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3.6 Magma Dynamics and Volcanic Processes

Volcanic eruptions are one of the most spectacular manifestations of mantle convection and plate tectonics. Every year, such events directly and indirectly result in significant damage to property and often loss of life. Factors that strongly affect the violence of individual eruptions include the chemistry and volatile content of the magmas. Sudden release of volatiles, notably water, from ascending magma can result in violent eruptions, including pyroclastic surges, clouds of ash and superheated steam that flow down the flanks of volcanoes at very high speeds. Thus, the rate of volatile loss from magma as it rises in the throat of a volcano can be very important in defining the nature of an ensuing eruption. In order to determine the effects of decompression rate on magma degassing, experiments have been performed on magma samples from Soufriere Hills volcano, Montserrat, in an attempt to understand the highly variable nature of eruptions in this volcano.

Magmas that are silica-rich are more viscous and hence more likely to result in violent eruptions. The silica content of magmas is affected by a number of parameters, including the temperature and chemistry of the rock in the source region, the amount and nature of fluid involved, and interactions between the magma and surrounding country rock during transport. Often pieces of country rock are entrained in the magma during transport, forming xenoliths and megacrysts, which are often used to infer the composition of rocks near the source region of the magma. From the reaction rims between xenoliths and the surrounding magma, one can infer much about the rates of chemical alteration of magmas (and the entrained xenoliths) during transport. Further information about the chemistry of rocks in the source region can be derived from studying the mineralogy and chemical composition of veins within megacrysts.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland
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