The earth is covered with a layer called Earth crust. This layer is pretty thin compared to the size of the Earth. Under the crust, there are enormous masses of mines that boil at a temperature where our dream will be inadequate. The movements of these melting masses cause earthquakes by shaking the crust of the Earth.
Generally, earthquakes linked to volcanic activity are signs of major temperature difference movements that affect the upper layers of the Earth's crust as well as the deeper layers. This activity causes cracks, crashes and sharpening. Thus, mountains and sea pits are formed.