viscosity

^{ALASKA VOLCANO OBSERVATORY GLOSSARY}

A measure of resistance to flow in a liquid (water has low viscosity while honey has a higher viscosity.)

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^{GLOSSARY OF VOLCANIC TERMS}

A measurement of the ratio of shear stress to the rate of shear strain in a fluid (Williams and McBirney, 1979, p. 20). In common language, how easily a fluid flows. Considered the most important physical property of a magma because it largely determines eruptive style as well as volcano morphology. Magma viscosity generally increases as the silica content of the magma increases (due to silica polymerization) and as the temperature of the magma decreases. Magma viscosity may also be affected by the presence of trace elements (e.g. Ti) or volatiles (e.g. H2O, CO2, SO2, etc.). In general, common magmas increase in viscosity in the following order: komatiite, basalt, andesite, dacite, rhyodacite, rhyolite.