Photoglossary

Rhyolite

Rhyolite is a light-colored rock with more than 68 weight percent of silica (SiO2). 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 850C.

Facts and Figures: The word rhyolite comes from the Greek word for stream (rhyax) and lithos (rock). 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. 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.

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