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Rhyolite

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Rhyolite is a high-silica lava that is chemically the same as granite but is extrusive rather than plutonic. (more below)
Stiff stuff
Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)
Click the photo for the full-size version. Rhyolite lava is too stiff and viscous to grow crystals except for isolated phenocrysts. The presence of phenocrysts means that rhyolite has a porphyritic texture. This rhyolite specimen, from the Sutter Buttes of northern California, has visible phenocrysts of quartz.

Rhyolite is typically dark and has a glassy groundmass. This is a less typical white example; it can also be reddish. Being high in silica, rhyolite is a stiff lava that tends to have a banded appearance. Indeed, "rhyolite" means "flow stone" in scientific Greek.

Rhyolite is typically found in continental settings where magmas have incorporated granitic rocks from the crust as they rise from the mantle. It tends to make lava domes when it erupts.

See other examples of rhyolite in the gallery of volcanic rocks.

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