User:Marshallsumter/Volcanoes/Volcanic rocks

"The behavior of a lava flow depends primarily on its viscosity (resistance to flow), slope of the ground over which it travels, and the rate of lava eruption. Because basalt contains the least amount of silica and erupts at the highest temperature compared to the other types of lava, it has the lowest viscosity (the least resistance to flow). Thus, basalt lava moves over the ground easily, even down gentle slopes. Dacite and rhyolite lava, however, tend to pile up around a vent to form short, stubby flows or mound-shaped domes."

Mineralogy
"When molten rock erupts onto the Earth's surface, it cools quickly, freezing the growth of existing minerals and preventing the development of new minerals. Such rapid cooling will typically produce lava rocks with a few small minerals suspended in a groundmass of volcanic glass. Molten rock that remains below the ground, however, cools very slowly so that existing minerals continue to grow and many new minerals develop. A slow rate of cooling will produce a coarse-grained plutonic rock that consists entirely of large crystals. Different names are given to such slow-cooling plutonic rocks on the basis of chemical composition and mineral proportions (for example, plutonic rocks of basaltic composition are called gabbro)."

"This graph [on the right] shows the volume percent of minerals present in a plutonic rock that consists entirely of crystals. For example, a granite with 70% SiO2 might have 22% quartz, 38% alkali feldspar, 28% plagioclase feldspar, and 12% biotite."

"The volume percent of minerals present in volcanic rocks typically varies 0-50%. To calculate the relative amounts of the crystals likely to be present, multiply the volume percent in the graph by the actual volume percent in the rock. For example, a rhyolite volcanic rock with 10% crystals, is likely to 2.2% quartz, 3.8% alkali feldspar, 2.8% plagioclase, and 1.2% biotite."

Igneous rocks
Def. "one of the major groups of rock that makes up the crust of the Earth; formed by the cooling of molten rock, either below the surface (intrusive) or on the surface (extrusive)" is called an igneous rock.

"An igneous rock is formed by the cooling and crystallization of molten rock."

Igneous "rocks [are divided] into two broad categories based on where the molten rock solidified."

The image on the right shows a collection of various igneous rocks: from top left to bottom right these are: granodiorite, andesite, syenite, gabbro, rhyolite, basalt, granite and an ignimbrite (a collective term for a special type of volcaniclastic sediment).

Volcanic rocks
Def. rock of "or pertaining to a volcano or volcanoes", produced "by a volcano, or, more generally, by igneous agencies", changed "or affected by the heat of a volcano" is called a volcanic rock.

Andesites
Def. a "class of fine-grained intermediate [..] rock [...] containing mostly plagioclase feldspar" is called an andesite.

"Andesite is a gray to black volcanic rock with between about 52 and 63 weight percent silica (SiO2). Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma commonly erupts from stratovolcanoes as thick lava flows, some reaching several km in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesites erupt at temperatures between 900 and 1100° C."

Anorthosites
Def. a "phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar" is called an anorthite.

Anorthosite on Earth can be divided into five types:
 * 1) Archean-age anorthosites
 * 2) Proterozoic anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth
 * 3) Layers within Layered Intrusions (e.g., Bushveld Igneous Complex and Stillwater igneous complex intrusions)
 * 4) Mid-ocean ridge and transform fault anorthosites
 * 5) Anorthosite xenoliths in other rocks (often granites, kimberlites, or basalts).

Plagioclase crystals are usually less dense than magma; so, as plagioclase crystallizes in a magma chamber, the plagioclase crystals float to the top, concentrating there.

Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.

Proterozoic anorthosites were emplaced during the Proterozoic Eon (ca. 2,500–542 Ma), though most were emplaced between 1,800 and 1,000 Ma.

Large volumes of ultramafic rocks are not found in association with Proterozoic anorthosites.

Aplites
Def. a "fine-grained granitic rock composed mostly of quartz and feldspars" is called an aplite.

Basalts
Def. a "hard mafic [...] rock of varied mineral content" is called a basalt.

"Basalt is a hard, black volcanic rock with less than about 52 weight percent silica (SiO2). Because of basalt's low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can flow quickly and easily move > 20 km from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions, however, still form explosive fountains hundreds of meters tall. Common minerals in basalt include olivine, pyroxene, and plagioclase. Basalt is erupted at temperatures between 1100 to 1250° C."

