Chemicals/Rocks

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Rock outcrop occurs along a mountain creek near Orosí, Costa Rica. Credit: Dirk van der Made.{{free media}}

Rocks are a bound aggregate of minerals with usually a large geographic extent.

Occasionally, a rock is composed of only one mineral. But a crystal of the mineral fluorite in your hand is a stone rather than a rock.

Minerals[edit | edit source]

This is a visual image of a forsterite crystal. Credit: Azuncha.
This is a specimen of obsidian, a mineraloid, from Lake County, Oregon. Credit: Locutus Borg.

Def. "a solid, homogeneous, crystalline chemical element or compound that results from natural inorganic processes" or "any naturally occurring inorganic material that has a (more or less) definite chemical composition and characteristic physical properties"[1] is called a mineral.

Def. a "substance that resembles a mineral but does not exhibit crystallinity"[2] is called a mineraloid.

Theory of rocks[edit | edit source]

Chalk strata in Cyprus - showing classic layered structure. Credit: MeanStreets.{{free media}}

Def. a "hard earthen substance that can form large rocks"[3] is called a stone.

Def. a "naturally occurring aggregate of minerals"[4] or any "natural material with a distinctive composition of minerals"[5] is called a rock.

The image on the right shows layers of rocks with large geographical extent in the form of strata.

Adakites[edit | edit source]

Closer view shows the adakite columns of Cerro Mackay, Chile. Credit: LBM1948.{{free media}}

Adakites are volcanic rocks of intermediate to felsic composition that have geochemical characteristics of magma originally thought to have formed by partial melting of altered basalt that is subducted below volcanic arcs.[6]

The geochemical characteristics of "slab melts" and the contention that melts can form from young and therefore warmer crust in subduction zones have been verified.[7]

Aeolianites[edit | edit source]

Holocene eolianite is on Long Island, Bahamas. Credit: Wilson44691.{{free media}}

Def. a "rock formed from dune sand, often calcareous"[8] is called an aeolianite.

Amphibolites[edit | edit source]

Garnet bearing amphibolite is from Val di Fleres, Italy. Credit: Bernabè Egon.
Amphibolite is from Cape Cod, Massachusetts. Credit: B.W. Hallett, V. F. Paskevich, L.J. Poppe, S.G. Brand, and D.S. Blackwood, USGS.

Def. any "of a class of [...] rock composed mainly of amphibole with some quartz etc"[9] is called an amphibolite.

On the left is foliated amphibolite, sample 81MW0005, a borehole sample from under Cape Cod in Massachusetts in USA. It is made of the minerals plagioclase (35%), hornblende (20%), biotite (20%), epidote (15%), quartz (9%), and trace oxides and sphene. Plagioclase is mostly fine grained and subhedral and occurs in the matrix. Fine-grained hornblende occurs as anhedral pleochroic green-tan crystals. Biotite is partly, but not entirely aligned in the foliation, suggesting that deformation took place before a secondary growth of biotite. Epidote is anhedral, and sometimes rimmed by biotite. Quartz occurs in 2 mm-thick aggregates and shows subgrain development.

Andesites[edit | edit source]

Close view is of andesite lava flow from Brokeoff Volcano, California. Credit: United States of America Geological Survey.

Def. a "class of fine-grained intermediate [..] rock [...] containing mostly plagioclase feldspar"[10] 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."[11]

Anorthosites[edit | edit source]

Anorthosite is a mafic intrusive igneous rock composed predominantly of plagioclase. Credit: Thamizhpparithi Maari.{{free media}}

Def. a "phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar"[12] is called an anorthite.

Anorthosite on Earth can be divided into five types:[13]

  1. Archean-age anorthosites
  2. Proterozoic anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth[14]
  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.[15][14][13]

Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.[16]

Proterozoic anorthosites were emplaced during the Proterozoic Eon (ca. 2,500–542 Ma), though most were emplaced between 1,800 and 1,000 Ma.[14]

Large volumes of ultramafic rocks are not found in association with Proterozoic anorthosites.[17]

Anthracites[edit | edit source]

Lump of anthracite was extracted from the Ibbenbüren underground coal mine, located in Ibbenbüren, Germany. Credit: Educerva.

Def. a "form of carbonized ancient plants; the hardest and cleanest-burning of all the coals; hard coal"[18] is called anthracite.

Def. a coal of a hard variety that contains relatively pure carbon is called an anthracite.

Aphanites[edit | edit source]

An aphanitic volcanic sand grain, with fine-grained groundmass is seen through a petrographic microscope. Credit: Qfl247.{{free media}}
This is a smooth textured, aphanitic, basaltic volcanic bomb found in the Mojave Desert National Preserve by Rob McConnell. Credit: Wilson44691.{{free media}}
IUGS classification of aphanitic igneous rocks according to their relative alkali (Na2O + K2O) and silica (SiO2) weight contents. Blue area is roughly where alkaline rocks plot; yellow area where subalkaline rocks plot. Credit: Woudloper.{{free media}}

International Union of Geological Sciences (IUGS) classification is of aphanitic extrusive igneous rocks according to their relative alkali (Na2O + K2O) and silica (SiO2) weight contents, where blue area is roughly alkaline rocks plot; yellow area where subalkaline rocks plot.[19]

Aphanite is a name given to certain igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye[20] (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geological texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. As a rule, the texture of these rocks is not the same as that of volcanic glass (e.g., obsidian), with volcanic glass being non-crystalline (amorphous), and having a glass-like appearance.[21]

Aphanites are commonly porphyritic, having large crystals embedded in the fine groundmass, or matrix. The larger inclusions are called phenocrysts.

They consist essentially of very small crystals of minerals such as plagioclase feldspar, with hornblende or augite, and may contain also biotite, quartz, and orthoclase.[20]

Def. certain "dark [...] rocks having grain so fine that the individual crystals cannot be seen with the naked eye"[22] is called an aphanite.

Aplites[edit | edit source]

This is an aplite sample from the NASA Rocklibrary. Credit: NASA.{{free media}}

Def. a "fine-grained granitic rock composed mostly of quartz and feldspars"[23] is called an aplite.

Argillites[edit | edit source]

This is a piece of black argillite from Haida Gwaii. Credit: Gbuchana.
Greyish chunks of graptolitic argillite in front of Pakri Cliff, yellowish and white chunks are limestone. Credit: Siim Sepp.

Def. a "fine-grained sedimentary rock, intermediate between shale and slate, sometimes used as a building material"[24] is called an argillite.

Arkoses[edit | edit source]

Arkose can have grains of K-feldspar (pinkish-orangish) and quartz (gray). Credit: James St. John.

Def. a "sedimentary rock consisting of small fragments of feldspar and quartz similar to a coarse sand"[25] is called an arkose.

Basalts[edit | edit source]

This is an example of a basalt. Credit: USGS.

Def. a "hard mafic [...] rock of varied mineral content"[26] 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."[27]

"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."[27]

"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."[27]

"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."[27]

"Shield volcanoes, such as those that make up the Islands of Hawai`i, are composed almost entirely of basalt."[27]

Basanites[edit | edit source]

Def. a "basaltic [...] rock, similar to chert"[28] is called a basanite.

Benmoreites[edit | edit source]

Benmoreite lava forms the "Dragon's Teeth" landmark on Maui, Hawaii.[29] Credit: Christopher Michel.{{free media}}

Benmoreite lava forms the "Dragon's Teeth" landmark on Maui, Hawaii[29]

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.[30] Nepheline benmoreite magmas derived from mantle sources, containing lherzolite xenoliths, display similarities to some plutonic nepheline syenites.[31]

Biofacies[edit | edit source]

Def. a "body of rock with characteristic biological features, such as certain kinds of fossil"[32] is called a biofacies.

Blairmorites[edit | edit source]

A specimen of blairmorite is from the Crowsnest Formation. Credit: Georgialh.{{free media}}

Blairmorite is a very rare[33] porphyritic volcanic rock named after the community of Blairmore in southwestern Alberta, Canada.[34][35] It is characterized by dominant analcime phenocrysts in a matrix of analcime, sanidine and alkalic pyroxene with accessory titanite, melanite and nepheline and is a leucocratic variety of analcimite (a foidite).[36] Blairmorite has also been described as an analcime-rich variety of phonolite.[37][38]

This extrusive igneous rock is known from only two geological formations worldwide: the foremost blairmorite occurrence is the Crowsnest Formation in the Canadian province of Alberta where it is associated with agglomerates and tuffs from explosive eruptions and the Lupata Gorge in Mozambique.[34]

Blueschists[edit | edit source]

This blueschist example is from Ile de Groix, France. Credit: Arlette1.{{free media}}

Def. a "rock containing glaucophane"[39] is called a blueschist.

Metamorphic facies blanc.svg

Eclogite
Blueschist
Greenschist
Prehnite-
Pumpellyite
Zeolite
Granulite
Amphibolite
Hornfels
Sanidinite
P
(kbar)
T (°C)
0
100
200
300
400
500
600
700
800
900
1000
0
2
4
6
8
10
12
14
16
18
20
Diagram showing metamorphic facies in pressure-temperature space. The domain of the graph corresponds to circumstances within the Earth's crust and upper mantle.

A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar pressures and temperatures.[40] The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure (See diagram at right).[40] Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in geological history of the area.[40] The boundaries between facies (and corresponding areas on the temperature v. pressure graph), are wide, because they are gradational and approximate.[40] The area on the graph corresponding to rock formation at the lowest values of temperature and pressure, is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.[40]

Blueschist is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers and 200 to ~500 degrees Celsius. The blue color of the rock comes from the presence of the mineral glaucophane. Blueschists are typically found within orogenic belts as terranes of lithology in faulted contact with greenschist or rarely eclogite facies rocks. ... Blueschist, as a rock type, is defined by the presence of the minerals glaucophane + ( lawsonite or epidote ) +/- jadeite +/- albite or chlorite +/- garnet +/- muscovite in a rock of roughly basaltic composition. Blueschist often has a lepidoblastic, nematoblastic or schistose rock microstructure defined primarily by chlorite, phengitic white mica, glaucophane, and other minerals with an elongate or platy shape. Grain size is rarely coarse, as mineral growth is retarded by the swiftness of the rock's metamorphic trajectory and perhaps more importantly, the low temperatures of metamorphism and in many cases the anhydrous state of the basalts. However, coarse varieties do occur. Blueschists may appear blue, black, gray, or blue-green in outcrop.

