rhyolite
American Heritage Dictionary:
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rhy·o·lite
(rī'ə-līt') 
n.
A fine-grained extrusive volcanic rock, similar to granite in composition and usually exhibiting flow lines.
n.
A fine-grained extrusive volcanic rock, similar to granite in composition and usually exhibiting flow lines.
[Greek rhuāx, stream (from rhein, to flow) + -LITE.]
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rhyolite
Britannica Concise Encyclopedia:
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rhyolite
Igneous rock that is the volcanic equivalent of granite, whose chemical composition is similar. Rhyolites are known from all parts of the Earth and from all geologic ages; they are found mostly on the continents or their immediate margins, but small quantities have been described from remote islands.
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A very light-colored, aphanitic (not visibly crystalline), volcanic rock that is rich in silica and broadly equivalent to granite in composition. Migration of rhyolitic magma through the Earth's crust, which causes much of the Earth's explosive and hazardous volcanic activity, represents a major process of chemical fractionation by which continental crust grows and evolves. See also Granite.
Rhyolites are formed by the process of molten silica-rich magma flowing toward the Earth's surface. Small differences in this process, notably those related to the release of gas from the magma at shallow depth, produce extremely diverse structural features. The high silica content gives rhyolitic lava a correspondingly high viscosity; this hinders crystallization and often causes young rhyolite to be a mixture of microcrystalline aggregates and glassy material. Because of the glassy nature of most rhyolites, they are best characterized by chemical analysis. They typically have 70–75 wt % silicon dioxide (SiO2) and more potassium oxide (K2O) than sodium oxide (Na2O). See also Lava; Magma; Volcanic glass; Volcano.
Rhyolite is one of the most common volcanic rocks in continental regions; it is virtually absent in the ocean basins. The rock often occurs in large quantities associated with andesite and basalt. It is common in environments ranging from accretionary prisms at continental margins to magmatic arcs related to subduction zones. Rhyolite is also prevalent in extensional regions and hot spots in continental interiors. See also Andesite; Basalt.
Columbia Encyclopedia:
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Rhyolite
rhyolite, fine-grained light-colored acidic volcanic rock. Rhyolite is chemically the equivalent of granite, and is thus composed primarily of quartz and orthoclase feldspar with subordinate amounts of plagioclase feldspar, biotite mica, amphiboles, and pyroxenes. Rhyolite lava exhibits a typical banded structure produced by its flow pattern. Rhyolite lavas occur in continental and submarine volcanoes, especially island arcs, and in igneous dikes. Rhyolite lavas are typically highly viscous and are explosively ejected from volcanoes. Rhyolites were formed in profusion in the Yellowstone Park area and throughout the southwestern portion of the United States.
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categories related to 'rhyolite'
- Materials, Formations, and Substances - rhyolite: fine-grained volcanic rock produced by lava flow
- Igneous rocks
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Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.
| 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. |
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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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.
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.
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.
See also
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Wikimedia Commons has media related to: Rhyolite |
- Comendite
- List of rock types
- Pantellerite
- Thunderegg
- Igneous rocks
References
- ^ Beckerman, Ira. National Register of Historic Places Inventory/Nomination: Carbaugh Run Rhyolite Quarry Site (36AD30). National Park Service, 1981, 2.
External links
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Rocks from the Bishop tuff, uncompressed with pumice on left; compressed with fiamme on right.
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A sample of Rhyolite from the Conical Hill dome at the head Lyttelton Harbour, Banks Peninsula, New Zealand
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