rhyolite

^{ALASKA VOLCANO OBSERVATORY GLOSSARY}

Volcanic rock (or lava) that characteristically is light in color, contains 69% silica or more, and is rich in potassium and sodium.

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^{GLOSSARY OF VOLCANIC TERMS}

A volcanic rock containing greater than 68% silica (by weight). Rhyolites are composed primarily of alkali feldspars (sanadine and orthoclase) and quartz (>10% by volume), with lesser amounts of sodic plagioclase (albite, oligoclase), hornblende, or biotite. Accessory minerals include zircon, apatite, and tourmaline. Due to their high silica content (and thus high degree of polymerization), rhyolite lava are very viscous and commonly form lava domes, mesa lavas, or coulees. Rhyolitic magmas with high gas contents typically explode violently to form pyroclastic flows, pyroclastic surges, and pyroclastic falls.

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^{USGS PHOTO GLOSSARY OF VOLCANIC TERMS}

Rhyolite is a light-colored rock with silica (SiO₂) content greater than about 68 weight percent. Sodium and potassium oxides both can reach about 5 weight percent. Common mineral types include quartz, feldspar and biotite and are often found in a glassy matrix. Rhyolite is erupted at temperatures of 700 to 850° C.

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Flow banding in rhyolite lava from Mono-Inyo Craters volcanic chain, California (black bands composed of obsidian)

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Did you know?

• The word rhyolite comes from the Greek word for stream (rhyax) + the suffix lite. Rhyolite was named streaming rock because of its beautiful flow bands, which are made of bubble- and crystal-rich layers that form as the lava flows onto the surface and advances.

• Rhyolite can look very different, depending on how it erupts. Explosive eruptions of rhyolite create pumice, which is white and full of bubbles. Effusive eruptions of rhyolite often produce obsidian, which is bubble-free and black.

• Some of the United States' largest and most active calderas formed during eruption of rhyolitic magmas (for example, Yellowstone in Wyoming, Long Valley in California and Valles in New Mexico).

• Rhyolite often erupts explosively because its high silica content results in extremely high viscosity (resistance to flow), which hinders degassing. When bubbles form, they can cause the magma to explode, fragmenting the rock into pumice and tiny particles of volcanic ash.