Boninites[edit | edit source]

Boninite is "a high-magnesium andesite that is uniquely predominant in Western Pacific forearc terranes and in select Tethyan ophiolites such as Oman and Troodos."[41]

A "low-calcium, high-silica boninite [occurs] in the middle Eocene Zambales ophiolite (Luzon island, Philippines)."[41]

"Olivine-orthopyroxene microphyric high-silica boninite, olivine-clinopyroxene-phyric low-silica boninite and boninitic basalt occur as lapilli fall deposits and pillow lava flows in the upper volcanic unit of the juvenile arc section (Barlo locality, Acoje Block) of Zambales ophiolite. This upper volcanic unit in turn overlies a lower volcanic unit consisting of basaltic andesite, andesite to dacitic lavas and explosive eruptives (subaqueous pahoehoe and lobate sheet flows, agglutinate, and spatter deposits) forming a low-silica boninite series."[41]

Bostonites[edit | edit source]

A composite dyke cuts through the hard jointed blocks. Credit: Colin Smith.{{free media}}

Def. a "fine-grained, pale-colored, grey or pinkish intrusive rock, which consists essentially of alkali-feldspar"[42] 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."[43]

Breccias[edit | edit source]

Tertiary breccia is at Resting Springs Pass, Mojave Desert, California. Credit: Wilson44691.
This unusual breccia is cemented by azurite and malachite, Morenci Mine, Arizona. Credit: James St. John.

Def. a "rock composed of angular fragments in a matrix that may be of a similar or a different material"[44] is called a breccia.

Calcarenites[edit | edit source]

The Pietra di Bismantova in the northern Appennine (Emilia Romagna region, northern Italy) is an example of calcarenite formation.

Def. a "form of limestone (or dolomite) composed of sand sized grains derived from the erosion of older rocks"[45] is called a calcarenite.

Carbonatites[edit | edit source]

Carbonatite from Jacupiranga, Brazil, is a rock composed of calcite, magnetite and olivine. Credit: Eurico Zimbres.{{free media}}
Carbonatite lava is at Ol Doinyo Lengai volcano, Tanzania. Credit: Thomas Kraft, Kufstein.{{free media}}
This magnesiocarbonatite is from Verity-Paradise Carbonatite Complex of British Columbia. Specimen is 75 mm wide. Credit: James St. John.{{free media}}
Okaite is from the Oka Carbonatite Complex, Oka Niobium Mine, Oka, Quebec. Credit: James St. John.{{free media}}

Def. any "rock having a majority of carbonate minerals"[46] 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.

Charnockites[edit | edit source]

Late-stage charnockite dykes cut anorthosite, Rogaland, Norway]. Credit: Mikenorton.{{free media}}

Charnockite is any orthopyroxene-bearing quartz-feldspar rock formed at high temperature and pressure, commonly found in granulite facies metamorphic regions, sensu stricto as an endmember of the charnockite series.[47]

Rocks of the charnockite series may be named by adding orthopyroxene to the normal igneous nomenclature (e.g. orthopyroxene-granite), but specific names are in widespread use such as norite, mangerite, enderbite, jotunite, farsundite, opdalite and charnockite (in the strict sense); equivalents of gabbro, monzonite, tonalite, monzodiorite, monzogranite, granodiorite and granite.[47]

Cinerites[edit | edit source]

The bank of yellow rock lower center is cinerite. Credit: Arlette1.{{free media}}

Def. a "rock composed mostly of [...] ash"[48] is called a cinerite.

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

Clastic rocks[edit | edit source]

Thin section is of a clast (sand grain), derived from a basalt scoria. Vesicles (air bubbles) can be seen throughout the clast. Plane light above, cross-polarized light below. Scale box is 0.25 mm. Credit: Qfl247.

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock, where a clast is a fragment of geological detritus,[49] chunks and smaller grains of rock broken off other rocks by physical weathering.[50]

Claystones[edit | edit source]

Glacial Lake Missoula claystone is shown. Credit: Qfl247.
Claystone is in Slovakia. Credit: Pelex.

Def. a "rock composed of fine, clay particles"[51] is called a claystone.

Clinopyroxenites[edit | edit source]

Pyroxenite banding exists at the boundary of a peridotite intrusion in the Western Gneiss Region, Otrøy, Møre og Romsdal, Norway. Credit: Woudloper.{{free media}}

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.

Coals[edit | edit source]

Bituminous coal is a sedimentary rock. Credit: USGS.
A piece of bituminous coal is displayed. Credit: Amcyrus2012.{{free media}}
Continental United States coal regions are mapped. Credit: USGS.

Def. "a black rock formed from prehistoric plant remains, composed largely of carbon and burned as a fuel"[52] is called a coal.

Def. a black coal having a relatively high volatile content is called a bituminous coal.

Conglomerates[edit | edit source]

The boulder is of conglomerate with cobble-sized clasts. Credit: Jstuby.
Carmelo Formation (Conglomerate) is at Point Lobos. Credit: Brocken Inaglory.

Def. a "rock consisting of gravel or pebbles embedded in a matrix"[53] is called a conglomerate.

Dacites[edit | edit source]

Close view is of dacite lava from the May 1915 eruption of Lassen Peak, California. Credit: USGS.{{fairuse}}

Def. a "rock with a high iron content"[54] 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."[55]

"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."[55]

Diabases[edit | edit source]

This is an image of a rock, a diabase with an aphanitic groundmass and plagioclase phenocrysts. Credit: Siim Sepp.{{free media}}

Def. a "fine-grained [...] rock composed mostly of pyroxene and feldspar"[56] is called a diabase.

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

Diamictites[edit | edit source]

Boulder of diamictite of the Precambrian Mineral Fork Formation is lithified glacial till, along the Elephant Head Trail, Antelope Island, Utah. Credit: Jstuby.{{free media}}

Def. "nonsorted, noncalcareous terrigenous deposits composed of sand and/or larger particles dispersed through a muddy matrix"[57] are called diamictons.

Def. a lithified diamicton is called a diamictite.[57]

"Such rocks have in common a mixed, ill-sorted, disperse-megaclastic lithology with a great to extreme range of size grades."[57] The definitions of these rocks are "without regard to origin".[57]

Def. a "sedimentary, calcareous conglomerate containing a mixture of particles; mixtite"[58] is called a diamictite.

Diorites[edit | edit source]

This is a piece of diorite from Massachusetts, USA. Credit: Amcyrus2012.{{free media}}

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"[59] is called a diorite.

Dolerites[edit | edit source]

Exposed after long erosion, we now see regular, steeply inclined layers of dolerite which rest at a high angle to the smooth dip-slope of the schistose rocks. Credit: Jonathan Wilkins.{{free media}}

Def. a "fine-grained basaltic rock"[60] is called a dolerite.

Dolomites[edit | edit source]

This is a dolomite from Ben Hogan Quarry (Black Rock Quarry), Black Rock, Lawrence County Zinc District, Lawrence County, Arkansas, USA. Credit: Didier Descouens.

Def. a "saline evaporite consisting of a mixed calcium and magnesium carbonate, with the chemical formula CaMg(CO3)2;"[61] is called a dolomite.

Dolostones[edit | edit source]

Def. a "carbonate rock that contains a high percentage of the mineral dolomite"[62] is called a dolostone.

Dunites[edit | edit source]

This is a dunite rock sample from Pilbara, Australia. Credit: Ebuhyo1.{{free media}}

Def. a "type of igneous rock with a coarse-grained or phaneritic texture"[63] 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.

Eclogites[edit | edit source]

Eclogite piece from Norway with a garnet (red) and omphacite (greyish-green) groundmass. Credit: Woudloper.

Def. a "coarse-grained [...] rock, a mixture of pyroxene, quartz, and feldspar with inclusions of red garnet"[64] is called an eclogite.

The sky-blue crystals are kyanite. Minor white quartz is present, presumably from the recrystallization of coesite. A few gold-white phengite patches can be seen at the top. A 23 millimetres (0.91 in) coin added for scale.

Enderbites[edit | edit source]

Enderbite is from Vinnytsia Oblast, Ukraine. Credit: בצלאל א..{{free media}}

In geology, enderbite is an igneous rock of the charnockite series, consisting essentially of quartz, antiperthite (or perthite), orthopyroxene (usually hypersthene) and magnetite, and is equivalent to an orthopyroxene bearing tonalite.[65] It is named for its occurrence in Enderby Land, Antarctica.

Essexites[edit | edit source]

300px[Craigleith is an island in Scotland composed of essexite. Credit: Lisa Jarvis.{{free media}}

Utilising the IUGS QAPF diagram of Streckeisen (1974) "essexite" is more formally known as nepheline monzodiorite or nepheline monzogabbro depending on the ratio of orthoclase to plagioclase and the abundance of nepheline.[66]

Geologically, Craigleith is a laccolith, a dome-shape igneous intrusion, composed of essexite.

Eucrites[edit | edit source]

A 175g individual is of the Millbillillie meteorite shower, a eucrite achondrite that fell in Australia in 1960. Credit: H. Raab.{{free media}}
Stannern eucrite was found in the Czech Republic. Credit: Claas Schembor.{{free media}}
GRA98033 is a brecciated eucrite about 5 cm across, found in the Graves Nunataks region of Antarctica. Credit: Astromaterials Curation at NASA Johnson Spaceflight Center.{{free media}}

Def. an "achondritic meteoritic rock consisting chiefly of pigeonite and anorthite"[67] is called a eucrite.

Eucrites consist of basaltic rock from the crust of 4 Vesta or a similar parent body. They are mostly composed of Ca-poor pyroxene, pigeonite, and Ca-rich plagioclase (anorthite).[68]

Eurites[edit | edit source]

Def. a "compact feldspathic rock"[69] is called a eurite.

Extrusives[edit | edit source]

Extrusive igneous rock lying on the slope of Mount Etna. Credit: Ekočlen.{{free media}}

"Volcanic rocks (also called extrusive igneous rocks) include all the products resulting from eruptions of lava (flows and fragmented debris called pyroclasts)."[70]

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

Felsites[edit | edit source]

Def. a "fine grained [...] rock, generally light in color, composed of felsic minerals"[71] is called a felsite.

Foidites[edit | edit source]

The volcanic equivalents of foidolites are foidite with phonolitic or tephritic foidites.

