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Rhyolite

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Rhyolite is felsic igneous extrusive rock and it is a fine-grained and dominated by quartz (>20%) and alkali feldspar (>35%).Due to the high silica content, rhyolite lava is very viscous.  It is often difficult to identify rhyolites without chemical analysis due to their glassy groundmasses. Many rhyolites consist mainly of glass, and are termed obsidian, or are partially devitrified, and termed pitchstones. Alkali rhyolites are those in which >90% of feldspars are alkali feldspars. These rocks are peralkaline and usually contain alkali amphiboles and/or pyroxenes.

Although lava flow structures are prominent, the riolite generally appears very uniform in the tissue. They are colored from white to gray. By virtue of its fine-grained nature, the separation of rolite from the aphanitic rocks of the different composition is not always certain only on a color basis, but the volcanic aphanitic rocks are likely to be a rolitic.

Group – Volcanic

Colour – Variable, but light coloured.

Texture – Aphanitic, Glassy, Porphyritic

Igneous Equivalent: Granite

Mineral Content – Groundmass generally of quartz and plagioclase, with lesser amounts oforthoclase, biotite, amphibole ( augite), pyroxene ( hornblende), and glass; phenocrysts of plagioclase and quartz, often with amphibole and / or biotite, sometimes orthoclase. Silica (SiO 2) content – 69%-77%.

Occurrence: Rhyolite has been found on islands far from land, but such oceanic occurrences are rare

Structure: Vesicles or amygdales may be present. (Pumice is a highly vesicular variety of rhyolite.) May contain spherulites which are spherical bodies, often coalescing, comprising radial aggregates of needles, usually of quartz or feldspar. Spherulites are generally less than 0,5 cm in diameter, but they may reach a meter or more across. They form by very rapid growth in quickly cooling magma, and the crystallization of glass. Mineralogy: As for granite, but rapid cooling results in minute crystals. Phenocrysts of quartz, feldspar, hornblende or mica occur.

Classification of Rhyolite

Rhyolite, with felsic minerals comprising >20% quartz and alkali feldspar/plagioclase 40-90%.

A group of extrusive igneous rocks, typically porphyritic and commonly exhibiting flow texture, with phenocrysts of quartz and alkali feldspar in a glassy to cryptocrystalline groundmass; also, any rock in that group; the extrusive equivalent of granite. It grades into rhyodacite with decreasing alkali feldspar content and into trachyte with a decrease in quartz.

Read to classification of igneous rock page

Rhyolite Composition

The mineralogical composition of rhyolite is defined as containing mostly quartz and feldspar with a total silica content of more than 68%. Quartz in rhyolite may be as low as 10% but is usually present in amounts of 25% to 30%. Feldspars often comprise 50% to 70% of rhyolite, with potassium feldspar present in at least twice the amount of plagioclase feldspar. Ferromagnesian, or dark, minerals are rare as phenocrysts, being mostly biotite when present. Trace accessory minerals may also include muscovite, pyroxenes, amphiboles, and oxides.

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.

Formation of the Rhyolite

Rhyolites erupt from the Earth’s surface at temperatures of 1382 to 1562 degrees Fahrenheit. The crystals are formed depending on the speed of the lava as well as the cooling period when it reaches the surface. Most rhyolites are uniform in texture, and their color ranges from gray to light-pink, depending on the striations made by the lava flow. These rocks have many shapes, ranging from pumice to porphyritic.

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.

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.

Where is Rhyolite Located

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

Characteristics and Properties

Rhyolite rocks bear a striking resemblance to granite, due to being classified as felsic rocks, except that rhyolite has a fine-grained texture with phenocrysts, which are small crystals sometimes embedded within the rock. The minerals that make up the composition of this rock are mica, feldspar, quartz, and hornblende. One of their distinct characteristics is the smooth appearance and high silica content.

Rhyolite Uses

  • Decorative Aggregates, Homes, Hotels, Interior Decoration, Kitchens
  • As Building Stone, As Facing Stone, Paving Stone, Office Buildings
  • Arrowheads, As Dimension Stone, Building houses or walls,
  • Construction Aggregate, Cutting Tool, for Road Aggregate, Knives
  • Artifacts
  • Gemstone, Laboratory bench tops, Jewelry

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.
  • It 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.
  • It 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.

References

Cite this article as: Geology Science. (2019). Rhyolite. [online] Available at: http://geologyscience.com/rocks/igneous-rocks/rhyolite/ [12th November 2019 ]
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