Dictionary: rhyolite   (rī'ə-līt')

A fine-grained extrusive volcanic rock, similar to granite in composition and usually exhibiting flow lines.

[Greek rhuāx, stream (from rhein, to flow) + –LITE.]

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.

For more information on rhyolite, visit Britannica.com.

This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.

Rhyolite

Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (acidic) composition (typically >69% SiO₂ — see the TAS classification. It may have any texture from aphanitic to porphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio > 1:2 — see the QAPF diagram). Biotite and pyroxene are common accessory minerals.

Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, due to their high content of silica and low iron and magnesium contents, rhyolites polymerize quickly and form highly viscous lavas. They can also occur as breccias or in volcanic necks 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.

Top stone is obsidian, below that is pumice and in lower right corner is rhyolite (light color)

See also

• List of rock types
• Granite

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