To understand what causes an earthquake you first need to understand that the surface of the earth is made up of a number of rigid tectonic plates. These tectonic plates are in constant motion and are slowly drifting across the magma that makes up the Earth’s core. These plates cover the entire surface of the Earth and as they are moving they often rub up against one another, sink under one another or spread apart from one another.
Causes of an Earthquake
There are two reasons why an earthquake occurs. The first reason is when the tectonic plates interact with one another due to the strain that they place on one another. Earthquakes occur along fault lines, which are places where the edges of tectonic plates exist. In places such as the San Andreas Fault in California the plates rub against one another as one is moving faster than the other causing earthquakes. In the Peru-Chile Trench along the western border of South America, the tectonic plates sink below each other causing earthquakes. The other reason earthquakes occur from tectonic plate movement is when the plates move away from each other such as in the Mid-Atlantic Ridge. In these places the plates don’t move smoothly pass or away from each other. The edges of the plates become “caught” on each other while the rest of the plate continues to move. This causes the rocks along the edge of the plates to distort and cause strain. The strain on the plates builds up to a point where the edges of the plates cannot take anymore distortion. Suddenly the plates break apart and begin to move. This lurch causes an earthquake that radiates out from the point where the plates have broken free from one another.
The second reason that causes an earthquake is the violent eruption of volcanoes. These are less common than tectonic earthquakes. When the volcano erupts it moves the ground around where it is located. This occurs in an acidic volcano eruption where the lava cools quickly upon contact with the air. Often in these cases the volcanic vent is blocked by solidified lava causing a blockage and a build up of pressure. Once the pressure has built up to a point that it has to escape there will be a violent eruption. It is this escape of pressure that causes the earthquake.