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Rhyolite Facts

Rhyolite Facts
An extrusive igneous rock with a very high silica content is rhyolite. It is found usually as pink or gray-colored with grains so small they are difficult to observe without a hand lens. It is made up of quartz, plagioclase, and sanidine, with tiny amounts of hornblende and biotite. When gases are trapped small to medium sized cavities, called vugs, inside the rock are formed through a variety of processes, which often contain crystals, opal, or glassy material. The rhyolites form from granitic magma that has partially cooled in the subsurface. When the magmas erupt, a rock with two grain sizes can form. Large crystals called phenocrysts beneath the surface form, and small crystals called groundmass form at the surface.
Interesting Rhyolite Facts:
The formation of rhyolite usually takes place in continental or continent-margin volcanic eruptions where the granitic magma reaches the surface. It rarely is produced during oceanic eruptions.
Due to the spontaneous release of large amounts of trapped gases, the eruptions of rhyolite may be highly explosive.
The eruptions not only produce rhyolite, but also can produce pumice, obsidian, or tuff. They all have similar compositions but different cooling conditions.
Effusive eruptions produce the rhyolite or the obsidian if the lava cools rapidly, but all the rocks can be found following a single eruption.
Rhyolite will often appear very uniform in texture, although lava flow structures may be evident.
Granitic eruptions, which are rich in silica, are rare and only three of them have occurred since 1900: St. Andrew Strait Volcano in Papua New Guinea, Novarupta Volcano in Alaska, and Chaiten Volcano in Chile.
Slow rhyolitic lava piles up around a vent as it slowly exudes from a volcano, and as a result, produces a mound-shaped structure called a "lava dome."
Gem deposits, such as red beryl, topaz, agate, jasper, and opal are sometimes hosted in rhyolite.
The thick granitic lava that forms rhyolite cools quickly, and pockets of gas remain trapped inside of the lava, eventually forming the vugs, where the materials precipitate as ground water or hydrothermal gases move through.
Rhyolite is rarely used in construction or manufacturing because it is too fractured with too many cavities, though it may be used in cements.
Rhyolite rocks have a hardness of 6 according to Mohs scale of hardness.
Rhyolite is sometimes used as crushed stone when other better materials are not available.
In the past, stone tools, scrapers, blades, hoes, axe heads, and projectiles points have been produced by ancient peoples using rhyolite, but most likely out of necessity.
The silica content of rhyolite is usually between 60% to 77%.
Rhyolite has the mineralogical composition of granite.
Rhyolite rocks can be found in many countries including New Zealand, Germany, Iceland, India, and China, and the deposits can be found near active or extinct volcanoes.


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