Product Code Database
Rhyolite ( ) is the most silica-rich of . It is generally or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral () in an otherwise fine-grained groundmass. The mineral assemblage is predominantly quartz, sanidine, and plagioclase. It is the extrusive equivalent of granite.
Its high silica content makes rhyolitic magma extremely viscosity. This favors explosive eruptions over effusive eruptions, so this type of magma is more often erupted as pyroclastic rock than as . Rhyolitic ash-flow are among the most voluminous of continental igneous rock formations.
Rhyolitic tuff has been used extensively for construction. Obsidian, which is rhyolitic volcanic glass, has been used for tools from prehistoric times to the present day because it can be shaped to an extremely sharp edge. Rhyolitic pumice finds use as an abrasive, in concrete, and as a soil amendment.
An extrusive igneous rock is classified as rhyolite when quartz constitutes 20% to 60% by volume of its total content of quartz, alkali feldspar, and plagioclase (QAPF diagram) and alkali feldspar makes up 35% to 90% of its total feldspar content. are not present. This makes rhyolite the extrusive equivalent of granite. However, while the IUGS recommends classifying volcanic rocks on the basis of their mineral composition whenever possible, volcanic rocks are often glassy or so fine-grained that mineral identification is impractical. The rock must then be classified chemically based on its content of silica and alkali metal oxides (Potassium oxide plus Sodium oxide). Rhyolite is high in silica and total alkali metal oxides, placing it in the R field of the TAS diagram. (2025). 9780521880060, Cambridge University Press. ISBN 9780521880060
The alkali feldspar in rhyolites is sanidine or, less commonly, orthoclase. It is rarely anorthoclase. These feldspar minerals sometimes are present as phenocrysts. The plagioclase is usually sodium-rich (oligoclase or andesine). Cristobalite and trydimite are sometimes present along with the quartz. Biotite, augite, fayalite, and hornblende are common accessory minerals.
Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. (1997). 9780697413406, McGraw-Hill Companies, Inc. ISBN 9780697413406 Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulite, nodular, and structures. Some rhyolite is highly vesicular pumice.
Peralkaline rock rhyolites (rhyolites unusually rich in alkali metals) include comendite and pantellerite. Peralkalinity has significant effects on lava morphology and mineralogy, such that peralkaline rhyolites can be 10–30 times more fluid than typical calc-alkaline rhyolites. As a result of their increased fluidity, they are able to form small-scale flow folds, and thin dikes. Peralkaline rhyolites erupt at relatively high temperatures of more than . They comprise bimodal at hotspots and (e.g. Rainbow Range, Ilgachuz Range and Level Mountain in British Columbia, Canada).
Eruptions of rhyolite lava are relatively rare compared to eruptions of less felsic lavas. Only four eruptions of rhyolite have been recorded since the start of the 20th century: at the St. Andrew Strait volcano in Papua New Guinea and Novarupta volcano in Alaska as well as at Chaitén and Cordón Caulle volcanoes in southern Chile. The eruption of Novarupta in 1912 was the largest volcanic eruption of the 20th century, and began with explosive volcanism that later transitioned to effusive volcanism and the formation of a rhyolite dome in the vent.
High-silica rhyolite (HSR), with a silica content of 75 to 77·8% , forms a distinctive subgroup within the rhyolites. HSRs are the most evolved of all igneous rocks, with a composition very close to the water-saturated granite Eutectic system and with extreme enrichment in most incompatible elements. However, they are highly depleted in strontium, barium, and europium. They are interpreted as products of repeated melting and freezing of granite in the subsurface. HSRs typically erupt in large caldera eruptions.
Rhyolite has been found on islands far from land, but such oceanic occurrences are rare.Farndon, John (2007) The Illustrated Encyclopedia of Rocks of the World. Southwater. p. 54. The tholeiitic magmas erupted at volcanic ocean islands, such as Iceland, can sometimes differentiate all the way to rhyolite, and about 8% of the volcanic rock in Iceland is rhyolite. However, this is unusual, and the Hawaiian Islands (for example) have no known occurrences of rhyolite. The alkaline magmas of volcanic ocean islands will very occasionally differentiate all the way to peralkaline rhyolites, but differentiation usually ends with trachyte.
Small volumes of rhyolite are sometimes erupted in association with , late in their history and where central volcanic complexes develop.
Obsidian is usually of rhyolitic composition, and it has been used for tools since prehistoric times. Obsidian scalpels have been investigated for use in delicate surgery. Pumice, also typically of rhyolitic composition, finds important uses as an abrasive, in concrete, (1990). 9783528020552, Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ). . ISBN 9783528020552 and as a soil amendment. Rhyolitic tuff was used extensively for construction in ancient Rome and has been used in construction in modern Europe.
|
|
