Eruption of high silica rhyolites - Textural Analysis

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Volcanic rocks are formed from magma produced at depths of many 10's of km. The magma is often extremely viscous and gas-rich. The way in which the gas separates and escapes from very viscous magma controls whether eruption is effusive or explosive, in like manner to the way which gas can be un-mixed from the liquid upon opening a champagne bottle. Viscous magmas cool during their ascent producing a variety of minerals such as quartz and feldspar. Just as Canadians know first hand, and recognize the variety of different crystalline forms that fall as ice from clouds, we know that the minerals of these volcanic systems form different patterns depending upon their conditions of formation. By examination of these mineral patterns we are able to infer some information about the nature of deep processes that led to eruption.