"Basalt is the most common rock type in the Earth's crust (the outer 10 to 50 km). In fact, most of the ocean floor is made of basalt."

"Huge outpourings of lava called "flood basalts" are found on many continents. The Columbia River basalts, erupted 15 to 17 million years ago, cover most of southeastern Washington and regions of adjacent Oregon and Idaho."

"Basaltic magma is commonly produced by direct melting of the Earth's mantle, the region of the Earth below the outer crust. On continents, the mantle begins at depths of 30 to 50 km."

"Shield volcanoes, such as those that make up the Islands of Hawai`i, are composed almost entirely of basalt."

Basanites
Def. a "basaltic [...] rock, similar to chert" is called a basanite.

Benmoreites
Benmoreite lava forms the "Dragon's Teeth" landmark on Maui, Hawaii

An origin by fractionation from basanite through nepheline hawaiite to nepheline benmoreite has been demonstrated for a volcanic suite in the McMurdo Volcanic Group of late Cenozoic age in McMurdo Sound area of Antarctica. Nepheline benmoreite magmas derived from mantle sources, containing lherzolite xenoliths, display similarities to some plutonic nepheline syenites.

Bostonites
Def. a "fine-grained, pale-colored, grey or pinkish intrusive rock, which consists essentially of alkali-feldspar" is called a bostonite.

In the image on the right, a composite dyke cuts through the hard jointed blocks. It has a camptonite centre and bostonite borders.

"Magmatic zircon in the syenite (bostonite) part of a composite NE–SW-trending cogenetic bostonite–camptonite dyke in Orkney, Scotland, yields a laser ablation inductively coupled plasma mass spectrometry age of 313 ± 4 Ma and εHf(313 Ma) values of +6 to +11."

Carbonatites
Def. any "rock having a majority of carbonate minerals" is called a carbonatite.

The specimen in the image on the right is 20 cm X 14 cm. Its mineralogical composition by color is the black minerals are magnetite, the white are calcite and the green ones are olivine.

Cinerites
Def. a "rock composed mostly of [...] ash" is called a cinerite.

In the image on the right, the bank of yellow rock is a very fine ash deposit.

Clinopyroxenites
In the image on the right a peridotite is seen above (mocca-colouring from weathered olivine; some purple garnets), followed by a clinopyroxenite band (greenish Cr-diopsite) and an orthopyroxenite (dark olive). The red minerals are garnets.

Dacites
Def. a "rock with a high iron content" is called a dacite.

"Dacite lava is most often light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68 percent silica (SiO2). Common minerals include plagioclase feldspar, pyroxene, and amphibole. Dacite generally erupts at temperatures between 800 and 1000°C. It is one of the most common rock types associated with enormous Plinian-style eruptions. When relatively gas-poor dacite erupts onto a volcano's surface, it typically forms thick rounded lava flow in the shape of a dome."

"Even though it contains less silica than rhyolite, dacite can be even more viscous (resistant to flow) and just as dangerous as rhyolites. These characteristics are a result of the high crystal content of many dacites, within a relatively high-silica melt matrix. Dacite was erupted from Mount St. Helens 1980-86, Mount Pinatubo in 1991, and Mount Unzen 1991-1996."

Diabases
Def. a "fine-grained [...] rock composed mostly of pyroxene and feldspar" is called a diabase.

On the right is an image of a rock, a diabase with an aphanitic groundmass and plagioclase phenocrysts.

Diorites
Def. a speckled, coarse-grained rock consisting essentially of plagioclase, feldspar, and hornblende or other mafic minerals is called a diorite.

Def. a "grey [...] rock composed mostly of plagioclase feldspar, biotite, hornblende and/or pyroxene" is called a diorite.

Dolerites
Def. a "fine-grained basaltic rock" is called a dolerite.

Dunites
Def. a "type of igneous rock with a coarse-grained or phaneritic texture" with 90 % or more by volume of olivine is called a dunite.

On the right is an image of a dunite rock sample from Pilbara, Australia. The green to dark green color indicates a high magnesium content and distinguishes it from the granitic rocks in the region.

