis.

     Earth has an average density of 5.52 g/cm³. The crustal material has a density of 2.8 g/cm³. The only explanation here is that it is proof for more denser material beneath the crust.

   Studying seismic waves from earthquakes have told us a great deal about Earth's interior. The waves react differently depending on the density of the matrial they are passing through. Knowing this, we are able to construct a basic model of the Earth. Studies have show that Earth's interior consists of a metallic core, a rocky mantle and a thin crust.

     The core is made up of iron and nickel and the density is estimated to be at least 14 g/cm³ and about 7000 K, hotter than the surface of the Sun! What keeps the core solid at such temperatures, are the tremendous pressures that exist at the center of the Earth. The outermost parts of the core are believed to be liquid. It is this liquid portion of the core that is responsible for the Earth's magnetic field.

     The mantle is a layer of dense rock that lies between the liquid core and the
surface. The mantle material is not quite liquid and not quite solid, but is more like a plastic. It will flow under pressure but has many of the properties of a solid.

     The crust is the thinnest layer, being only about 35 km deep under continents and 5 km deep under the oceans. Because of the crust's low density (2.5 to 3.5 g/cm3) the material floats on the denser mantle. The crust is divided into sections called plates.
 
 

Moving Plates