Rhyolite
Encyclopedia
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada
, and for the satellite system, see Rhyolite/Aquacade
.
Rhyolite is an igneous
, volcanic (extrusive) rock
, of felsic
(silica-rich) composition (typically > 69% SiO2 — see the TAS classification
). It may have any texture from glassy to aphanitic to porphyritic
. The mineral assemblage is usually quartz
, alkali
feldspar
and plagioclase
(in a ratio > 1:2 — see the QAPF diagram
). Biotite
and hornblende
are common accessory minerals.
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
s. They can also occur as breccia
s or in volcanic plug
s 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 lithophysa
l 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 ignimbrite
s.
, rhyolite was quarried extensively in eastern Pennsylvania
in the United States
. Among the leading quarries was the Carbaugh Run Rhyolite Quarry Site
in Adams County
, where as many as fifty small quarry pits are known.
The name rhyolite was introduced into science
by the German traveler and geologist Ferdinand von Richthofen
after his explorations in the Rocky Mountains
in the 1860s.
Rhyolite, Nevada
Rhyolite is a ghost town in Nye County, in the U.S. state of Nevada. It is located in the Bullfrog Hills, about northwest of Las Vegas, near the eastern edge of Death Valley. The town began in early 1905 as one of several mining camps that sprang up after a prospecting discovery in the surrounding...
, and for the satellite system, see Rhyolite/Aquacade
Rhyolite/Aquacade
Rhyolite and later, Aquacade are reportedly code names for a class of SIGINT spy satellites operated by the National Reconnaissance Office for the United States Central Intelligence Agency. The National Security Agency was also reportedly involved...
.
Rhyolite is an igneous
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
, volcanic (extrusive) rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
, of felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
(silica-rich) composition (typically > 69% SiO2 — see the TAS classification
TAS classification
The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali content and the silica content. These chemical parameters are useful, because the relative proportions of alkalis and silica play an important role in...
). It may have any texture from glassy to aphanitic to porphyritic
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...
. The mineral assemblage is usually quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
and plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
(in a ratio > 1:2 — see the QAPF diagram
QAPF diagram
A QAPF diagram is a double triangle diagram which is used to classify igneous rocks based on mineralogic composition. The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid ". These are the mineral groups used for classification in QAPF diagram...
). Biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
and hornblende
Hornblende
Hornblende is a complex 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....
are common accessory minerals.
Geology
Rhyolite can be considered as the extrusive equivalent to the plutonic graniteGranite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
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
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
s. They can also occur as breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s or in volcanic plug
Volcanic plug
A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma hardens within a vent on an active volcano. When forming, a plug can cause an extreme build-up of pressure if volatile-charged magma is trapped beneath it, and this can sometimes lead to an...
s and dikes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian
Obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
, spherulitic
Spherulite
In petrology, spherulites are small, rounded bodies that commonly occur in vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone and rhyolite as globules about the size of millet seed or rice grain, with a duller luster than the surrounding glassy base of the rock,...
, nodular
Nodule (geology)
A nodule in petrology or mineralogy is a secondary structure, generally spherical or irregularly rounded in shape. Nodules are typically solid replacement bodies of chert or iron oxides formed during diagenesis of a sedimentary rock...
, and lithophysa
Lithophysa
A lithophysa is a small cavity found in felsic volcanic rocks believed to be caused by expanding gases in tuffs before solidification. If it has become lined with crystals it may be referred to as a geode or a thunderegg. The word lithophysae is from the Latin for rock bubble...
l structures. Some rhyolite is highly vesicular
Vesicular texture
Vesicular texture is a volcanic rock texture characterised by a rock being pitted with many cavities at its surface and inside. The texture is often found in extrusive aphanitic, or glassy, igneous rock...
pumice
Pumice
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...
. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
/tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
or of ignimbrite
Ignimbrite
An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere....
s.
History
In North American pre-historic timesPre-Columbian era
The pre-Columbian era incorporates all period subdivisions in the history and prehistory of the Americas before the appearance of significant European influences on the American continents, spanning the time of the original settlement in the Upper Paleolithic period to European colonization during...
, rhyolite was quarried extensively in eastern Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. Among the leading quarries was the Carbaugh Run Rhyolite Quarry Site
Carbaugh Run Rhyolite Quarry Site
The Carbaugh Run Rhyolite Quarry Site is an archaeological site in Franklin Township, Adams County, Pennsylvania, United States. It is composed of two smaller sites: a group of small quarries on top of Snaggy Ridge, and the remnants of what may have been a campsite along Carbaugh Run below the...
in Adams County
Adams County, Pennsylvania
Adams County is a county in the U.S. state of Pennsylvania. As of the 2010 census, the population was 101,407. It was created on January 22, 1800, from part of York County and named in honor of the second President of the United States, John Adams...
, where as many as fifty small quarry pits are known.
Name
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...
by the German traveler and geologist Ferdinand von Richthofen
Ferdinand von Richthofen
Ferdinand Freiherr von Richthofen was a German traveller, geographer, and scientist.-Biography:He was born in Carlsruhe, Prussian Silesia, and was educated in Breslau and Berlin. He traveled or studied in the Alps of Tyrol and the Carpathians in Transylvania...
after his explorations in the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
in the 1860s.