Gabbronorites
The image on the right shows a 2 x 109 b2k gabbronorite from South Africa.

Gabbros
Def. a dark, coarse-grained plutonic rock of crystalline texture, consisting mainly of pyroxene, plagioclase feldspar, and often olivine is called a gabbro.

Def. "a coarsely crystalline, igneous rock consisting of lamellar pyroxene and labradorite" is called a gabbro.

As with diamictites, rock definitions should be without regard to origin.

Granites
Def. a very hard, granular, crystalline, rock consisting mainly of quartz, mica, and feldspar is called a granite.

On the right is a mineral phase diagram for classifying phaneritic igneous rocks. The minerals represented are
 * 1) Q - quartz
 * 2) A - alkali feldspar such as microcline (KAlSi3O8), and
 * 3) P - plagioclase.

Granitoids
On the right is a granitoid, albeit weathered, with the grus sand below that resulted from the weathering.

Granodiorites
Def. a "rock similar to granite, but containing more plagioclase than potassium feldspar" is called a granodiorite.

Harzburgites
Def. an "ultramafic igneous rock, a variety of peridotite consisting mostly of olivine and low-calcium pyroxene" is called a harzburgite.

Hawaiites
Def. an "olivine basalt intermediate between alkali olivine and mugearite" is called a hawaiite.

Hornblendites
This hornblendite on the right has a cumulitic structure, ipidiomorphic texture composed of hornblende (darkish) and interstitial plagioclase (bright).

Hyaloclastites
Def. a "rock containing glassy fragments" is called a hyaloclastite.

A hyaloclastite sample on the right is on display at the House of the Volcano, Reunion Island.

Ijolites
Def. a "rare igneous rock consisting essentially of nepheline and augite" is called an ijolite.

Such an ijolite is shown in the image on the right.

Kimberlites
Def. a "variety of peridotite containing a high proportion of carbon dioxide; often contains diamonds" is called a kimberlite.

Lamproites
Def. any "of several volcanic rocks having a high potassium content" is called a lamproite.

Lamprophyres
Def. an "uncommon, small-volume ultrapotassic igneous rock primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions" is called a lamprophyre.

Latites
"Igneous rocks are classified on the basis of mineralogy, chemistry, and texture."

A latite has between 35 % and 65 % plagioclase by volume, and up to 5 % by volume of quartz.

Lherzolites
Def. a "dark-green igneous rock consisting largely of chrysolite, with pyroxene and picotite" is called a lherzolite.

Monzogranites
Rochovce granite, composing the coring on the right, is a coarse-grained biotite monzogranite.

Monzonites
Def. an "intrusive igneous rock composed mostly of plagioclase and orthoclase" is called a monzonite.

Mugearites
Def. a "kind of orthoclase-bearing basalt that is made up of olivine, apatite, and opaque oxides" is called is called a mugearite.

The main feldspar in mugearite is oligoclase.

In the TAS classification of volcanic rock, mugearite is classified as sodium-rich basaltic trachyandesite.

Analysis of a Martian rock found by the Curiosity rover and named "Jake Matijevic" (or "Jake M"), after a NASA engineer, determined that this Martian rock is very similar to mugearite erupted on Earth.

Nephelinites
Def. "a dark, finely crystalline rock of volcanic origin, being a mixture of nepheline and pyroxene" is called a nephelinite.

Norites
Def. a "granular crystalline rock consisting essentially of a triclinic feldspar (such as labradorite) and hypersthene" is called a norite.

Obsidian
Def. "a type of black glass produced by volcanoes" is called an obsidian.

Orthopyroxenites
Def. "an ultramafic and ultrabasic rock that is almost exclusively made from the mineral orthopyroxene" is called an orthopyroxenite.

Peridotites
Def. a "rock consisting of small crystals of olivine, pyroxene and hornblende" is called a peridotite.

Phonolites
Def. "a light-coloured rock of volcanic origin composed mostly of alkali feldspars" is called a phonolite.

The dark needle-like minerals are aegirine phenocrysts.

Picrites
Def. a "variety of high-magnesium olivine basalt" is called a picrite.

