Rhyolite is an extrusive igneous rock, a felsic, silica-rich volcanic rock that includes obsidian and pumice. It has a composition similar to granite but has a much smaller grain size. It is composed of the light-colored silicates and is usually buff to pink in color. Occasionally it is light gray. Rhyolite is very fine grained with an aphanitic texture — a crystalline structure that is very fine. Because of rapid cooling, it frequently contains voids and glassy fragments.
Rhyolite weighs roughly one-half as much as limestone and granite, does not crumble and is virtually impervious to the elements. It is commonly used as surface veneer that provides a colorful and rough-hewn surface. It is sometimes used as a decorative rock in landscaping.
Rhyolite is much less common than granite but there are large lava flows and deposits of rhyolite in Yellowstone National Park, WY. Rhyolite was used for the lower walls of the Eagle Store in West Yellowstone. In the Amargosa Desert, the gold mining ghost town of Rhyolite is named for the prevailing rocks from a southwestern Nevada volcanic field, which contain gold ore deposits in a series of rhyolitic lava flows. Castle Rock Rhyolite or Wall Mountain Tuff has been mined from Castle Rock, CO since the 1870s.
PHOTOS:
- Rhyolite (flow banded) volcanic rock. www.flexiblelearning.auckland.ac.
- Rhyolite 1-piece stone table slab from Big Springs. www.lyngsogarden.com
- Eagle Store, W Yellowstone, MT. (see rhyolite inset) www.gly.uga.edu