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