Rhyolite is a fine-grained to glassy, light-colored, silica-rich extrusive volcanic rock that often has a porphyritic texture. It forms when highly evolved and differentiated silicic magma cools quickly on or near the Earth’s surface.
This rock is of felsic composition and has mainly quartz, sanidine, and plagioclase with small amounts of mafic minerals such as biotite, fayalite, augite, and less often hornblende.
The term ‘rhyolite’ comes from the Greek word rhýax + lite, a name introduced by Ferdinand von Richthofen, a German geologist and traveler, in 1860. Rhýax means streams of lava, while the suffix lite means a rock. Before that, it was known as liparite.
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Description: Texture, properties and appearance
Rhyolite is a fine-grained to glassy, light-colored, felsic extrusive igneous rock. It has a Mohs hardness scale of 6-7 and a specific gravity (density) of about 2.4 (2.4 g/cm3).
Rhyolite is usually pinkish, buff, light-gray, blue-gray, yellowish, reddish, reddish purple, and rarely blackish or dark in color. The light color is from the high content of felsic minerals or elements like feldspar, aluminum, sodium, and potassium.
What about its texture? Rhyolite has a fine-grained or aphanitic texture, often porphyritic. Some specimens may, however, have a glassy texture when cooling is rapid or be pumice-like if high in volatiles.
Porphyritic rhyolite has large crystals (phenocrysts) of quartz, oligoclase, K-feldspar, and less often biotite, amphibole, or pyroxene in a fine-grained matrix.
The fine-grained matrix has mainly quartz, alkali feldspar, and sometimes glass with less of the mafic minerals. Nevadite is a porphyritic rhyolite variety with mainly quartz phenocrysts, often confused for granite.
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Finally, rhyolite may show flow banding if it forms from lava flows. This rock may also have spherulitic or perlitic structures, thundereggs, or even geodes.
Chemical composition
Rhyolite is an acidic rock with 69-80% silica (SiO2). It is high in sodium and potassium and lower in calcium, magnesium, and iron oxides.
Typical percentage chemical composition of rhyolite by weight is 69-80% SiO2, >12% Al2O3, >7% Na2O + K2O, < 3% iron oxides, < 1% MgO, and < 2% CaO. This rock also has tiny amounts of P2O5, MnO, and TiO2. The exact composition of these minerals may slightly vary but will remain within these ranges.
Depending on silica content, USGS further classifies rhyolite into low silica rhyolite (HSR) with 69-74% SiO2 and high silica rhyolite (HSR) with 75-80% SiO2.
The HSR variety is high in incompatible elements but is strontium, barium, and europium depleted. This variety is also highly evolved and has a water-saturated granite eutectic-like composition.
Lastly, on the Total Alkali Silica (TAS) classification diagram, rhyolite is a rock with >69% silica. Lowering silica content grades this rock into dacite or trachyte depending on the Na2O + K2O content.
Mineral composition
Rhyolite is a felsic rock dominated by quartz, sanidine, and plagioclase. It also has a minor amount of mafic minerals like mica (biotite), amphibole (hornblende), fayalite (olivine), and less often pyroxene (augite).
Quartz will only occur together with tridymite and cristobalite, the two high-temperature silica polymorphs, while the accessory minerals in this rock are hypersthene, magnetite, and ilmenite.
On the QAPF diagram for extrusive volcanic rock, rhyolite is a rock in which quartz is 20–60% of the QAPF content by volume and alkali feldspar accounts for 30–90% of the total feldspars. This rock doesn’t have feldspathoids.
How is rhyolite formed?
Rhyolite rocks form from highly viscous, silica-rich magmas relatively low in magnesium and iron that erupt on or near the Earth’s surface. The rapid cooling doesn’t give time for larger crystals to grow. These rocks will, therefore, have a fine-grained texture.
Rhyolitic magmas that form this rock are highly evolved and differentiated. They form from partial melting of upper crustal rock or fractional crystallization of basaltic magma, derived from the upper mantle.
Fractional crystallization can occur with or without incorporating crustal rocks, and the coexistence of this rock with andesite confirms this origin.
