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Terms to Know

The terms listed below have been defined as they are referred to in the text.  Definitions are from the dictionary of geology at geology.com unless marked by an asterisk.

Felsic

A term used to describe an igneous rock that has a large percentage of light-colored minerals such as quartz, feldspar, and muscovite. Also used in reference to the magmas from which these rocks crystallize. Felsic rocks are generally rich in silicon and aluminum and contain only small amounts of magnesium and iron. Granite and rhyolite are examples of felsic rocks. (See mafic to contrast.)

Flank Collapse* –

Flank Collapse is the failure of a large section of unstable slope.  Although similar to a landslide, the magnitude of these events is many magnitudes greater.  These events occur under conditions of extreme instability often created by rising magma bodies causing bulging of already steep sided volcanoes.  Collapse volumes can reach hundreds of cubic kilometers.

Overpressure*

As magma moves toward the earth’s surface, it pushes up on the crust creating great confining pressure.  As this pressure builds, the volcano becomes less and less stable until a state of overpressure is reached where the volcano is ready to erupt.  At this point, a triggering event may cause the eruption of the volcano.  Overpressure may take tens of years or hundreds of thousands of years to build depending on the properties of the volcano.

Mafic

A term used to describe an igneous rock that has a large percentage of dark-colored minerals such as amphibole, pyroxene and olivine. Also used in reference to the magmas from which these rocks crystallize. Mafic rocks are generally rich in iron and magnesium. Basalt and gabbro are examples of mafic rocks. (See felsic to contrast.)

Phreatomagmatic

An explosive volcanic eruption initiated by the interaction of magma and water (usually either meteoric or groundwater).

Viscosity

The resistance of a fluid to flow. Fluids with a high viscosity resist flow. Fluids with a low viscosity flow freely

Volatiles*

A volatile is a material with a tendency to vaporize.  In volcanism, these materials are very critical to the behavior of magma. Magma invariably contains small amounts of dissolved gas (water, CO2 etc) which is released as pressure is removed. Magmas formed by melting of mantle rocks have generally low volatile contents, but those formed by partial melting of crustal rocks are often volatile-rich. A high volatile content decreases viscosity (like adding water to treacle), and is probably the main factor in enabling some highly viscous (but also volatile-rich) melts to reach the surface at all. The release of gas during eruption is particularly likely to be explosive if the magma is both viscous (as gas is released, so viscosity is immediately increased) and volatile rich.    (Source:  The Geological Society)

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