Why does this happen

Earthquakes occur very quickly but the conditions necessary for an earthquake take years to become just right. Can you explain why this happens?

Earthquakes are caused when tension is released from the rocks in the Earth's crust and upper mantle. This tension is due to friction between what scientists believe are large 'plates' floating on magma on the Earth's surface. Sometimes earthquakes happen when the rocks in the earth's crust bend and break. This causes shock waves to travel on the earth's surface, resulting in widespread destruction.

Also, plate tectonics move slowly (at the rate your finger nails grow). With the slow movement, pressure builds until the built up energy waves are released. The waves move very rapidly in relation to the plate tectonics.

It’s a very complex equation of friction and pressure. As plates move against each other they get stuck and pressure builds up until it eventually overcomes the friction between the rocks that are holding the plates in place. The plates then slip and we get an earthquake. Because plates move so slowly that pressure building phase can take a very long time to reach that critical point.

Edit: Yes, tension. I should have included tension as that is a better descriptor of what is happening than pressure.