Foidolites[edit | edit source]

Foidolite (sodalitolite variety) is from Namibia. Credit: Lysippos.{{free media}}
Thin section of foidolite under polarizing microscope shows in the foreground a large yellow sodium pyroxene grain surrounded by fine grains. Credit: Elena manul.{{free media}}

Foidolite is a rare phaneritic (coarse-grained) intrusive igneous rock in which more than 60% (by volume) of light-coloured minerals are feldspathoids.[36] Crystals of alkali feldspar, plagioclase, biotite, amphibole, pyroxene, and/or olivine may be present within the rock.

Gabbronorites[edit | edit source]

This is the Impala Black Granite - an attractive, 2 billion year old gabbronorite from South Africa. Credit: James St. John.{{free media}}

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

Gabbros[edit | edit source]

Gabbro specimen is from Rock Creek Canyon, eastern Sierra Nevada, California. Credit: Mark A. Wilson, Department of Geology, The College of Wooster.{{free media}}
Gabbro is from the NASA rock library classroom specimen collection for Mars. Credit: Unknown author.{{free media}}
Mineralogical composition of igneous rocks with decreasing silica (SiO2) content are shown. Credit: Woudloper.{{free media}}
Photomicrograph with crossed polars has a width of the view at approximately 0.5 cm, where the main minerals are plagioclase, clinopyroxene and olivine. Credit: Siim.{{free media}}

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"[72] is called a gabbro.

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

The diagram at lower right shows the rock names for volcanic/extrusive and plutonic/intrusive rocks. Notice that this is a rough way to determine a rock name: the exact difference between the given igneous rocks does not only depend on silica content.

Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene and plagioclase, or as part of a layered intrusion as a cumulate formed by settling of pyroxene and plagioclase.[73]

Gabbro may be found as a coarse-grained interior facies of certain thick lavas.[74][75]

Gneisses[edit | edit source]

This gneiss is the property of museum of geology at the University of Tartu. Credit: Siim Sepp.

Def. a "rock having bands or veins, but not schistose"[76] is called a gneiss.

Granites[edit | edit source]

This is a highly modified IUGS classification of phaneritic igneous rocks. Credit: NASA/CSU Pomona.{{fairuse}}
View is of polished granite. Credit: Dake.{{free media}}
The color of a granite usually comes from the color of the feldspar. Credit: Luis Fernández García.{{free media}}
Granite such as this contains potassium feldspar, plagioclase feldspar, quartz, biotite and/or amphibole. Credit: Friman.{{free media}}
This is a polished native red granite from China. Credit: Rola Wang.{{fairuse}}
QAPF diagram has granite field highlighted in yellow. Credit: Kent G. Budge.{{free media}}

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.

The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline (completely crystalline) rock.[77]

Granites can be predominantly white, pink, or gray in color, depending on their mineralogy.[78]

Phenocrysts are usually alkali feldspar.[79]

True granite (according to modern petrologic convention) contains between 20% and 60% quartz by volume, with 35% to 90% of the total feldspar consisting of alkali feldspar. Granitic rocks poorer in quartz are classified as syenites or monzonites, while granitic rocks dominated by plagioclase are classified as granodiorites or tonalites. Granitic rocks with over 90% alkali feldspar are classified as alkali feldspar granites. Granitic rock with more than 60% quartz, which is uncommon, is classified simply as quartz-rich granitoid or, if composed almost entirely of quartz, as quartzolite.[80][81][82]

True granites are further classified by the percentage of their total feldspar that is alkali feldspar. Granites whose feldspar is 65% to 90% alkali feldspar are syenogranites, while the feldspar in monzogranite is 35% to 65% alkali feldspar.[81][82] A granite containing both muscovite and biotite micas is called a binary or two-mica granite. Two-mica granites are typically high in potassium and low in plagioclase, and are usually S-type granites or A-type granites, as described in the Alphabet classification system.[83][84]

Another aspect of granite classification is the ratios of metals that potentially form feldspars. Most granites have a composition such that almost all their aluminum and alkali metals (sodium and potassium) are combined as feldspar. This is the case when K2O + Na2O + CaO > Al2O3 > K2O + Na2O. Such granites are described as normal or metaluminous. Granites in which there is not enough aluminum to combine with all the alkali oxides as feldspar (Al2O3 < K2O + Na2O) are described as peralkaline, and they contain unusual sodium amphiboles such as riebeckite. Granites in which there is an excess of aluminum beyond what can be taken up in feldspars (Al2O3 > CaO + K2O + Na2O) are described as peraluminous, and they contain aluminum-rich minerals such as muscovite.[79]

Granitoids[edit | edit source]

The image shows a weathered granitoid above and the resulting grus sand below. Credit: Qfl247.{{free media}}

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

Granodiorites[edit | edit source]

Here's a photo of a granodiorite. Credit: Zerohuman.{{free media}}
A sample of granodiorite is from Massif Central, France. Credit: Rudolf Pohl.{{free media}}
QAPF diagram is for classification of plutonic rocks. Credit: -xfi-.{{free media}}
Photomicrograph thin section of granodiorite from Slovakia (in crossed polarised light). Credit: Siim.{{free media}}

Def. an "intrusive igneous rock similar to granite, but containing more plagioclase than potassium feldspar"[85] is called a granodiorite.

Granophyres[edit | edit source]

Polarized light microscope image shows an intergrowth of quartz and alkali feldspar in a granophyre (Muskox intrusion), as seen in thin section (Long dimension is 1.5 mm). Credit: Omphacite~commonswiki.{{free media}}

Def. a "subvolcanic rock with angular intergrowths of quartz and alkali feldspar"[86] is called a granophyre.

The texture can be similar to micrographic texture and to the coarser graphic intergrowths of quartz and alkali feldspar common in pegmatite to document simultaneous crystallization of quartz and feldspar from a silicate melt at the eutectic point, perhaps in the presence of a water-rich phase, that may also be formed by crystallization when the magma is significantly undercooled, not necessarily under eutectic conditions.[87]

Granophyres typically are intrusive rocks that crystallized at shallow depths, and many have compositions similar to those of granites.[88]

Granulites[edit | edit source]

This is a granulite from Slovakia. Credit: Helix84.

Def. "fine-grained [...] rock composed chiefly of feldspar, quartz, and garnets"[89] is called a granulite.

Greenlandites[edit | edit source]

Eoarchaean (3.8 b.y.) Greenlandite specimen (fuchsite-quartz gneiss) is from Nuup Kangerlua, Greenland. Credit: James St. John.{{free media}}

East of Nuuk, Greenland, the banded iron ore region of Isukasia, over three billion years old, contains the world's oldest rocks, such as greenlandite (a rock composed predominantly of hornblende and hyperthene), formed 3.8 billion years ago,[90] and nuummite.

Greensands[edit | edit source]

A roadcut within the Llano Uplift on Texas Highway 1431 about 18 km northwest Marble Falls, Texas, exposes greensand of the Lion Mountain Sandstone (Cambrian) in the lower unit. Notice the normal fault cutting through the formation. Credit: Erimus.{{free media}}
Occurrence of glauconitic siltstone is in the Serra da Saudade ridge, in the Alto Paranaíba region, Minas Gerais, Brazil. Credit: Verde Fertilizantes.{{free media}}
Greensand (glauconitic sandstone). Credit: Mai Seppel.{{free media}}

Def. a "greenish sandstone containing glauconite"[91] is called a greensand.

Greensand or green sand is a sand or sandstone which has a greenish color, specifically applied to shallow marine sediment that contains noticeable quantities of rounded greenish grains called glauconies and consist of a mixture of mixed-layer clay minerals, such as smectite and glauconite mica also loosely applied to any glauconitic sediment.[92][93][94]

Greensand forms in anoxic marine environments that are rich in organic detritus and low in sedimentary input.[92] Having accumulated in marine environments, greensands can be fossil-rich, such as in the late-Cretaceous deposits of New Jersey.[95]

Greisens[edit | edit source]

Def. a "highly altered granitic rock containing quartz and mica"[96] is called a greisen.

Greywackes[edit | edit source]

This is a sample of greywacke. Credit: www.mineraly.sk.

Def. a "hard dark sandstone with poorly sorted angular grains of quartz, feldspar, and small rock fragments in a compact, clay-fine matrix"[97] is called a greywacke.

Gritstones[edit | edit source]

The Salt Cellar, a gritstone tor on Derwent Edge in the Peak District, England. Credit: Mick Knapton.

Def. a "form of sedimentary rock, similar to sandstone but coarser"[98] is called a gritstone.

Harzburgites[edit | edit source]

This is an IUGS igneous rock classification diagram for ultramafic rocks. Credit: Richard Harwood.{{fairuse}}
This harzburgite is from a small intrusion 35 km south of Ambositra, Madagascar. Credit: Diorit.{{free media}}

Def. an "ultramafic igneous rock, a variety of peridotite consisting mostly of olivine and low-calcium pyroxene"[99] is called a harzburgite.

Harzburgite is "an olivine-rich, olivine-orthopyroxene [ultramafic] rock"[100]

It most "commonly contains chromian Al-spinel as a minor or accessory, although garnet may also occur."[100]

Hawaiites[edit | edit source]

Geological sample is on display at the House of the Volcano, Reunion Island. Credit: David Monniaux.

Def. an "olivine basalt intermediate between alkali olivine and mugearite"[101] is called a hawaiite.

Hornblendites[edit | edit source]

This is a hornblendite. Credit: Khruner.{{free media}}
Hornblendite includes apatite and pyroxenes from Bystrzyca Górna in the Owl Mountains, Lower Silesia, Poland. Credit: Piotr Sosnowski{{free media}}

Def. a "plutonic, ultramafic igneous rock composed mostly of hornblende"[102] is called a hornblendite.

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

The hornblendite on the left is the oldest igneous rock from Poland, dated at 1.2 billion years.

Hornfels[edit | edit source]

This is a sample of banded hornfels from Borok quarry in Novosibirsk. Credit: Fed.

The hornfels shown on the right were formed from the heating of sandstones and siltstones by the Insskoy series of granite intrusions.

Hyaloclastites[edit | edit source]

This is a hyaloclastite sample on display at the House of the Volcano, Reunion Island. Credit: David Monniaux.{{free media}}
Hyaloclastite occurs between pillows of lava in Montana. Credit: USGS.{{free media}}

Def. a "rock containing glassy fragments"[103] is called a hyaloclastite.

