When a volcano erupts, magma from beneath the Earth is forced out of the volcano's mouth in the form of molten lava. Large amounts of gases and ash can also be released with the lava, depending on the type of eruption.
Magma is molten rock formed by the high temperatures underneath the Earth´s surface in the lithosphere, the lower part of the Earth´s crust and the upper part of its mantle. The molten rock is less dense than the rock above it, causing it to rise and eventually be released above the Earth´s surface. There are about 1,500 active or potentially active volcanoes on Earth.