Some magmas that produce explosive eruptions above subduction zones contain enormous amounts of dissolved gas. These magmas can contain several percent dissolved gas by weight! This gas can have several origins, examples of which include the following:

• Water vapor produced when ocean-floor sediments on an oceanic plate are heated in a subduction zone.
• Water vapor produced when hydrous minerals dehydrate in the heat of a subduction zone.
• Carbon dioxide produced when rising magma encounters carbonate rocks, such as limestone, marble, or dolomite.
• Water vapor produced when a rising magma chamber encounters groundwater.

At depth, these gases can be dissolved in the magma like carbon dioxide dissolved in a can of cold beer. If that can of beer is shaken and suddenly depressurized by opening the can, the gas and the beer will erupt from the opening.

A volcano behaves in a similar manner. A rising magma chamber can instantly be depressurized by a landslide, faulting, or other event and an enormous volume of rapidly expanding gas will explode through the overlying rock.

Many volcanic plumes and ash eruptions occur when gas-charged andesitic magmas erupt. The gas pressure that causes the eruption blows large amounts of tiny rock and magma particles into the atmosphere.

These particles, known as volcanic ash, can be blown high into the atmosphere and carried long distances by the wind. They often cause problems for aircraft operating downwind from the volcano.

Catastrophic eruptions like Mount St. Helens, Pinatubo, Redoubt, and Novarupta were produced by andesitic magmas with enormous amounts of dissolved gas under high pressure. It is difficult to imagine how a magma can contain enough dissolved gas to produce one of these eruptions. Magma is Earth’s most powerful solvent.