Rhyolitic magmas can also form from the interstitial fluids extracted from long-lived crystalline magma mushes.
last but not least, the eruptions that form these rocks occur at temperatures of 800-1000°C (1,470 to 1,830°F) and can be explosive, effusive, or hybrid with both effusive and explosive elements depending on volatile content.
1. Explosive eruptions
Explosive eruptions occur in rhyolite magma high in volatiles (4–8%). The high viscosity of these magmas prevents gas escape, resulting in some of the most violent and devastating eruptions.
These eruptions are often voluminous and will eject ash, pumice lapilli, blocks, and bombs or tephra. Some will deposit tephra over large areas, and others can trigger pyroclastic density currents.
Explosive eruptions will form rhyolite rocks, ash deposits, breccia, ignimbrite, or welded tuff that may cover extensive areas. For instance, the 20Ka Old Oruanui eruption in New Zealand created 320 cubic kilometers of ignimbrite sheets.
2. Effusive eruptions
If these magmas are low in volatiles, they will ooze out slowly from the vent, forming slow-moving lava flows. These lava flows are 10–100 meters thick and cover a small area. Some effusive eruptions may form rhyolite lava domes and plugs.
The margins of effusively erupted rhyolite will have obsidian if degassing effectively happens.
Finally, rhyolitic dome collapse may result in auto brecciation and deadly pyroclastic flows.
Where is rhyolite found?
Rhyolite rocks of all ages occur in continental margins, less often in interior continents, and rarely in intra-oceanic islands where one oceanic crust overrides the other.
These rocks occur mostly on Andean-type subduction on convergent boundaries and sometimes in intracontinental hotspots and rifts.
Thick continental crusts above subduction zones and some continental hotspots favor magma differentiation and assimilation of crustal rocks to form rhyolitic magmas.
In the United States of America, significant rhyolite deposits are in Yellowstone in Wyoming, Valles in New Mexico, the Rio Grande Rift in Colorado and New Mexico, and the Long Valley Systems and Coso Volcanic Field in California. Mount Kineo in Maine, Palisade in Lake Superior, and Utah also have considerable rhyolite deposits.
Other places with rhyolite include Aden volcano in Yemen, the Taupo volcanic zone, and Great Barrier Island in New Zealand. East African Rift Valley also has this rock, and over 8% of the rocks in Iceland are rhyolite.
Uses
The vuggy and fractured nature of rhyolite limits its use to when there are no better alternatives. This rock is also high in silica, making it unsuitable for making concrete.
Some of the ancient uses of rhyolite were to make scrappers, arrowheads, blades, statues, and sculptures.
Modern uses of these rocks include low-quality aggregates used in road construction or land fill. Other uses are landscaping, decorating gardens, and controlling soil erosion.
Some rhyolite rocks may contain valuable precious or semiprecious gemstones like topaz, jasper, red beryl, agate, and opal. These gemstones occur in vugs, geodes, spherulites, lithophysae, or thundereggs.
Additionally, some amygdaloidal rhyolites may also have secondary semiprecious minerals. These minerals include leopard skin, galaxy, orbicular, kambaba (rainforest rhyolite jasper), rainbow, blue, skin, sage dendritic, flower, wonderstone, and bird’s eye mushroom.
Uses of these precious and semiprecious gemstones or minerals include making necklaces, bracelets, pendants, brooches, and other ornamental or decorative items.
Frequently Asked Questions
Rhyolite differs from granite because it has a fine-grained texture, soda-rich sanidine, and less often muscovite. In contrast, granite has a coarse-grained texture, muscovite, and soda-poor microcline or microcline perthite.
Peralkaline rhyolites refer to rhyolite, pantellerite, and comendite rocks, which are unusually high in sodium and potassium than needed by feldspar. These rocks are also aluminum deficient.
Due to their high sodium and potassium, peralkaline rhyolites have alkaline mafic minerals like aegirine and riebeckite.
Comendite differs from pantellerite because it is higher in aluminum and lower in iron than pantellerite.
Magmas that form peralkaline rhyolites are up to 30 times less fluid and erupt at higher temperatures. Their low viscosity allows these rocks to form inside lava tubes, small-scale flow folds, or thin dikes.
Finally, peralkaline rhyolites erupt in shield volcanoes, especially rifts and hotspots like the Kenyan Rift Valley, the Rainbow Range in Canada, and the Australian Glass House Mountains.