Rhyolite is an igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO2—see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio > 1:2—see the QAPF diagram). Biotite and hornblende are common accessory minerals.
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, rhyolite melts are highly polymerized and form highly viscous lavas. They can 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.
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, where as many as fifty small quarry pits are known.
Eruptions of this advanced form of Igneous rock are rare, only three eruptions of rhyolite have been recorded since the 20th century—the eruptions were at the St. Andrew Strait Volcano in Papua New Guinea, Novarupta Volcano in Alaska, United States and Chaiten in Southern Chile.
Rhyolite quarry, Löbejün
Rhyolite in the Kaldaklofsfjöll, Landmannalaugar, Iceland
- the North Island of New Zealand has the largest concentration of young rhyolite-volcanoes
- the Gondwana Rainforests of Australia World Heritage Area contains rhyolite-restricted flora along the Great Dividing Range
The name rhyolite was introduced into science by the German traveler and geologist Ferdinand von Richthofen after his explorations in the Rocky Mountains in the 1860s.
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