Pronunciation: RYE•oh•light
Description*:
Rhyolite is an extrusive igneous rock composed primarily of alkalai feldspar (e.g., the potassium-rich feldspar orthoclase, which commonly has a salmon pink color), sodium-rich plagioclase feldspar (which is commonly white to gray), and quartz (which is commonly colorless). Rhyolite commonly has a aphanitic (fine-grained) to aphanitic-porphyritic (with some large crystals) texture. Rhyolites and granites (their intrusive equivalents) commonly have a pinkish tone due to the pink potassium feldspar. Granite and rhyolite are said to be felsic (or sialic) rocks because of their light tone, in contrast to the dark tone of the mafic rocks basalt and gabbro.
Rhyolitic-andesitic volcanoes like Mt. St. Helens are found on continental crust above subduction zones, and generally erupt explosively.
*Description from Cronin, V.S., 2001, Geology laboratory projects for group learning: Primis McGraw-Hill, 156 pp., ISBN 0-07-252348-4
Used by permission of the author.
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