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

Hyaloclastite ridges, formed by subglacial eruptions during the last glacial period, are a prominent landscape feature of Iceland and the Canadian province of British Columbia. Hyaloclastite is usually found at subglacial volcanoes, such as tuyas, which is a type of distinctive, flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet.

In lava deltas, hyaloclastites form the main constituent of foresets formed ahead of the expanding delta. The foresets fill in the seabed topography, eventually building up to sea level, allowing the subaerial flow to move forwards until it reaches the sea again.[104]

Icelandites[edit | edit source]

Icelandite is a type of volcanic rock, an iron rich, aluminium lacking andesite.[105] Icelandites are between rhyodacite and tholeiitic basalt in composition and contain andesine, hypersthene and augite, with a silica (SiO2) content greater than 60%.[106]

Igneous[edit | edit source]

This rock shows a common facies of the Piégut-Pluviers granodiorite, northwestern Massif Central, France. Credit: Rudolf Pohl.{{free media}}
Samples of various igneous rocks from the Arequipa region of Peru are shown. Credit: Rojinegro81.{{free media}}

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)"[107] is called an igneous rock.

"An igneous rock is formed by the cooling and crystallization of molten rock."[70]

Igneous "rocks [are divided] into two broad categories based on where the molten rock solidified."[70]

The image on the left 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).

Ignimbrites[edit | edit source]

Rocks from the Bishop tuff are from California, United States, uncompressed with pumice on left; compressed with fiamme on right. Credit: .{{free media}}
The caprock in this photo is the ignimbrite layer of the Rattlesnake Formation in Oregon. Credit: .{{free media}}

Def. a "deposit left by the pyroclastic flow from a volcano, consisting of ash, pumice lapilli, and lithic fragments"[108] is called an ignimbrite.

Ignimbrite is a variety of hardened tuff.[36] Ignimbrites are igneous rocks made up of crystal and rock fragments in a glass-shard groundmass, albeit the original texture of the groundmass might be obliterated due to high degrees of welding. The term ignimbrite is not recommended by the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks.[36]

Ijolites[edit | edit source]

This is an ijolite sample. Credit: NASA.{{fairuse}}
Ijolite (Oka Carbonatite Complex, Early Cretaceous, 124-125 Ma; is from the Oka Niobium Mine, Quebec, Canada). Credit: James St. John.{{free media}}

Def. a "rare igneous rock consisting essentially of nepheline and augite"[109] is called an ijolite.

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

Ijolite is an igneous rock consisting essentially of nepheline and augite.[110] Ijolite is a rare rock type of considerable importance from a mineralogical and petrological standpoint. Ijolite occurs in various parts of the Kainuu region of eastern Finland and in the Kola Peninsula of northwest Russia on the shores of the White Sea, first defined and named by Finnish geologist Wilhelm Ramsay.[111]

The Oka Carbonatite Complex is located in Quebec, Canada. It’s a large body of alkaline igneous rocks intruded through Precambrian metamorphics. The Oka occurs in the western part of the Canadian Shield’s Monteregian Hills Province. Published research indicates that Oka rocks cooled from magma produced by partial melting of upper mantle rocks (inferred to be metasomatized garnet lherzolites). The rocks in the complex contain some rare elements, including economic concentrations of niobium (Nb). Several mines exploit Oka rocks for their Nb content. Oka rocks include coarsely-crystalline calciocarbonatites (a.k.a. sövites; a.k.a. C1 calciocarbonatites), alnoites, ijolites, and okaites. Age: mid-Barremian Stage, mid-Early Cretaceous, 124-125 million years.

Intrusives[edit | edit source]

The image shows hardened intrusive igneous rocks. Credit: Adityamadhav83.{{free media}}
QAPF diagram is for the classification of plutonic rocks. Credit: Antonio Ciccolella.{{free media}}
Devils Tower is an igneous intrusion exposed when the surrounding softer rock eroded away. Credit: A7x.{{free media}}

"Plutonic rocks (also called intrusive igneous rocks) are those that have solidified below ground; plutonic comes from Pluto, the Greek god of the underworld."[70]

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

Jets[edit | edit source]

This is a piece of jet. Credit: Ewa Jastrzębska.{{free media}}
Sample is unworked jet, about 15 mm long. Credit: Geni.{{free media}}

Def. a "hard, black form of coal"[112] is called jet.

Jet is a type of lignite,[113] the lowest rank of coal, and is a gemstone, but, unlike many gemstones, jet is a mineraloid.[114]

Kimberlites[edit | edit source]

Picture is of a diamond-bearing kimberlite rock, from a mine somewhere in the US. Credit: Woudloper.{{free media}}

Def. a "variety of peridotite containing a high proportion of carbon dioxide; often contains diamonds"[115] is called is called a kimberlite.

Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes, as well as igneous dykes and horizontal sills.[116]

Komatiites[edit | edit source]

Komatiite lava is at the type locality in the Komati Valley, Barberton Mountainland, South Africa. Credit: .{{free media}}

Komatiite lava at the type locality in the Komati Valley, Barberton Mountainland, South Africa, shows the distinctive "spinifex texture" formed by dendritic plates of olivine (scale shown by a hammer on the right edge of the photo on the right).

Def. any "class of ultramafic mantle-derived volcanic rocks that have a high magnesium content"[117] is called a komatiite.

Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO.[118] Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa,[119] and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.[120]

Komatiites are rare rocks; almost all komatiites were formed during the Archaean Eon (4.0–2.5 billion years ago), with few younger (Proterozoic or Phanerozoic) examples known. This restriction in age is thought to be due to cooling of the mantle, which may have been 100–250 °C (212–482 °F) hotter during the Archaean.[121][122]

Lamproites[edit | edit source]

This is a photograph of a sample of Lamproite with ruler for scale. Credit: JPL/NASA.{{free media}}

Def. any "of several volcanic rocks having a high potassium content"[123] is called a lamproite.

The lamproites at Gaussberg in Antarctica, and lead-lead isotope geochemistry have revealed that the source of lamproites may be transition zone melts of subducted lithosphere which has become trapped at the base of the lithospheric mantle.[124]

Lamprophyres[edit | edit source]

This lamprophyre has an ophitic fabric. Credit: Lysippos.{{free media}}
Microscope view (long dimension 2 mm) is a thin section of minette from the Colorado Plateau. Credit: Omphacite~commonswiki.{{free media}}

Def. an "uncommon, small-volume ultrapotassic igneous rock primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions"[125] is called a lamprophyre.

In the thin section, magnesium-rich biotite (P, phlogopite) and clinopyroxene (C) phenocrysts occur in a groundmass of alkali feldspar, pyroxene, and iron-titanium oxides.

Latites[edit | edit source]

International Union of Geological Sciences (IUGS) classification of volcanic rocks is diagrammed. Credit: NASA/CSU Pomona.{{fairuse}}
Latite is from Boxberg, High-Eifel, Germany. Credit: Ra'ike.{{free media}}
Photomicrograph is a thin section of latite (in plane polarised light). Credit: Chiara Groppo.{{free media}}

]

Photomicrograph is a thin section of latite (in cross polarised light). Credit: Chiara Groppo.{{free media}}

"Igneous rocks are classified on the basis of mineralogy, chemistry, and texture."[126]

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

Def. "an igneous, volcanic (extrusive) rock, with aphanitic to porphyritic texture"[127] is called a latite.

Quartz is less than five percent and is absent in a feldspathoid-bearing latite, and olivine is absent in a quartz-bearing latite, but when quartz content is greater than five percent the rock is classified as quartz latite.[128]

Biotite, hornblende, pyroxene and scarce olivine or quartz are common accessory minerals, where feldspathoid-bearing latite is sometimes referred to as tristanite.[129]

Lherzolites[edit | edit source]

This lherzolite is from Dreiser Weiher, Eifel, Germany. Credit: Woudloper.{{free media}}
This lherzolite is from the Estonian Museum of Natural History. Credit: Tõnis Saadre.{{free media}}
Lherzolite is at Etang de Lers, Ariège, France. Credit: .{{free media}}
Garnet lherzolite is a xenolith from a kimberlite pipe, Kimberley, South Africa, field of view ~1.6 cm across: purplish red = pyrope garnet, bright green = chromian diopside, dark greenish-black = orthopyroxene, olive-green= olivine. Credit: Asabengurtza.{{free media}}

Def. a "dark-green igneous rock consisting largely of chrysolite, with pyroxene and picotite"[130] is called a lherzolite.

The Moon’s lower mantle may be composed of lherzolite.[131]

Lias[edit | edit source]

System Series Stage Age (Ma) European lithostratigraphy
Jurassic Lower Hettangian younger Lias
Triassic Upper Rhaetian 199.6–203.6
Keuper
Norian 203.6–216.5
Carnian 216.5–228.7
Middle Ladinian 228.7–237.0
Muschelkalk
Anisian 237.0–245.9
Bunter or Buntsandstein
Lower Olenekian 245.9–249.5
Induan 249.5–251.0
Permian Lopingian Changhsingian older
Zechstein
Major lithostratigraphic units of northwest Europe with the International Commission on Stratigraphy (ICS)'s geologic timescale of the Triassic.[132]
Lower Lias sequence is exposed at Nash Point, Glamorgan, Wales. Credit: Wilson44691.

Def. a "stratigraphic group from the lower Jurassic period, consisting of thin layers of blue limestone [present in parts of southern England]"[133] is called a lias.

Lignites[edit | edit source]

This is a sample of lignite. Credit: Saupreiß.
Lignite seams are interlayered with calcareous mud strata. Credit: Nadirrias.

Def. a "low-grade, brownish-black coal"[134] is called a lignite.

Lignite, often referred to as brown coal,[135] is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat has a carbon content around 25–35%,[135][136] and is considered the lowest rank of coal due to its relatively low heat content. Lignite is mined all around the world and is used almost exclusively as a fuel for steam-electric power generation.

The combustion of lignite produces less heat for the amount of carbon dioxide and sulfur released than other ranks of coal. As a result, environmental advocates have characterized lignite as the most harmful coal to human health.[137]

Lignite is brownish-black in color and has a carbon content of 60-70 percent on a dry ash-free basis. However, its inherent moisture content is sometimes as high as 75 percent[135] and its ash content ranges from 6–19 percent, compared with 6–12 percent for bituminous coal.[138] As a result, its carbon content on the as-received basis (i.e., containing both inherent moisture and mineral matter) is typically just 25-35 percent.[136]

The energy content of lignite ranges from 10 to 20 MJ/kg (9–17 million British thermal unit (BTU) per short ton) on a moist, mineral-matter-free basis. The energy content of lignite consumed in the United States averages 15 MJ/kg (13 million BTU/ton), on the as-received basis.[139] The energy content of lignite consumed in Victoria, Australia, averages 8.6 MJ/kg (8.2 million BTU/ton) on a net wet basis.[140]

Lignite can be separated into two types. The first is xyloid lignite or fossil wood and the second form is the compact lignite or perfect lignite.

