Volcanism Explained — How Do Volcanoes Form and Why Do They Erupt? | NEXT IAS In this quick GeoConcepts lesson, we explore volcanism — the powerful natural process responsible for forming volcanoes, new landforms, and even oceanic crust. Volcanoes form when heat and pressure deep inside the Earth melt the mantle, pushing magma upward through vents, especially at tectonic plate boundaries and hotspots. Depending on lava thickness and gas content, volcanoes can become shield, composite, or cinder