Pumices
Def. a "light, porous type of pyroclastic igneous rock, formed during explosive volcanic eruptions when liquid lava is ejected into the air as a froth containing masses of gas bubbles" is called a pumice.

Pyroxenites
Def. a "heavy, dark igneous rock consisting mostly of pyroxene minerals with smaller amounts of olivine and hornblende" is called a pyroxenite.

Rhyolites
Def. a rock "of felsic composition, with aphanitic to porphyritic texture" is called a rhyolite.

"Rhyolite is a light-colored rock with silica (SiO2) content greater than about 68 weight percent. Sodium and potassium oxides both can reach about 5 weight percent. Common mineral types include quartz, feldspar and biotite and are often found in a glassy matrix. Rhyolite is erupted at temperatures of 700 to 850° C."

"Rhyolite can look very different, depending on how it erupts. Explosive eruptions of rhyolite create pumice, which is white and full of bubbles. Effusive eruptions of rhyolite often produce obsidian, which is bubble-free and black."

"Some of the United States' largest and most active calderas formed during eruption of rhyolitic magmas (for example, Yellowstone in Wyoming, Long Valley in California and Valles in New Mexico)."

"Rhyolite often erupts explosively because its high silica content results in extremely high viscosity (resistance to flow), which hinders degassing. When bubbles form, they can cause the magma to explode, fragmenting the rock into pumice and tiny particles of volcanic ash."

Scorias
Def. rough "masses of rock formed by solidified lava, and which can be found around a volcano's crater" is called a scoria.

The piece of scoria in the image on the right is from the Golan Heights.

Silexites
On the left is a classification phase diagram of rocks showing the composition range for silexite.

On the right are large pieces of silexite at the motorway service station at La Lozère.

Syenites
Def. an "igneous rock composed of feldspar and hornblende" is called a syenite.

On the left is a leucocratic variety of nepheline syenite from Sweden called särnaite.

Tephrites
Def. an "igneous rock consisting essentially of plagioclase and either leucite or nephelite, or both" is called a tephrite.

Tonalites
Def. an "igneous, plutonic rock composed mainly of plagioclase" is called a tonalite.

Trachyandesites
Def. roche "volcanique intermédiaire entre les trachytes et les andésites" is called a trachyandesite.

Trachytes
Def. a "pale igneous rock consisting mostly of potassium feldspar and plagioclase" is called a trachyte.

Troctolites
Def. a "rare type of ultramafic intrusive rock, consisting primarily of olivine and calcic plagioclase" is called a troctolite.

This sample in the image on the right has a mass of 156 grams and is up to 5 centimeters across. It was collected as part of a rake sample at station 6 at the foot of the North Massif. Another view of this sample was used in Figure 7-16 of the Apollo 17 Preliminary Science Report (SP-330, 1973), which has the following caption: "Coarse-grained norite (sample 76535) with fresh-appearing plagioclase (white to light gray) that has typical striations of albite twinning. Although pyroxenes (medium gray) are fractured along cleavage planes, they do not appear badly crushed."

Trondhjemites
On the right, is a rock cut displaying tonalite, trondhjemite and gneiss.

Websterites
Def. an "ultramafic and ultrabasic rock that consists of roughly equal proportions of orthopyroxene and clinopyroxene" is called a websterite.

Wehrlites
Def. an "ultramafic and ultrabasic rock that is a mixture of olivine and clinopyroxene" is called a wehrlite.

Extrusives
"Volcanic rocks (also called extrusive igneous rocks) include all the products resulting from eruptions of lava (flows and fragmented debris called pyroclasts)."

On the right are extrusive igneous rocks lying on the slope of Mount Etna.

Intrusives
"Plutonic rocks (also called intrusive igneous rocks) are those that have solidified below ground; plutonic comes from Pluto, the Greek god of the underworld."

Hardened intrusive igneous rocks at Tenneti park in Visakhapatnam, Vizag, are shown in the image on the right.

Petrography
"The initial distinction between volcanic and plutonic rocks is made on the basis of texture (fine-grained volcanic vs. coarse-grained plutonic)."

The image on the right shows a display of kimberlitic rocks from the Buffalo Head Hills in Alberta, Canada.