Although xyloid lignite may sometimes have the tenacity and the appearance of ordinary wood, it can be seen that the combustible woody tissue has experienced a great modification. It is reducible to a fine powder by trituration, and if submitted to the action of a weak solution of potash, it yields a considerable quantity of humic acid.[141] Leonardite is an oxidized form of lignite, which also contains high levels of humic acid.[142]

Jet is a hardened, gem-like form of lignite used in various types of jewelry.[113]

Limestones[edit | edit source]

Layers of alpine limestone are dated to the Triassic. Credit: Gikü.{{free media}}
Limestone outcrop is in the Torcal de Antequera nature reserve of Málaga, Spain. Credit: Fernando Domínguez Cerejido.{{free media}}
This limestone deposit in the karst of Dinaric Alps near Sinj, Croatia was formed in the Eocene. Credit: Ponor.{{free media}}

Def. a rock "primarily composed of calcite (CaCO); it occurs in a variety of forms, both crystalline and amorphous"[143] is called a limestone.

About 20% to 25% of sedimentary rock is carbonate rock, and most of this is limestone.[144][145]

Magnesian limestone is an obsolete and poorly-defined term used variously for dolomite, for limestone containing significant dolomite (dolomitic limestone), or for any other limestone containing a significant percentage of magnesium.[146]

Much dolomite is secondary dolomite, formed by chemical alteration of limestone.[147][148]

Limonites[edit | edit source]

Limonite is an amorphous mineraloid of a range of hydrated iron oxides. Credit: USGS.

Limonite is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·nH2O, although this is not entirely accurate as the ratio of oxide to hydroxide can vary quite widely. Limonite is one of the two principle iron ores, the other being hematite, and has been mined for the production of iron since at least 2500 BCE.[149][150] Although originally defined as a single mineral, limonite is now recognized as a mixture of related hydrated iron oxide minerals, among them goethite, akaganeite, lepidocrocite, and jarosite. Individual minerals in limonite may form crystals, but limonite does not, although specimens may show a fibrous or microcrystalline structure,[151] and limonite often occurs in concretionary forms or in compact and earthy masses; sometimes mammillary, botryoidal, reniform or stalactitic. Because of its amorphous nature, and occurrence in hydrated areas limonite often presents as a clay or mudstone. However there are limonite pseudomorphs after other minerals such as pyrite.[152] This means that chemical weathering transforms the crystals of pyrite into limonite by hydrating the molecules, but the external shape of the pyrite crystal remains. Limonite pseudomorphs have also been formed from other iron oxides, hematite and magnetite; from the carbonate siderite and from iron rich silicates such as almandine garnets. Limonite usually forms from the hydration of hematite and magnetite, from the oxidation and hydration of iron rich sulfide minerals, and chemical weathering of other iron rich minerals such as olivine, pyroxene, amphibole, and biotite. It is often the major iron component in lateritic soils. One of the first uses was as a pigment. The yellow form produced yellow ochre for which Cyprus was famous.[153]

Lithification[edit | edit source]

Differentially cemented & eroded sandstone occur in the Eocene of Fantasy Canyon, Utah, USA. Credit: James St. John.

Def. the "compaction and cementation of sediment into rock"[154] is called lithification.

Def. a "subdivision of any stratigraphic unit that has characteristic lithologic features"[155] is called a lithofacies.

Def. the "formation of sedimentary rock"[156] is called lithogenesis.

Def. "an element that forms silicates or oxides and is concentrated in the minerals of the Earth's crust"[157] is called a lithophile.

"The rocks are quartzose sandstones that were deposited on the eastern shore of ancient Lake Uinta, which existed during the Eocene. Some wisps and ribbons of dark-colored, magnetite-rich sand are present in the sandstone. The variety of chaotic rockforms at Fantasy Canyon are quite diverse - these cannot be explained by ordinary weathering and erosion. Close examination shows that erosion has acted upon differentially cemented sandstone. The sandstone has not undergone complete lithification and diagenesis - groundwater lobes have preferentially cemented portions of the sandstone, especially immediately adjacent to joint planes. The poorly-cemented sandstone was easily eroded & the better-cemented sandstone remains."[158]

"Fantasy Canyon [is located] between Red Wash & Coyote Wash, Chapita Wells Gas Field, west-northwest of the town of Bonanza & south-southeast of the town of Vernal & east of the town of Ouray, northeastern Utah, USA."[158]

Lithosomes[edit | edit source]

Def. a "uniform mass of rock that has different lithography to that of the surrounding rock"[159] is called a lithosome.

Luxullianites[edit | edit source]

Luxullianite from Cornwall, shows dark patches of tourmaline and pink crystals of orthoclase. Credit: kevinzim / Kevin Walsh.{{free media}}
Thin section of luxullianite from Cornwall, shows clusters of radially-arranged, acicular, greenish tourmaline crystals. Credit: Strekeisen.

Luxullianite (also Luxulyanite, Luxulianite) is a rare type of porphyritic granite, notable for the presence of clusters of radially-arranged acicular tourmaline crystals enclosed by phenocrysts of orthoclase and quartz in a matrix of quartz, tourmaline, alkali feldspar, brown mica, and cassiterite.[160]

Mafics[edit | edit source]

Def. "rocks, which contain relatively high concentrations of magnesium and iron"[161] are called mafics.

Marbles[edit | edit source]

This is a block of white marble. Credit: USGS.
Mississippian marble is in Big Cottonwood Canyon, Wasatch Mountains, Utah. Credit: commons:User:Wilson44691:Mark A. Wilson.

On the right is a block of white marble.

The left shows Mississippian marble in Big Cottonwood Canyon, Wasatch Mountains, Utah.

Maw sit sit[edit | edit source]

Maw sit sit is a very rare, complex, polymineralic metamorphic rock. Credit: James St. John.{{free media}}

Maw sit sit, also known as jade-albite, is a gemstone found exclusively in northern Burma, first identified in 1963 by the late Swiss gemologist, Edward Gubelin, and was named after the village close to where it was first found in the foothills of the Himalayas.[162]

Typically maw sit sit is green with distinctive dark-green to black veins, is technically a rock rather than a mineral, composed of several different minerals, including kosmochlor (also known as ureyite), varying amounts of chromium-enriched jadeite, and albite feldspar.[163]

Maw sit sit can have a refractive index ranging from 1.52-1.68,[164][165] and a hardness rating of 6 to 7 on the Mohs scale.

Metamorphics[edit | edit source]

Def. "one of the major groups of rock that makes up the crust of the Earth; consists of pre-existing rock mass in which new minerals or textures are formed at higher temperatures and greater pressures than those present on the Earth's surface"[166] is called a metamorphic rock.

Monzogranites[edit | edit source]

Core sample is of Rochovce granite, coarse-grained biotite monzogranite (75.6 ± 1.1 Ma - Cretacous). Credit: Pelex.

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

Monzonites[edit | edit source]

This is a monzonite sample. Credit: Khruner.{{free media}}

Def. an "intrusive igneous rock composed mostly of plagioclase and orthoclase"[167] is called a monzonite.

Mudstones[edit | edit source]

Mudstone formation is on Lyme Regis East Beach. Credit: Ballista.
Sample is of mudstone. Credit: Manishwiki15.
Red mudrock is in the Ragged Reef Formation (Pennsylvanian), Cumberland Basin, Nova Scotia. Credit: Michael C. Rygel.

Def. a "fine-grained sedimentary rock whose original constituents were clays or muds"[168] is called a mudstone.

Mugearites[edit | edit source]

Mugearite lava flow (on right); flow on left is hawaiite; at North Berwick, Scotland. Credit: Anne Burgess.{{free media}}

Def. a "kind of orthoclase-bearing basalt that is made up of olivine, apatite, and opaque oxides"[169] is called is called a mugearite.

The main feldspar in mugearite is oligoclase.[170]

In the TAS classification of volcanic rock, mugearite is classified as sodium-rich basaltic trachyandesite.[171]

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.[172][173][174][175]

Napoleonites[edit | edit source]

A fragment of napoleonite shows its orbicular structure. Credit: Emmanuel Douzery.{{free media}}

Though mostly rounded, the spheroids may be elliptical or subangular; sometimes they are in contact with one another but usually they are separated by small areas of massive diorite.[176]

Nephelinites[edit | edit source]

A nephelinite lava flow is in Kaiserstuhl, SW Germany. Credit: Derhammer.{{free media}}

Def. "a dark, finely crystalline rock of volcanic origin, being a mixture of nepheline and pyroxene"[177] is called a nephelinite.

Norites[edit | edit source]

Sulfidic norite (field of view ~4.5 cm across) is from the Johns-Manville Reef, Lower Banded Series, Stillwater Complex. Credit: James St. John.{{free media}}

Def. a "granular crystalline rock consisting essentially of a triclinic feldspar (such as labradorite) and hypersthene"[178] is called a norite.

Obsidian[edit | edit source]

A specimen of obsidian is from Lake County, Oregon. Credit: Locutus Borg.{{free media}}

Def. "a type of black glass produced by volcanoes"[179] is called an obsidian.

Oolites[edit | edit source]

Ooids occur on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern Utah. Credit: .
Thin-section is of calcitic ooids from an oolite within the Carmel Formation (Middle Jurassic) of southern Utah. Credit: .

Def. a "rock consisting of spherical grains within a mineral cortex accreted around a nucleus, often of quartz grains"[180] is called an oolite.

Ophites[edit | edit source]

Ophite column in the Church of Saints Sergius and Bacchus (now the Küçuk Ayasofya Camii) is in Istanbul, Turkey (527-536 CE). Credit: Valerius Tygart.{{free media}}

Ophite is a serpentinite breccia[181] popular since ancient times as a decorative facing stone. It is a dark, dull green, white-mottled (or white-veined) serpentine, mixed with calcite, dolomite, or magnesite, which takes a high polish.[182]

Orthopyroxenites[edit | edit source]

ALH 84001 is an orthopyroxenite achondrite meteorite from Mars. Credit: Jstuby.{{free media}}

Def. "an ultramafic and ultrabasic rock that is almost exclusively made from the mineral orthopyroxene"[183] is called an orthopyroxenite.

