What is a Volcano? A volcano is where molten rock (that is magma) has rises through the Earth's crust, reached the surface, and erupts. How do volcanoes form? Deep inside Earth, between the molten iron core and the thin crust at the surface, there is the mantle, a large layer of rock that is largely solid, but flows like plastic. When, for various reasons, rock from the mantle melts, it sometimes moves to the Earth’s surface through weak spots in the crust, releasing heat, gasses, and rock--a volcanic eruption. But why does this solid rock melt and come to the surface? From Magma to Lava. Extremely high temperature and pressure can cause the lower mantle to melt and become liquid rock, or magma. When a lot of magma rises through the denser rock layers toward Earth's surface, some of it remains liquid. Magma that has reached the surface is called lava. How do Volcanos erupt? A volcano erupts when the pressure of magma in the volcano gets so high that it fractures its way to the surface. When molten rock is below the surface it's called "magma" . Types of Volcanos. Cinder Cone Volcanoes Composite Volcanoes Shield Volcanoes FUN FACTS!!!!! More than 80 percent of the earth's surface is volcanic in origin. The sea floor and some mountains were formed by volcanic eruptions. Gaseous emissions from volcano formed the earth's atmosphere.