Deep inside Earth, between the molten iron core and the thin crust at the surface, there is a solid body of rock called the mantle. When rock from the mantle melts, moves to the surface through the crust, and releases pent-up gases, then volcanoes erupt. Extremely high temperature and pressure cause the rock to melt and become liquid rock which we usually refer to as "magma". When a large body of magma has formed, it rises through the denser rock layers toward Earth's surface.  Magma that has reached the surface is called "lava".