Fall 2009 » UA Q & A » Why do some volcanoes erupt explosively?

Why do some volcanoes erupt explosively?

December, 2007

Glen S. Mattioli, associate professor of geosciences, replies:

Volcanoes are surface manifestations of processes that originate deep within the Earth. As depth increases in the Earth, temperature also increases. At Earth's surface pressure, one atmosphere, most rocks melt at about 700 to 1200 degrees centigrade, and their melting points rise with increasing pressure. Given that the rate of temperature and pressure increase starts at one atmosphere pressure - well within the solid state field - and that its slope remains below that of the melting curve, this implies that common rocks melt within the Earth only under special conditions.

Three scenarios can lead to melting: the temperature or pressure at some fixed depth increases, the pressure decreases so that the melting curve is crossed from a higher pressure/temperature regime, or the melting curve is depressed by the addition of water.

All cases can lead to the formation of magmas, or molten rock. The exact melting temperature and pressure and the composition of the melt depend upon the chemical composition of the rock.

The style and frequency of volcanic eruptions are related to the rate of motion between adjacent plates; the chemical composition of the magma; the dissolved volatile (water and carbon dioxide) content within the molten rock; and the rate at which magma moves toward the Earth's surface.

Both water and carbon dioxide can dissolve into magmas of various compositions. If the magma comes slowly to the Earth's surface, then the dissolved volatiles escape slowly, subtly changing the physical properties of the magma en route. However, if volatile-rich magma rises rapidly toward the surface, then the dissolved gases cannot passively escape. Bubbles form, quickly expand, coalesce, and break apart the magma at some shallow depth - usually less than one kilometer below the surface. This imparts substantial momentum to the rising magma and molten particles of magma explosively eject from the volcanic vent at close to super-sonic speeds.

A similar process occurs when you open a soda bottle rapidly and incompletely and the dissolved carbon dioxide forms a foam that forces its way out of the tiny opening, usually causing a large mess and much embarrassment.