The Earth, our home is unique in the Solar System, it has the right range of temperatures and atmospheric pressure to allow liquid water to exist on its surface. It is also currently the only definite planet in the universe that is known to support life. Earth is the largest of the four rocky planets that make up the inner Solar System. It is also the only planet that is known to have plate tectonics. It has been suggested that the fact that water exists on the surface of the planet may lubricate Earth's crust, which could help the plates to move.

This image was taken by Apollo 17 astronauts during the last moon mission. Since then it has become an icon to humanity.

This theory would also explain why Venus and Mars have no plate tectonics. The Earth is the only planet in the Solar System that is capable of supporting life (at least complex life, with the possible exception of Jupiter's moon Europa). But it was life itself that made this possible. The oxygen in the atmosphere was built up over billions of years by primitive Cyano bacteria, making Earth habitable for us.

Diagram of a subduction zone. The subducting plate melts as it moves through the mantle.

The theory of plate tectonics was originally regarded as "ridiculous". Since then however, many pieces of evidence have lead to this theory now being regarded as fact. One major piece of evidence is the fact that fossils of specific species of animals and plants have been found on continents that are on the other side of the world. This can be explained if all the continents were once combined into a supercontinent, called Pengaea.

The theory of plate tectonics states that the lithosphere floats on the asthenosphere. (see left.) This is driven by convection currents in the mantle. The lithosphere is about 100 km thick, the top seven or so kilometers make up the crust of ocean plates and the top 30 is the crust of continental plates. The asthenosphere is the liquid layer under the lithosphere which is around 150 km thick.

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