Pegmatites[edit | edit source]

Pegmatite has blue corundum crystals. Credit: Zimbres.{{free media}}
This pegmatite contains lepidolite, tourmaline, and quartz from the White Elephant Mine in the Black Hills, South Dakota. Credit: Jstuby.{{free media}}
Proterozoic pegmatite swarm is in the headwall of the cirque of a small mountain glacier, northeastern Baffin Island, Nunavut. Credit: Mike Beauregard from Nunavut, Canada.{{free media}}

A pegmatite is an igneous rock, formed by slow crystallization at high temperature and pressure at depth, and exhibiting large interlocking crystals usually greater in size than 25 mm (0.98 in), an intrusive rock found in sheets of rock (dikes and veins) near large masses of igneous rocks called batholiths.[184]

Pegmatite means "to bind together", in reference to the intertwined crystals of quartz and feldspar in the texture known as graphic granite.[185]

Crystal size is the most striking feature of pegmatites, with crystals usually over 50 mm (2.0 in) in size, individual crystals over 10 m (33 ft) long have been found, and many of the world's largest crystals were found within pegmatites, including spodumene, microcline, beryl, and tourmaline.[186] The largest crystal ever found is a beryl crystal from Malakialina on Madagascar, weighing about 380 tons, with a length of 18 m (59 ft) and a crosscut of 3.5 m (11 ft).[187]

Pelites[edit | edit source]

Def. a "sedimentary rock containing very fine particles"[188] is called a pelites.

Peridotites[edit | edit source]

Peridotite specimen is displayed. Credit: USGS.{{free media}}

Def. a "rock consisting of small crystals of olivine, pyroxene and hornblende"[189] is called a peridotite.

Phanerites[edit | edit source]

Close-up is of granite, a phanerite rock, from Yosemite National Park in California, U.S. Credit: David Monniaux.{{free media}}
Close-up is of phaneritic granite exposed in Chennai, India. Credit: Mark A. Wilson, Department of Geology, The College of Wooster.{{free media}}

A phanerite[190] is an igneous rock whose microstructure is made up of crystals large enough to be distinguished with the unaided human eye.

Phonolites[edit | edit source]

Rock name is tinguaite (variety of phonolite) and it is from Sweden. Credit: Siim Sepp.{{free media}}

Def. "a light-coloured rock of volcanic origin composed mostly of alkali feldspars"[191] is called a phonolite.

The dark needle-like minerals are aegirine phenocrysts.

Phosphorites[edit | edit source]

Peloidal phosphorite is from the Phosphoria Formation, Simplot Mine, Idaho, specimen 4.6 cm wide. Credit: James St. John.
Fossiliferous peloidal phosphorite specimen is 4.7 cm across, from Yunnan Province, China. Credit: James St. John.

Def. "a sedimentary rock rich in phosphate minerals such as apatite"[192] is called a phosphorite.

Phyllites[edit | edit source]

This is a sample of phyllite, a metamorphic rock. Credit: USGS.

A sample of a phyllite is on the right.

Picrites[edit | edit source]

This is an IUGS Igneous Rock Classification Diagram using a Le Maitre plot. Credit: Richard Harwood.{{fairuse}}

Def. a "variety of high-magnesium olivine basalt"[193] is called a picrite.

Porphyry[edit | edit source]

Porphyritic texture is shown in a granite; eastern Sierra Nevada, Rock Creek Canyon, California. Credit: Wilson44691.{{free media}}
A porphyritic volcanic sand grain is seen under the petrographic microscope. Scale box in millimeters. Credit: Qfl247. {{free media}}
Andesite porphyry is from the summit of O'Leary Peak. Credit: Jstuby.{{free media}}

On the right is an intrusive porphyritic granite. The white, square feldspar phenocrysts are much larger than crystals in the surrounding matrix.

On the left, the large grain in the middle is of a much different size class than the small needle-like crystals around it.

Second down on the right is an extrusive rock porphyritic rock, as the pink (and black) phenocrysts are clearly visible, in contrast to the grey groundmass with its microscopic crystals.

For igneous rocks, a porphyry has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group.[194] Porphyritic rocks may be aphanites or extrusives, with large crystals or phenocrysts floating in a fine-grained groundmass of non-visible crystals, as in a porphyritic basalt, or phanerites or intrusive rock, with individual crystals of the groundmass easily distinguished with the eye, but one group of crystals clearly much bigger than the rest, as in a porphyritic granite.

Pumices[edit | edit source]

This piece of pumice is from the Teide volcano. Credit: MPF.{{fairuse}}

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"[195] is called a pumice.

Pyroxenites[edit | edit source]

This is an IUGS Igneous Rock Classification Diagram for Ultramafic Rocks with Hornblende. Credit: Richard Harwood.{{fairuse}}

Def. a "heavy, dark igneous rock consisting mostly of pyroxene minerals with smaller amounts of olivine and hornblende"[196] is called a pyroxenite.

Quartzites[edit | edit source]

This quartzite shows banding. Credit: Siim Sepp.

Def. "a [...] rock consisting of interlocking grains of quartz"[197] is called a quartzite.

In a quartzite, fractures occur through the quartz grains. In a sedimentary rock composed of quartz grains, the rock fractures around the quartz grains.

Radiolarites[edit | edit source]

Radiolarian chert outcrop is near Cambria, California, where individual beds range from about 2 to 5 cm thick Credit: Peter D. Tillman.

Def. "the sedimentary rock formed from" "radiolarian ooze"[198] is called a radiolarite.

Rhyodacites[edit | edit source]

This sample of rhyodacite is from Slovakia. Credit: Helix84.{{free media}}
QAPF diagram has the approximate location of rhyodacite highlighted]. Credit: Kent G. Budge.{{free media}}
TAS diagram has the approximate location of rhyodacite highlighted. Credit: Kent G. Budge.{{free media}}

Rhyodacite then describes a fine-grained igneous rock containing between 20% and 60% quartz and in which plagioclase makes up about two-thirds of the total feldspar content.[199][82] Such a rock will contain between 69% and 72% silica by weight.[200]

The U.S. Geological Survey defines rhyodacite as volcanic rock containing 20% to 60% quartz and with plagioclase making up 35% to 65% of the total feldspar content, making rhyodacite the extrusive equivalent of monzogranite.[201]

In addition to its quartz and feldspar content, rhyodacite commonly contains phenocrysts of andesine, biotite, hornblende, and pyroxene, where quartz and sanidine phenocrysts are less common than in rhyolite, and rhyolite has only scant mafic phenocrysts.[202]

Like other silica-rich compositions, rhyodacites are more abundant as pyroclastic rock than as lava flows.[203] These include extensive ash flows and fallout sheets that are important tephrochronology (stratigraphic markers).[202]

Rhyolites[edit | edit source]

A rhyolite boulder near Carn Alw shows the characteristic pattern of swirling or parallel layers called flow banding caused by the molten magma meeting a hard surface before cooling and setting. Credit: ceridwen.{{free media}}
Flow banding is in rhyolite lava from Mono-Inyo Craters volcanic chain, California (black bands composed of obsidian). Credit: USGS.{{free media}}

Def. a rock "of felsic composition, with aphanitic to porphyritic texture"[204] 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."[205]

"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."[205]

"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)."[205]

"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."[205]

Sandstones[edit | edit source]

Bunter Sandstone (detail), Hopstone, Shropshire, has layers of coloured pebbles often found in this Triassic rock, where this image shows about one metre (height) of sandstone. Credit: Roger Kidd.
Outcrop of Kangaroo Creek Sandstone is in Clarence Moreton Basin, New South Wales. Credit: Yendor of yinn.

Def. a "sedimentary rock produced by the consolidation and compaction of sand, cemented with clay etc"[206] is called a sandstone.

Sandstone classification by the Dott scheme uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains.[207]

Saprolites[edit | edit source]

Saprolite of weathered quartzite is on Aranmore. Credit: Geologician.{{fairuse}}

Def. a chemically weathered rock is called a saprolite.

In the image on the right, is a transition from tectonized quartzite through saprolite to laterite. The weathered section is covered by glacial drift with scattered erratics, Holocene sandy soil and thin bog.

Schists[edit | edit source]

This is a detail of schist, a foliated metamorphic rock. Credit: Michael C. Rygel.{{free media}}

At right is an image of schist.

Schist is a medium-grained metamorphic rock that shows well-developed schistosity.[208] Schistosity is a thin layering of the rock produced by metamorphism (a foliation) that permits the rock to easily be split into flakes or slabs less than 5 to 10 millimeters (0.2 to 0.4 in) thick.[209][210] The mineral grains in a schist are typically from 0.25 to 2 millimeters (0.01 to 0.08 in) in size[210] and so are easily seen with a 10× hand lens.[211] Typically, over half the mineral grains in a schist show a preferred orientation. Schists make up one of the three divisions of metamorphic rock by texture, with the other two divisions being gneiss, which has poorly developed schistosity and thicker layering, and granofels, which has no discernible schistosity.[209][210]

Scorias[edit | edit source]

This is a piece of scoria from the Golan Heights. Credit: Daniel Ventura.{{free media}}

Def. rough "masses of rock formed by solidified lava, and which can be found around a volcano's crater"[212] is called a scoria.

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

Sedimentary[edit | edit source]

This image shows the sedimentary rock layers at Zabriskie Point in Death Valley, USA. Credit: Brigitte Werner (werner22brigitte).

Def. "one of the major groups of rock that makes up the crust of the Earth; formed by the deposition of either the weathered remains of other rocks, the results of biological activity, or precipitation from solution"[213] is called a sedimentary rock.

Shales[edit | edit source]

The outcrop's striking layers, some at angles to each other, is a pattern called crossbedding. Credit: NASA/JPL-Caltech/MSSS.
Marcellus shale shown along Rt 174 just south of Slate Hill Rd, Marcellus, NY. Credit: Lvklock.
Black Shale occurs with pyrite. Credit: Rob Lavinsky.

Def. a "fine-grained [...] rock of a thin, laminated, and often friable, structure"[214] is called a shale.

Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite.[79] Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility.[79] Shale is the most common sedimentary rock.[215]

The term shale is sometimes applied more broadly, as essentially a synonym for mudrock, rather than in the more narrow sense of clay-rich fissile mudrock.[216]

Shergottites[edit | edit source]

Martian meteorite EETA79001 is a shergottite. Credit: NASA.{{free media}}
NWA 6963,[217] a shergottite, was found in Morocco, September 2011. Credit: Steve Jurvetson.{{free media}}

Roughly three-quarters of all Martian meteorites can be classified as shergottites, named after the Shergotty meteorite, which fell at Sherghati, India in 1865.[218] Shergottites are igneous rocks of mafic to ultramafic lithology that fall into three main groups, the basaltic, olivine-phyric (such as the Tissint group found in Morocco in 2011[219][220]) and lherzolite, (lherzolitic) shergottites, based on their crystal size and mineral content, alternatively categorized into three or four groups based on their rare-earth element content.[221]

Shoshonites[edit | edit source]

Shoshonite is a type of igneous rock, more specifically, it is a potassium-rich variety of basaltic trachyandesite,[36] composed of olivine, augite and plagioclase phenocrysts in a groundmass with calcic plagioclase and sanidine and some dark-colored volcanic glass. Shoshonite gives its name to the shoshonite series and grades into absarokite with the loss of plagioclase phenocrysts and into banakite with an increase in sanidine.[222]

Silexites[edit | edit source]

This is an IUGS Igneous Rock Classification Diagram for plutonic rocks. Credit: Richard Harwood and IUGS.{{fairuse}}
These are large pieces of silexite at the motorway service station, La Lozère. Credit: Clem Rutter, Rochester, Kent, England.{{free media}}

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.

Siltstones[edit | edit source]

A well-developed veined network, a fossilised soil structure, extends down from the top of a greyish red siltstone unit, and is underlain by a zone of calcareous nodules. Credit: P. J. Barrett, B. P. Kohn, R. A. Askin & J. G. McPherson.
Siltstone is at UAT, Estonia. Credit: Siim Sepp.

At the upper right is a small portion of the stratigraphic column between the Hatherton and MacKay glaciers in Antarctica. The top rock layer is a greyish red siltstone. The next downward is a greenish grey siltstone penetrated by sinuous tubes that may be roots or root-like structures. Underlaying this is "a zone of calcareous nodules."[223]

"The Beacon Supergroup (Barrett, 1970) in the Transantarctic Mountains is largely a flat-lying, nonmarine sequence from Devonian or older to Jurassic in age. It consists of the Taylor Group (Devonian or older), a quartzose sandstone sequence, and the Victoria Group (Permian and Triassic), dominantly a coal-bearing sandstone-siltstone sequence (Harrington, 1965)."[223]

"The Taylor Group comprises up to 1,450 m of quartzose sandstone, with smaller conglomerate, arkosic and shaly units [...]. [The] youngest Taylor Group unit [is] the Aztec Siltstone [of which the image at the right exhibits]."[223]

Def. a "sedimentary rock whose composition is intermediate in grain size between the coarser sandstone and the finer mudstone"[224] is called a siltstone.

Slates[edit | edit source]

The image shows finely layered slate perhaps with occasional dolomite layers exposed on a beach in Cornwall, UK. Credit: Si Griffiths.{{free media}}
The image shows folds in slate and quartzite of the Meguma Group near the Ovens, Nova Scotia, Canada. Credit: Michael C. Rygel.{{free media}}
This is a cyan colored slate. USGS.{{free media}}

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock.[225] Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression.[225] Slate is frequently grey in color, especially when seen, en masse, covering roofs. However, slate occurs in a variety of colors even from a single locality; for example, slate from North Wales can be found in many shades of grey, from pale to dark, and may also be purple, green or cyan.

Def. a "fine-grained homogeneous [...] rock composed of clay or [...] ash which [...] cleaves easily into thin layers"[226] is called a slate.

Stocks[edit | edit source]

A stock of nordmarkite (quartz-alkali syenite) of Triassic age, is in the Gevanim Valley, Makhtesh Ramon, southern Israel. Credit: Wilson44691.{{free media}}

In geology, a stock is an igneous intrusion that has a surface exposure of less than 100 square kilometres (40 sq mi),[227][228] differing from batholiths only in being smaller. A stock has a discordant relationship with the rocks that it intrudes. Many stocks are cupolas of hidden batholiths.[229][228] Some circular or elliptical stocks may be volcanic plugs, which fill the vents of now extinct volcanoes.[230][231]

A boss is a small stock.[232]

Def. a "pipe (vertical cylinder of ore)"[233] is called a stock.

Syenites[edit | edit source]

This is a piece of syenite. Credit: USGS.
Rock name is särnaite (leucocratic variety of nepheline syenite) and it is from Sweden. Credit: Siim Sepp.

Def. an "igneous rock composed of feldspar and hornblende"[234] is called a syenite.

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

Taconites[edit | edit source]

Taconite, in the United States, is a hard, silica-rich iron ore mined in the Lake Superior region. Credit: USGS.{{free media}}

"This rock is widely spread over the whole length of the Mesabi, and being different from anything found elsewhere and peculiar to this horizon of the Taconic, has been called taconyte by the writer."[235]

Taconite is a variety of iron formation, an iron-bearing (over 15% iron) sedimentary rock, in which the iron minerals are interlayered with quartz, chert, or carbonate, of the Precambrian Biwabik Iron Formation of northeastern Minnesota, bearing a superficial resemblance to iron-bearing rocks from the Taconic Mountains of New York state.[235]

Tephrites[edit | edit source]

This is an image of a leucite tephrite lava used as a building facade. Credit: Roll-Stone.{{free media}}

Def. an "igneous rock consisting essentially of plagioclase and either leucite or nephelite, or both"[236] is called a tephrite.

Tonalites[edit | edit source]

A piece of tonalite on red granite gneiss from Tjörn in Sweden. Credit: Ingwik.{{free media}}

Def. an "igneous, plutonic rock composed mainly of plagioclase"[237] is called a tonalite.

Trachyandesites[edit | edit source]

A cut block of trachyandesite lava, used as a type of building stone. Credit: zarmel.{{free media}}
Trachyandesite is field S3 in the TAS diagram. Credit: Kent G. Budge.{{free media}}

Def. "volcanique intermédiaire entre les trachytes et les andésites"[238] is called a trachyandesite.

Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite, having little or no free quartz, but dominated by sodic plagioclase and alkali feldspar, formed from the cooling of lava enriched in alkali metals and with an intermediate content of silica.[239][240]

Trachytes[edit | edit source]

This is an IUGS Igneous Rock Classification Diagram for volcanic rocks. Credit: Richard Harwood.{{fairuse}}

Def. a "pale igneous rock consisting mostly of potassium feldspar and plagioclase"[241] is called a trachyte.

Travertines[edit | edit source]

This is an example of a travertine. Credit: USGS.

Def. "light, porous form of concretionary limestone (or calcite)"[242] is called a travertine.

Troctolites[edit | edit source]

Troctolite sample 76535 brought back from the Moon by Apollo 17. Credit: NASA/Johnson Space Center photograph S73-19456.{{free media}}

Def. a "rare type of ultramafic intrusive rock, consisting primarily of olivine and calcic plagioclase"[243] 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[edit | edit source]

The profiles consist of tonalite, trondhjemite, and gneiss. Credit: Bjoertvedt.{{free media}}

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

Turbidites[edit | edit source]

Turbidites (interbedded with mudstones/siltstones) from the Ross Sandstone Formation. Credit: USGS.
Turbidite (Gorgoglione Flysch) is from Miocene, South Italy. Credit: Geologist.

Def. "sea-bottom deposits formed by massive slope failures where rivers have deposited large deltas"[244] are called turbidites.

"Turbidites [shown in the image on the right] are sea-bottom deposits formed by massive slope failures where rivers have deposited large deltas. These slopes fail in response to earthquake shaking or excessive sedimentation load. The temporal correlation of turbidite occurrence for some deltas of the Pacific Northwest suggests that these deposits have been formed by earthquakes."[244]

"Turbidites (interbedded with mudstones/siltstones) from the Ross Sandstone Formation Turbidite system of Namurian age in County Clare, Western Ireland. The sandstone beds were formed in a deep basin by turbidites coming from a delta area."[244]

Ultramafics[edit | edit source]

A mantle xenolith of peridotite shows chemical weathering of olivine (green) to iddingsite (brown). Credit: Matt Affolter.{{free media}}

Def. "rocks that contain magnesium and iron and only a very small amount of silica"[245] are called ultramafics.

Variolites[edit | edit source]

Beach pebble of variolitic pillow lava (varolite) is from the Olympic Peninsula, Washington state. Credit: Schaferpho.{{free media}}

Variolites are mafic, igneous, and typically volcanic rocks, e.g. tholeiite, basalt or komatiite, that contain centimeter-scale spherical or globular structures, called varioles, in a fine-grained matrix, lighter colored than the host rock and typically range in diameter from 0.05 mm to over 5 cm.[246][247][248] In 1648, Aldrovandi created the term variolite for aphanitic or fine-grained igneous rocks containing varioles.[249][250] The weathering of varioles often cause variolites to have a pock-marked appearance. In allusion to the pock-marked appearance of weathered surfaces of variolite, this term is derived from the Latin word, variola, for smallpox.[247][248]

Varioles are millimeter- to centimeter-scale, light-colored, globular to spherical structures, that are conspicuously observable within aphanitic, mafic igneous rocks, such as basalt, komatiite, and tachylite, that comprise either pillow lavas, subaerial lava flows, or volcanic dykes, typically, less resistant to weathering than the enclosing aphanitic rock and, as a result, form pock-marks on the weathered surfaces of mafic rocks.[251][252]

The alteration, specifically mineralization, and deformation associated with many Precambrian volcanic rocks, particularly Archean volcanic rocks, makes the determination of their origin difficult, if not impossible, without further laboratory analyses.[251][253] Phillips (1973)[254] provides a detailed review of the nomenclature of different types of varioles that have been proposed. Confusingly, a few Earth scientists use variolite as if it is synonymous with variole.[255]

Volcanics[edit | edit source]

The diagram shows a classification and flow characteristics of volcanic rocks. Credit: J. Johnson (USGS).{{fairuse}}

Def. rock of "or pertaining to a volcano or volcanoes",[256] produced "by a volcano, or, more generally, by igneous agencies",[256] changed "or affected by the heat of a volcano"[256] is called a volcanic rock.

