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Formation
Aphanitic rocks form when molten rock, called magma, solidifies near the surface of the earth where cooling is relatively rapid. Under these conditions, there is insufficient time for large crystals to grow, so the texture of aphanitic rock is extremely fine. Rhyolite is an example of a felsic, aphanitic rock. Rich in silica (SiOs), its mineral assemblage includes quartz, alkali feldspar and plagioclase.
Identification
Because aphanitic rocks contain no visible mineral grains, identification often relies upon color or thin-section analysis. The color spectrum ranges from felsic (light color) to mafic (dark color). Within this spectrum, rhyolite is considered to be a felsic rock, and basalt is considered to be mafic. You can also produce thin sections of rocks and examine their mineral composition under a microscope. For example, a thin section of a light-colored aphanitic rock that contains abundant quartz crystals would likely be rhyolite.
Location
Rhyolite, an example of an aphanitic rock that contains quartz, is less common than granite but can still be found in many locations. For example, there are vast solidified lava flows in Yellowstone Park in Wyoming that contain rhyolite. There are also rhyolite formations along the Snake River in Hell's Canyon on the Idaho-Oregon border.
Usage
Jewelers use rhyolite to make jewelry. Landscape architects and gardeners use rhyolite to decorate outdoor spaces. Engineers and architects also use rhyolite when constructing surface veneers. It weighs half as much as limestone and granite, does not crumble, and it is extremely tolerant to erosion by wind and rain.