Rhyolite ( ) is the most
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
-rich of
volcanic rock
Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...
s. It is generally
glass
Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
y or fine-grained (
aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geo ...
) in
texture, but may be
porphyritic
Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning ...
, containing larger mineral
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s (
phenocrysts) in an otherwise fine-grained
groundmass. The
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...
assemblage is predominantly
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
,
sanidine, and
plagioclase. It is the
extrusive equivalent of
granite
Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...
.
Its high silica content makes rhyolitic
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
extremely
viscous. This favors
explosive eruptions over
effusive eruptions, so this type of magma is more often erupted as
pyroclastic rock than as
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
s. Rhyolitic ash-flow
tuffs are among the most voluminous of continental
igneous rock formations.
Rhyolitic tuff has been used extensively for construction.
Obsidian
Obsidian ( ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter element ...
, 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
Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...
, and as a
soil amendment.
Description
Rhyolite is an
extrusive igneous rock, formed from magma rich in
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
that is extruded from a volcanic vent to cool quickly on the surface rather than slowly in the subsurface. It is generally light in color due to its low content of
mafic minerals, and it is typically very fine-grained (
aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geo ...
) or
glassy.
An extrusive igneous rock is classified as rhyolite when
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
constitutes 20% to 60% by volume of its total content of quartz,
alkali feldspar, and
plagioclase (
QAPF) and alkali feldspar makes up 35% to 90% of its total feldspar content.
Feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspar
Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, pota ...
s 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 (
K2O plus
Na2O). Rhyolite is high in silica and total alkali metal oxides, placing it in the R field of the
TAS diagram.
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
Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
-rich (
oligoclase or
andesine).
Cristobalite
Cristobalite ( ) is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, Si O2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members o ...
and
trydimite are sometimes present along with the quartz.
Biotite,
augite,
fayalite, and
hornblende
Hornblende is a complex silicate minerals#Inosilicates, inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common ...
are common accessory minerals.
Geology
Due to their high content of silica and low
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
and
magnesium
Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...
contents, rhyolitic
magmas form highly viscous
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
s.
As a result, many eruptions of rhyolite are highly explosive, and rhyolite occurs more frequently as
pyroclastic rock than as
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
s.
Rhyolitic ash flow
tuffs are the only volcanic product with volumes rivaling those of
flood basalt
A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot (geolo ...
s.
Rhyolites also occur as
breccias or in
lava domes,
volcanic plugs, and
dikes.
Rhyolitic lavas erupt at a relatively low temperature of , significantly cooler than basaltic lavas, which typically erupt at temperatures of .
Rhyolites that cool too quickly to grow
crystals form a natural glass or vitrophyre, also called
obsidian
Obsidian ( ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter element ...
. 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.
Peralkaline rhyolites (rhyolites unusually rich in alkali metals) include
comendite and
pantellerite. Peralkalinity has significant effects on
lava flow morphology and
mineralogy
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and mineralized artifact (archaeology), artifacts. Specific s ...
, 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,
lava tubes and thin dikes. Peralkaline rhyolites erupt at relatively high temperatures of more than . They comprise
bimodal shield volcanoes at
hotspots and
rifts (e.g.
Rainbow Range,
Ilgachuz Range and
Level Mountain in
British Columbia
British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...
, 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
Papua New Guinea, officially the Independent State of Papua New Guinea, is an island country in Oceania that comprises the eastern half of the island of New Guinea and offshore islands in Melanesia, a region of the southwestern Pacific Ocean n ...
and
Novarupta volcano in
Alaska
Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...
as well as at
Chaitén and
Cordón Caulle volcanoes in southern
Chile
Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...
.
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.
Petrogenesis
Rhyolite magmas can be produced by
igneous differentiation of a more mafic (silica-poor) magma, through
fractional crystallization or by assimilation of melted crustal rock (
anatexis). Associations of
andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...
s,
dacites, and rhyolites in similar tectonic settings and with similar chemistry suggests that the rhyolite members were formed by differentiation of mantle-derived
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
ic magmas at shallow depths. In other cases, the rhyolite appears to be a product of melting of crustal sedimentary rock.
Water vapor plays an important role in lowering the melting point of silicic rock,
and some rhyolitic magmas may have a water content as high as 7–8 weight percent.
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 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
A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...
eruptions.
Occurrence

Rhyolite is common along
convergent plate boundaries, where a slab of
oceanic lithosphere is being
subducted into the
Earth's mantle beneath overriding oceanic or
continental lithosphere. It can sometimes be the predominant igneous rock type in these settings. Rhyolite is more common when the overriding
lithosphere
A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...
is continental rather than oceanic. The thicker continental crust gives the rising magma more opportunity to differentiate and assimilate crustal rock.
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
Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...
, 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
flood basalt
A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot (geolo ...
s, late in their history and where central volcanic complexes develop.
Name
The name rhyolite was introduced into
geology
Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
in 1860 by the German traveler and geologist
Ferdinand von Richthofen from the Greek word ''rhýax'' ("a stream of lava") and the rock name suffix "-lite".
Uses
In
North American pre-historic times, rhyolite was quarried extensively in what is now eastern
Pennsylvania
Pennsylvania, officially the Commonwealth of Pennsylvania, is a U.S. state, state spanning the Mid-Atlantic (United States), Mid-Atlantic, Northeastern United States, Northeastern, Appalachian, and Great Lakes region, Great Lakes regions o ...
. Among the leading quarries was the
Carbaugh Run Rhyolite Quarry Site in
Adams County. Rhyolite was mined there starting 11,500 years ago.
Tons of rhyolite were traded across the
Delmarva Peninsula,
[ because the rhyolite kept a sharp point when knapped and was used to make spear points and arrowheads.
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
Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...
, and as a soil amendment. Rhyolitic tuff was used extensively for construction in ancient Rome and has been used in construction in modern Europe.
See also
*
*
References
External links
Geology.com - Rhyolite
Gemstones.com - guide and properties chart - Rhyolite
{{Volcanoes