how is volcano formed
- When this pressure is released, eg as a result of plate movement, magma explodes to the surface causing a volcanic eruption. The lava from the eruption cools to form new crust. Over time, after several eruptions, the rock builds up and a volcano forms. Volcanic Features - Intrusive & Extrusive The formation of volcanoes. 1  Magma rises through cracks or weaknesses in the Earth's crust. 2  Pressure builds up inside the Earth. 3  When this pressure is released, eg as a result of plate movement, magma explodes to the surface causing a volcanic eruption. 4  The lava from the eruption cools to form new crust.