So how do they happen?

Earthquakes are caused by the movement of tectonic plates, which are moved by convection currents in the mantle of the Earth's core. When two plates, that are made of land, collide they push up and cause mountain ranges or singular mountains.

  When two are pushed in opposite directions and two pieces of land get stuck, and then suddenly fly apart, this causes a large release of energy and this type of movement can only cause earthquakes.

  When an 'Oceanic Plate' and a land place are pushed together the Oceanic plate is forced under the other, which then causes friction. The friction then produces heat, which reacts with the magma in the mantle, and this produces volcanic eruptions.