The outer shell of the Earth, or Crust (continental and oceanic) and the upper part of the Mantle, is made up of a number of rigid segments called tectonic plates. These plates are continually moving at rates of a few centimetres per year (about as fast as your fingernails grow), driven by forces deep within the Earth.
Below the tectonic plates, lies the Earth’s asthenosphere. The asthenosphere behaves like a fluid over very long time scales. There are a number of competing theories that attempt to explain the movement of tectonic plates
At the boundaries between the plates, where they are moving together, apart or past each other, tremendous stresses build up, and are where most earthquakes occur.
