Rhyolite
| Igneous Rock | |
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| Composition | |
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| Felsic: igneous quartz and alkali feldspar (orthoclase, sanidine and sodic plagioclase), biotite and hornblende. |
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.
Rhyolite is an igneous, volcanic (extrusive) 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.
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[edit] 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, 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.
[edit] History
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.[1]
Eruptions of this advanced form of Igneous rock are rare, only 3 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.
[edit] Name
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.
[edit] See also
[edit] References
- ^ Beckerman, Ira. National Register of Historic Places Inventory/Nomination: Carbaugh Run Rhyolite Quarry Site (36AD30). National Park Service, 1981, 2.
[edit] External links
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Wikimedia Commons has media related to: Rhyolite |
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