The Rock - Rhyolite
Igneous Rock Type: Extrusive
Related to: Granite, pumice, obsidian
Color: White, gray, light black
(crystals too small to see)
(a mixture of crystal sizes)
Origins: Volcanic arcs
Common Minerals: Quartz, feldspars
Accessory Minerals: Pyroxenes
Uses: Decorative stones, scouring stones and abrasives,
Rhyolite is a
relatively common volcanic rock. It is the chemical equivalent of granite.
Although the two rock types have the same chemistry, rhyolite is extrusive
and granite is intrusive. While granite has crystals that are generally
easy to see, in rhyolite the crystals are often too small to see. This
is due to the more rapid cooling of the rhyolite lava compared to granite's
slower cooling magma.
In general, the slower a magma cools the larger
the crystal size. Although crystals in rhyolite are usually hard to
see, they are there, but as microscopic crystals often surrounded by a glassy
matrix. If the lava fails to form crystals and is essentially all
glass, then it is more correctly called an obsidian.
At times some crystals can grow large enough to see and then the
texture is call porphyritic. Porphyritic texture means that there
are larger crystals surrounded by a fine grained or glassy matrix. Sometimes
there are rounded sphericules of quartz or feldspar in the matrix. If
the rock contains numerous holes or vesicules, then the rhyolite is called pumice.
Rhyolite is found in volcanic arcs where crustal rocks have been subducted
under continental crust and melted into a lighter magma rich in silica.
Rhyolite contains over 70% silica or SiO2.
This high silica content gives the rock its general light color, low
density and a high viscosity to the lava. Viscosity is a measure of
how resistant to flow a liquid is. The higher the viscosity, the slower
and more "thick" the lava is.
Rhyolitic lavas are often more explosive
and slower moving than the less viscous basalt lavas such as those that
erupt on the island of Hawaii. Rhyolite often is found with flow banding
"frozen" into the rock. This lends to uses as decorative rocks
and even ornamental stones for jewelry.