The earth's crust is not a complete sphere, it is cracked into number of sections, called Tectonic plates. As Warm magma rises from the centre
of the earth and then sinks down as it cools, it carries the plates slowly arround the earth. (See Diagrams above)
There are several types of Plate Margin (The place where one plate meets another).
The Constructive plate margin
The plates are being pulled apart by the convection currents. This means that the atlantic ocean grows by several metres per year.
The Destructive plate margin
One plate is pushed under another. The plate is melted and the molten rock rises and forms volcanic mountains.
The Conservative plate margin
The plates slide against each other. At points, the plates encounter resistance. At these points, earthquakes are formed.
The Collision zone
Two plates are pressed into each other and mountains are pressed up where the plates fold each other over. This has formed mountain ranges such as the alpes.
The city of San Francisco is situated on the San Andreas fault line. This is a conservative plate margin between the North American plate and the Pacific plate. At 1300km long, it is one of the most notable landmarks in eastern USA.
The San Andreas fault line
Every now and again, the two plates rub together and devastation hits the nearest city to the epicentre. Before 1906, the San Andreas fault line had been relatively quiet. The plates were building up to an enormous quake.
On april 17th 1906, the plates finaly gave way. A massive shock wave hit the city of San Francisco.