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 large 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. 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. |
The Rock - RhyoliteCopyright ©1995-2007 by Amethyst Galleries, Inc.
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