Rhyolite


Rhyolite is an igneous, volcanic rock, of felsic composition. It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase. Biotite and hornblende are common accessory minerals. It is the extrusive equivalent to granite.

Geology

Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolitic magmas form highly viscous lavas. They also occur as breccias or in volcanic plugs and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures. Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra/tuff or of ignimbrites.
Eruptions of rhyolite are relatively rare compared to eruptions of less felsic lavas. Only three eruptions of rhyolite have been recorded since the start of the 20th century: at the St. Andrew Strait volcano in Papua New Guinea, Novarupta volcano in Alaska, and Chaiten in southern Chile.

Occurrence

Rhyolite has been found on islands far from land, but such oceanic occurrences are rare.
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Europe

The name rhyolite was introduced into geology in 1860 by the German traveler and geologist Ferdinand von Richthofen from the Greek word rhýax and the rock name suffix "-lite".

Quarrying by Native Americans

In North American pre-historic times, rhyolite was quarried extensively in eastern Pennsylvania in the United States. Among the leading quarries was the Carbaugh Run Rhyolite Quarry Site in Adams County. Rhyolite was mined there starting 11,500 years ago. Tons of rhyolite were traded across the Delmarva Peninsula, because the rhyolite kept a sharp point when knapped and was used to make spear points and arrowheads.

Characteristics