Pronunciation: AN•deh•sight
Description*: Andesite is an aphanitic to aphanitic-porphyritic extrusive igneous rock composed primarily of intermediate-composition plagioclase feldspar (which is commonly white), amphibole and various minor minerals such as pyroxene, biotite, a white alkalai feldspar, and perhaps a very small amount of quartz. Andesites and diorites (their intrusive equivalent) commonly have a grayish to salt-and-pepper appearance. Lighter-toned andesites may be mistaken for rhyolite; however, rhyolite commonly has pink potassium feldspar and always has quartz. As with many aphanitic rocks, precise identification often requires microscopic or chemical analysis. Andesitic-rhyolitic volcanoes are found on continental crust above subduction zones.
*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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