How a Volcano Erupts

What causes a volcanic eruption? The process is similar to what happens when you quickly open a bottle of soda that has been shaken. Like the soda in the bottle, magma is under pressure and contains dissolved gases. In the case of magma, the gases include carbon dioxide and water vapor. As magma approaches the surface, lower pressure allows the gases in magma to expand rapidly. An eruption occurs when the gases bubble out through a crack in the crust, propelling magma to the surface.

Structure of a Volcano

Before an eruption, magma often collects in a pocket called a magma chamber, shown in Figure 39. Magma slowly accumulates in the magma chamber until enough pressure builds up to start an eruption. Then, magma rises to the surface in a narrow, vertical channel called a pipe.

An opening in the ground where magma escapes to the surface is called a vent. Often there is one central vent at the top of a volcano. Sometimes there are other vents that open along a volcano's side. At the top of the central vent in most volcanoes is a bowl-shaped pit called a crater.

After an eruption, a volcano's magma chamber and main vent may empty of magma, creating a hollow shell. If this shell collapses inward, it creates a huge depression, called a caldera, at the top of the volcano.

Figure 39 When a volcanic mountain erupts, magma under pressure is forced upward from the magma chamber. It passes through the pipe, and out the vent. Magma flows onto the surface as lava. Predicting What might happen to a volcano if its pipe became plugged with hardened magma?