The Rock - Rhyolite

  • Igneous Rock Type: Extrusive volcanic
  • Related to: Granite, pumiceobsidian
  • Chemistry: Acidic
  • Color: White, gray, light black
  • Texture: Aphanitic (crystals too small to see) to porphyritic (a mixture of crystal sizes)
  • Origins: Volcanic arcs
  • Common Minerals: Quartz, feldspars and hornblende
  • Accessory Minerals: Pyroxenes and biotite
  • Uses: Decorative stones, scouring stones and abrasives, ornamental stone

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.

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