When do earthquakes occur ?

Our planet is very restless place. Every 30 seconds, the ground suddenly rumbles and trembles. Most of the movements are so slight that they are not felt. Others can be so large they cause complete disaster. Big cracks appear in the land, streets buckle and buildings simply crumbel. In fact whole towns and cities can be destroyed. These are called earthquakes and the reason they occur is because the earth's crust is made up of moving parts called plates. When these plates slide past or into each other, the rocks jolt and send out shock waves.