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Rhyolite

An extrusive igneous rock with a very high silica content.

rhyolite

Rhyolite: The specimen shown is about two inches across.

What is Rhyolite?

Rhyolite is an extrusive igneous rock with a very high silica content. It is usually pink or gray in color with grains so small that they are difficult to observe without a hand lens. Rhyolite is made up of quartz, plagioclase and sanidine, with minor amounts of hornblende and biotite. Trapped gases often produce vugs in the rock. These often contain crystals, opal or glassy material.

Many rhyolites form from granitic magma that has partially cooled in the subsurface. When these magmas erupt, a rock with two grain sizes can form. The large crystals that formed beneath the surface are called phenocrysts and the small crystals formed at the surface are called groundmass.Rhyolite forms in continental or continent-margin volcanic eruptions where granitic magma reaches the surface. Rhyolite is rarely produced at oceanic eruptions.



rhyolite porphyry

Rhyolite Porphyry: Several specimens of rhyolite porphyry, each about three inches across. Click the image to enlarge.

Eruptions of Granitic Magma

Eruptions of granitic magma can produce rhyolite, pumice, obsidian, or tuff. These rocks have similar compositions but different cooling conditions. Explosive eruptions produce tuff or pumice. Effusive eruptions produce rhyolite or obsidian if the lava cools rapidly. These different rock types can all be found in the products of a single eruption.

Eruptions of granitic magma are rare. Since 1900 only three are known to have occurred. These were at St. Andrew Strait Volcano in Papua New Guinea, Novarupta Volcano in Alaska, and Chaiten Volcano in Chile.

Granitic magmas are rich in silica and often contain up to several percent gas by weight. (Think about that - several percent gas by weight is a LOT of gas!) As these magmas cool the silica starts to connect into complex molecules. This gives the magma a high viscosity and causes it to move very sluggishly.

The high gas content and high viscosity of these magmas are perfect for producing an explosive eruption. The viscosity can be so high that the gas can only escape by blasting the magma from the vent.

Granitic magmas have produced some of the world's most explosive volcanic eruptions. Examples include Yellowstone in Wyoming, Long Valley in California and Valles in New Mexico. The sites of their eruption are often marked by large calderas.



rhyolite arrowhead

Rhyolite Arrowheads: Rhyolite was often used to make stone tools and weapons when more suitable materials where not available. It could be fashioned into scrapers, hoes, axe heads, spear points and arrowheads.

Lava Domes and Vugs

Sluggish rhyolitic lavas can pile up around their eruption vents to produce a lava dome instead of flowing freely. Rhyolite lava flows can be so sluggish that they have difficulty flowing downhill!

The thick lava often cools while pockets of gas are still trapped inside. These produce voids within the lava flow known as "vugs". Later, when the lava flow has cooled and gases or groundwater move thorugh, material can preciptate in the vugs. This is how some of the worlds best deposits of red beryl, topaz, agate, jasper and opal have formed.



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