Geochemistry
"Volcanic and plutonic rocks are divided further on the basis of chemistry and mineral composition. The classification scheme [above] is based on chemistry, and is perhaps the simplest method; there are many other classification methods for igneous rocks."

"These rock types all have different characteristics, including temperature when fluid, viscosity (resistance to flow), composition, explosiveness, and types, amounts, and sizes of minerals."

"Volcanic rocks are typically divided into four basic types according to the amount of silica (SiO2) in the rock (see [the bar graph above] at bottom").

"Components of Igneous Rocks":
 * 1) "rhyolite consists of more than 68% silica"
 * 2) "dacite consists of about 63-68% silica"
 * 3) "andesite consists of about 52-63% silica"
 * 4) "basalt consists of about 48-52% silica".

"Other major elements in varying proportion include titanium (TiO2), aluminum (Al2O3), iron (FeO or Fe2O3), manganese (MnO), magnesium (MgO), calcium (CaO), sodium (Na2O), potassium (K2O, and phosphorous (P2O5). The bar graph [above] shows the average concentration of each major element for the four basic types of volcanic rock."

QAPF classification
A "classification of volcanic rocks when modal analyses are lacking [...] is on a non-genetic basis using the total alkali-silica (TAS) diagram, and is as nearly consistent as possible with the [Quartz, Alkali feldspar, Plagioclase, Feldspathoid] QAPF modal classification. The diagram is divided into 15 fields, two of which contain two root names which are separated according to other chemical criteria, giving the following 17 root names: basalt, basaltic andesite, andesite, dacite, rhyolite, trachybasalt, basaltic trachyandesite, trachyandesite, trachyte, trachydacite, picrobasalt, basanite, tephrite, phonotephrite, tephriphonolite, phonolite and foidite. Using Na-K criteria, trachybasalt may be further divided into the sub-root names hawaiite and potassic trachybasalt, basaltic trachyandesite into the sub-root names mugearite and shoshonite, and trachyandesite into the sub-root names benmoreite and latite."

TAS classification
"Many and peculiar are the classifications that have been proposed for igneous rocks. Their variability depends in part upon the purpose for which each was intended, and in part upon the difficulties arising from the characters of the rocks themselves. The trouble is not with the classifications but with nature which did not make things right. … Rocks must be classified in order to compare them with others, previously described, of similar composition and appearance. If this cannot be done on a genetic basis, then an artificial system must answer in order to serve as a card index to rock descriptions. Although this may be an evil thing, it is, at least, the least of several evils."

The classification cannot be applied to all volcanic rocks: certain rocks cannot be named using the diagram, for others, additional chemical, mineralogic, or textural criteria must be used, as for lamprophyres.

The TAS classification should be applied only to rocks for which the mineral mode cannot be determined (otherwise, use a scheme based on mineralogy, such as the QAPF diagram or one of the other diagrams presented for igneous rocks).

Before classifying rocks using the TAS diagram, the chemical analyses must be recalculated to 100% excluding water and carbon dioxide.

The names provided for fields in the TAS diagram are listed below.

B (Basalt) (Use normative mineralogy to subdivide)

O1 (Basaltic andesite)

O2 (Andesite)

O3 (Dacite)

R (Rhyolite)

T (Trachyte or Trachydacite) (Use normative mineralogy to decide)

Ph (Phonolite)

S1 (Trachybasalt) *Sodic and potassic variants are Hawaiite and Potassic Trachybasalt

S2 (Basaltic trachyandesite) *Sodic and potassic variants are Mugearite and Shoshonite

S3 (Trachyandesite *Sodic and potassic variants are Benmoreite and Latite

Pc (Picrobasalt)

U1 (Basanite or Tephrite) (Use normative mineralogy to decide)

U2 (Phonotephrite)

U3 (Tephriphonolite)

F (Foidite) (Name according to dominant feldspathoid when possible. Melilitites also plot in this area and can be distinguished by additional chemical criteria.)


 * Sodic as used above means that Na2O - 2 is greater than K2O, and potassic that Na2O - 2 is less than K2O. Yet other names have been applied to rocks particularly rich in either sodium or potassium (as are ultrapotassic igneous rocks).

Hypotheses

 * 1) The temperatures necessary to melt rocks and produce volcanic rocks can come from high resistance locally to electrical flow.