RHYOLITE
Rhyolite:
Rhyolite is very closely related to
granite.
The difference is rhyolite has
much finer crystals.
These crystals are so small that they can not be seen by the
naked eye.
Rhyolite is an
extrusive igneous rock
having cooled much more
rapidly than granite, giving it a glassy appearance.
Composition:
Rhyolite is a felsic (rich in silica and aluminum),
rock, light gray, typically pink to red, fine-grained, sugary texture, usually has a brecciaed texture
(tuff) and sometimes contains phenocrysts of feldspar (Na-
rich varieties), quartz,
hornblende, and biotite mica.
Rhyolite's
silica (SiO2)
content is
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.
Eruption Temperatures:
Rhyolite is erupted at temperatures of 700 to 850 degrees C.
-- Excerpt from:
Philip Stoffer, 2002, Rocks and Geology in the San Francisco Bay Region, USGS Bulletin 2195,
Volcano World Website, July 2001,
and
USGS Volcano Hazards Program Photoglossary, 2003
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