This causes the lithospheric plate to melt and being less dense than the rock in the mantle it will rise. This rising magma will produce a volcano. Volcanoes can form at a mid-ocean ridge. When the two plates separate magma fills the void and a volcano is produced. These chains of volcanic mountains are the longest mountain chain in the world.
Volcanoes can also form at a hot spot.
Earthquakes occur because the Earth's plate are in motion. The plates do not move smoothly and evenly. Great stresses build up along the plate boundaries. When a plate moves suddenly a great amount of energy is released in the form of wave energy. These waves are what cause the damage from an earthquake.