Volcanic Rhyolite
Volcano Terms and Definition

volcanism
Volcanism
Volcanic eruptions are the clear and dramatic expression of dynamic processes going on in planet Earth. The author, one of the most profound specialists in the field of volcanology, explains in a concise and easy way how to understand manner the basics and most recent findings in the field of volcanology.

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

Rhyolite is a light-colored rock with silica (SiO 2 ) 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.



  • 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.

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Source:
U.S. Department of the Interior