Rhyolite ( ) is an RY-?-lyte, RY-oh- igneous, volcanic rock, of felsic ( silica-rich) composition (typically > 69% SiO 2 - see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase (in a ratio > 2:1 - see the QAPF diagram). Biotite and hornblende are common accessory minerals. It is the extrusive equivalent to granite.
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.
Rhyolite has been found on islands far from land, but such oceanic occurrences are rare.
Rhyolite in the Kaldaklofsfjöll, Landmannalaugar, Iceland
Cascade Range 
Castle Rock, Colorado
Cobalt, Ontario 
Crater Lake, Oregon 
Mount Jasper, Berlin, New Hampshire
Mount Kineo, a volcanic plug; an iconic mountain feature located in Moosehead Lake, Rockwood, Maine
Palisade Head, a formation found at Tettegouche State Park, Minnesota
Rhyolite, Nevada was named after a rhyolite deposit that characterises the area. 
San Juan volcanic field 
Sheep Creek, Idaho
St. Francois Mountains
Wichita Mountains within the Southern Oklahoma Aulacogen Yellowstone
The name rhyolite was introduced into
geology in 1860 by the German traveler and geologist Ferdinand von Richthofen   from the Greek word  rhýax ("a stream of lava") and the rock name suffix "-lite".  
Quarrying by Native Americans
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. 
Comendite – A hard, peralkaline igneous rock, a type of light blue grey rhyolite
List of rock types – A list of rock types recognized by geologists
Pantellerite – A peralkaline rhyolite type of volcanic rock Thunderegg – A nodule-like rock, that is formed within rhyolitic volcanic ash layers
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