How a volcano erupts. First a volcano makes something called magma from melted rock.The magma goes through the circulation. The main vent is a hole that is in the volcano and when the volcano is ready to erupt the magma is at the top of the main vent. The magma goes up the main vent slowly while it is still getting hotter.When the magma is about half way up the main vent then the magma turns into lave and then BOOM!