In the beginning the earth was not as it is today. It was one continent called Pangaea. Over the years the continents drifted apart, hence the continental drift theory. The reason why the continents started to drift apart was because of the plates that the continents are lying on. The reason why the plates are moving is because of convection. What happens is that when the magma is heated from the core, it circulates, because heat rises and coolness lowers. The circulation in the mantle causes the crust to move. Sometimes the plates move apart, against each other, and other times into each other. Now the Earth is not just one solid piece of rock, it actually has several different insides. On the surface it is called the crust, what we walk on. The crust can be anywhere from 5 miles to 25 miles thick. The next layer is the mantle. The mantle is full of pockets of magma. Extremely hot! The magma can sometimes reach the surface either from pressure from below or there is a movement in two or more plates, when it does come to the surface it comes out of a volcano. The San Andres Fault is an example of a fault that gave suddenly. At Point Reyes there is trail that shows exactly how far it gave. There are fence posts that use to ran perpendicular to the fault and when it moved the fence broke but not the fence posts. Leaving it to show how far it moved. Another example is the San Francisco Earthquake. An Earthquake is caused when a sudden movement is made at a fault line. Hopefully the two plates can move peacefully but not all the time. Sometimes, the get stuck and tension builds and builds until it is released, in the form of an earthquake. All the above examples are related to plates and faults and the movement of the continents.