Why do Volcanoes Happen?

Volcanoes happen because of a build up of magma (molten rock) deep with the earth’s core. Depending on how thick and runny this magma is will determine how the volcano expels the magma. Thin and runny magma usually flows through a vent or fissure on the earth’s surface. However when the magma is thick and sticky gases are not allowed to be released so pressure builds up and eventually an explosion or eruption occurs.