Upheavals in the Earth
What causes volcanoes?
The solid rock layers which form the earth's crust are thicker in some places than in others, and directly underneath them, the earth's internal temperature is hot enough to melt rock. This molten rock is known as magma (MAG-ma).

At certain places under the crust, magma collects in reservoirs or pools. As this magma wells up out of the insides of the earth, it pushes gases ahead of it. As the gases become more and more tightly compressed, they exert a tremendous pressure against the underside of the crust. If this occurs at a place where the crust is weak, or where an ancient earthquake has created a break in the rock, the mixture of gas and magma breaks through the crack and erupts on the surface in the form of lava.

As the lava gushes out through the crack in the earth, it cools and solidifies and, in time, forms a cone around the opening that grows higher and higher. In the end it becomes a volcanic mountain.

Sometimes a volcano keeps erupting and throwing out great clouds of smoke and ashes and streams of lava for many years at a time. Then, when enough of the internal gases have been released to ease the pressure from far underground, the eruption stops. The lava inside the cone then cools and plugs up the crack in the crust. Often, many years later, the pressure builds up again, blows out the plug of solid lava, and the volcano erupts once more.
What causes volcanoes?