Rhyolite

What is Rhyolite?

Rhyolite is an extrusive igneous rock with a very high silica content. It is usually pink or gray in colour 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 usually forms in continental or continent-margin volcanic eruptions where granitic magma reaches the surface. Rhyolite is rarely produced at oceanic eruptions. Rhyolite forms of the lava with high silica content so the lava is very viscous. If rhyolitic magma is rich in gases, it can erupt explosively which will solidify quickly forming pumice. Certain times extremely porous rhyolite lava flows occur and such flow allow degassing and subsequent collapse of lava which forms obsidian.

Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts are highly polymerized and form highly viscous lava. They also occur as breccias or in volcanic plugs and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures. Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra/tuff or of ignimbrites.

Eruptions of rhyolite are relatively rare compared to eruptions of less felsic lava. Only three eruptions of rhyolite have been recorded since the start of the 20th century: at the St. Andrew Strait volcano in Papua New Guinea, Novarupta volcano in Alaska, and Chaiten in southern Chile.

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. 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 most explosive volcanic eruptions in Earth's history. 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.

Photo of a lava dome in the caldera of Mount St. Helens. Activity at St. Helens slowly extrudes thick lavas that gradually build domes in the caldera. This dome is composed of dacite, a rock that is intermediate in composition between rhyolite and andesite.

Lava Domes

Sluggish rhyolitic lava can slowly exude from a volcano and pile up around the vent. This can produce a mound-shaped structure known as a "lava dome." Some lava domes have grown to a height of several hundred meters.

Lava domes can be dangerous. As additional magma extrudes, the brittle dome can become highly fractured and unstable. The ground can also change slope as the volcano inflates and contracts. This activity can trigger a dome collapse. A dome collapse can lower the pressure on the extruding magma. This sudden lowering of pressure can result in an explosion. It can also result in a debris avalanche of material falling from the tall collapsing dome. Many pyroclastic flows and volcanic debris avalanches have been triggered by a lava dome collapse.

Composition of Rhyolite

Rhyolite has composition similar to that of granite but with much smaller grains. It is composed of light colour silicates. Generally composition is quartz and plagioclase with less amount of orthoclase, biotite, amphibole, pyroxene and glass.

Rhyolite and Gemstones

Many gem deposits are hosted in rhyolite. These occur for a logical reason. The thick granitic lava that forms rhyolite often cools quickly while pockets of gas are still trapped inside of the lava. As the lava quickly cools, the trapped gas is unable to escape and forms cavities known as "vugs." Later, when the lava flow has cooled and hydrothermal gases or ground water move through, material can precipitate in the vugs. This is how some of the world's best deposits of red beryl, topaz, agate, jasper, and opal are formed. Gem hunters have learned this and are always on the lookout for vuggy rhyolite.

Occurrence of Rhyolite

Rhyolite in Europe

• Etsch Valley Vulcanite Group near Bolzano and the surrounding area
• Gréixer rhyolitic complex at Moixeró range (Catalonia, Spain)
• Vosges
• Iceland: all active and extinct central volcanoes, e.g. Torfajökull, Leirhnjúkur / Krafla, Breiddalur central volcano
• Papa Stour in Shetland
• Copper Coast Geopark in southeast Ireland
• various locations around Snowdonia, Wales
• Massif de l'Esterel, France

Rhyolite in Germany

• the Thuringian Forest consists mainly of rhyolites, latites and pyroclastic rocks of the Rotliegendes
• Saxony, especially the north west
• Saxony-Anhalt north of Halle
• Saar-Nahe Basin e.g. the Königstuhl (Pfalz) on the Donnersberg mountain
• Black Forest e.g. on the Karlsruher Grat
• Odenwald

Rhyolite in America

• Andes
• Cascade Range
• Cobalt, Ontario Canada
• Rocky Mountains
• Jemez Mountains
• Rhyolite, Nevada was named after a rhyolite deposit that characterised the area
• St. Francois Mountains
• Jasper Beach - Machiasport, Maine

Rhyolite in Oceania

• the Taupo Volcanic Zone in New Zealand has a large concentration of young rhyolite volcanoes
• the Gondwana Rain forests of Australia World Heritage Area contains rhyolite-restricted flora along the Great Dividing Range

Rhyolite in Asia

• The Malani Igneous Suite, Rajasthan, India.
• The Yandang Shan mountain chain, near the town of Wenzhou, Zhejiang province, China

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