Earth

The Earth is the densest major body in the solar system. The other terrestrial planets probably have similar structures and compositions with some differences the Moon has at most a small core Mercury has an extra large core relative to its diameter the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.
Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle.

There is also transverse motion at some plate boundaries . the San Andreas Fault in California and collisions between continental plates . India Eurasia. There are at present eight major plates: There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries.


The Earth's surface is very young. In the relatively short by astronomical standards period of years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history such as impact craters. Thus the very early history of the Earth has mostly been erased. The Earth is billion years old, but the oldest known rocks are about 4 billion years old and rocks older than billion years are rare. The oldest fossils of living organisms are less than billion years old. There is no record of the critical period when life was first getting started.