"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."[70]

Websterites[edit | edit source]

Extremely coarse-grained, ultrahigh pressure garnet websterite is found near Selje, Norway. Credit: Simon Cuthbert.{{fairuse}}

Def. an "ultramafic and ultrabasic rock that consists of roughly equal proportions of orthopyroxene and clinopyroxene"[257] is called a websterite.

Wehrlites[edit | edit source]

The boundary of wehrlite and gabbro xenolith is ambiguous, suggesting both wehrlite and gabbro were still ductile. Credit: Niigata University.{{fairuse}}

Def. an "ultramafic and ultrabasic rock that is a mixture of olivine and clinopyroxene"[258] is called a wehrlite.

Rocky objects[edit | edit source]

This image is of asteroid 2012 LZ1 by the Arecibo Observatory in Puerto Rico using the Arecibo Planetary Radar. Credit: Arecibo Observatory.{{fairuse}}

A rocky object is any object, including astronomical objects, composed of one or more types of rocks.

Def.

1.a: "something that is or is capable of being seen, touched, or otherwise sensed",
1.b: "something physical or mental of which a subject is cognitively aware",
2. "something that arouses an emotion in an observer", or
3. "a thing that forms an element of or constitutes the subject matter of an investigation or science"

is called an object.[259]

Def. full "of, or abounding in, rocks; consisting of rocks... [l]ike a rock"[260] is called rocky.

Arêtes[edit | edit source]

This is an image of Punta dell'Alp from monte Pelvo. Credit: F Ceragioli.

Def. a "very thin ridge of rock"[261] is called an arête.

"An earlier [...] glacial episode, herein termed the Altonah Glaciation, is indicated by an extensive lateral moraine beyond the mouth of Yellowstone canyon as well as moraines in Lake Fork and Uinta River canyons. At higher elevations, alpine glacial landforms, including cirques, rock glaciers, arêtes, and hanging valleys are ubiquitous."[262]

"Arêtes and cols are most common in the south-central and southwestern Uinta Mountains, where accumulation areas of glaciers were largest and the development of ice caps that drained into multiple valleys was common. In the south-central Uintas, the most prominent arêtes have more than 450 m of relief and are more than 10 km in length. In the southeastern Uintas, rounded unglaciated divides locally termed “bollies” are more common than narrow arêtes. Examples of these features include broad divides that separate glacial valleys in the headwaters of Dry Fork and Ashley Creeks [...]."[262]

Canyons[edit | edit source]

This is a view of the Grand Canyon in Arizona, USA. Credit: Mike Buchheit.

On the right is an example of a river canyon, specifically the Grand Canyon in Arizona, USA.

Def. a "valley, especially a long, narrow, steep valley, cut in rock by a river"[263] is called a canyon, or a river canyon.

Def. a deep gorge is called a canyon.

Cliffs[edit | edit source]

This cliff is part of a karst landscape formed by water and plants on calcareous (probably) limestone. Credit: Pavle Cikovac.
An eroded boulder clay cliff is shown. Credit: Eric Jones.
A red cliff along US287 between Lander and Dubois in Wyoming, near Wind Canyon. Credit: Wing-Chi Poon.
Cliffs of Moher is in East, Ireland. Credit: Tobias Helfrich.

The cliff in the image on the right is part of a karst landscape formed by water and plants on calcareous (probably) limestone.

"This photo [on the left] shows clearly why these glacial deposits are called boulder clay, consisting as they are of clay and stones of various sizes up to and including large boulders. Till is another name for boulder clay."[264] The image is of an eroded boulder-clay cliff near to Trefor, Gwynedd, Great Britain.

Def. a "vertical (or nearly vertical) rock face"[265] is called a cliff.

Glaciers[edit | edit source]

Briksdalsbreen is a part of the Jostedalsbreen glacier in Norway. Credit: Donarreiskoffer.

Def. "a mass of ice that originates on land, usually having an area larger than one tenth of a square kilometer"[266] is called a glacier.

Mountains[edit | edit source]

Low winter sun reflects off Skopphornet and Sunnmøre alps (Sykkylven) in Norway. Credit: "color line".

Def. a "large mass of earth and rock, rising above the common level of the earth or adjacent land, usually given by geographers as above 1000 feet in height (or 304.8 metres), though such masses may still be described as hills in comparison with larger mountains"[267] is called a mountain.

Pavements[edit | edit source]

Limestone pavement is above Malham Cove in the Yorkshire Dales. Credit: Lupin.
This shows limestone pavement in Haute Savoie, France. Credit: f.corageoud.
Desert pavement is near Barstow, California. Credit: Wilson44691.
The "Tesselated Pavement“ is the result of an orthogonal joint pattern in the rock. Credit: JJ Harrison.
Grooves and striations are on exhumed Permian glacial pavement. Credit: Bahudhara.

Def. a more or less horizontal, hard expanse of bare rock as a surface is called a pavement.

On the right are two images of a limestone pavement which is part of a karst topography.

On the left is a desert pavement in southeastern California.

A pavement such as the one on the second left covered with pieces that are similarly shaped is referred to as a tessellated pavement.

Volcanic bombs[edit | edit source]

This is an accretionary lava ball. Credit: J. D. Griggs, USGS HVO.
This is a volcanic bomb found in the Mojave Desert National Preserve by Rob McConnell. Credit: Wilson44691.
This is a picture of a lavabomb at Strohn, Germany. Credit: Jhintzbe.

Def. "distinctively shaped [natural] projectiles ... which acquired their shape essentially before landing"[268] are called bombs.

Def. a bomb "ejected from a volcanic vent"[268] is called a volcanic bomb.

Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight.

The image at top right is an "[a]ccretionary lava ball [coming] to rest on the grass after rolling off the top of an ‘a‘a flow in Royal Gardens subdivision. Accretionary lava balls form as viscous lava is molded around a core of already solidified lava."[269]

Volcanic bombs cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they do not have grains making them extrusive igneous rocks. Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight.

Volcanic bombs can be extremely large; the 1935 eruption of Mount Asama in Japan expelled bombs measuring 5–6 m in diameter up to 600 m from the vent. A large volcanic bomb is shown in the third image at right from Strohn, Germany.

Volcanic bombs are known to occasionally explode from internal gas pressure as they cool, but explosions are rare. Bomb explosions are most often observed in 'bread-crust' type bombs.

Ribbon or cylindrical bombs form from highly to moderately fluid magma, ejected as irregular strings and blobs. The strings break up into small segments which fall to the ground intact and look like ribbons. Hence, the name "ribbon bombs". These bombs are circular or flattened in cross section, are fluted along their length, and have tabular vesicles.

Spherical bombs also form from high to moderately fluid magma. In the case of spherical bombs, surface tension plays a major role in pulling the ejecta into spheres.

Spindle, fusiform, or almond/rotational bombs are formed by the same processes as spherical bombs, though the major difference being the partial nature of the spherical shape. Spinning during flight leaves these bombs looking elongated or almond shaped; the spinning theory behind these bombs' development has also given them the name 'fusiform bombs'. Spindle bombs are characterised by longitudinal fluting, one side slightly smoother and broader than the other. This smooth side represents the underside of the bomb as it fell through the air.

Cow pie bombs are formed when highly fluid magma falls from moderate height; so the bombs do not solidify before impact (they are still liquid when they strike the ground). They consequently flatten or splash and form irregular roundish disks, which resemble cow-dung.

Bread-crust bombs are formed if the outside of the lava bombs solidifies during their flights. They may develop cracked outer surfaces as the interiors continue to expand.

Cored bombs are bombs that have rinds of lava enclosing a core of previously consolidated lava. The core consists of accessory fragments of an earlier eruption, accidental fragments of country rock or, in rare cases, bits of lava formed earlier during the same eruption.

Rock strata[edit | edit source]

The image shows rock strata in Cafayate, Argentina. Credit: travelwayoflife.
The image shows an anticline in the Barstow Formation (Miocene) at Calico Ghost Town near Barstow, California USA. Credit: Wilson44691.

The image at the right shows rock strata in Cafayate, Argentina, the subject of stratigraphy.

Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories.

Marginal marines[edit | edit source]

This is a marginal marine sequence from southwestern Utah, USA. Credit: Wilson44691.

The marginal marine sequence on the right has been dated to the Middle Triassic.

Hadean[edit | edit source]

Def.

  1. "the geologic eon from about 4,600 to 3,800 million years ago; marked by the formation of the solar system, a stable Earth-Moon orbit and the first rocks"[270] or
  2. the "eon before 4,000 Ma"[271]

is called the Hadean.

"[U]ranium-lead dating [has been conducted] on fragments of the mineral zircon extracted from Apollo 14 lunar samples. The pieces of zircon were minuscule — no bigger than a grain of sand."[272]

"Size doesn't matter, they record amazing information nonetheless! The moon holds "so much magic ... the key to understand how our beautiful Earth formed and evolved."[273]

More "zircons from Apollo 14 samples [from the moon's Fra Mauro highlands collected in February 1971 are being studied], but [are not expected] to change [the] estimate of 4.51 billion years for the moon's age, possibly 4.52 billion years at the most."[272]

"It would be more a double-checking than anything else."[273]

Regardless "of how the moon came to be — one big strike at Earth, many smaller ones or even none at all — you still end up at the end solidifying the moon as we know it today."[273]

"We finally pinned down a minimum age for the moon formation, regardless of how it formed."[273]

Petrology[edit | edit source]

The photomicrographs show of a sand grain held in an amorphous matrix, in plane-polarized light on top, cross-polarized light on bottom. Scale box in mm. Credit: Qfl247.{{free media}}

Def. the "study of the origin, composition and structure of rock"[274] is called petrology.

Petrography[edit | edit source]

Specimens of kimberlitic rocks from the Buffalo Head Hills in Alberta. Credit: Georgialh of Display by Alberta Geological Survey.{{free media}}

Def. "the branch of petrology that deals with the scientific"[275] "description and classification of rocks"[276] is called petrography.

"The initial distinction between volcanic and plutonic rocks is made on the basis of texture (fine-grained volcanic vs. coarse-grained plutonic)."[70]

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

Geochemistry[edit | edit source]

The bar graph shows the major chemical elements forming igneous rocks. Credit: J. Johnson (USGS).{{fairuse}}

"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."[70]

"These rock types all have different characteristics, including temperature when fluid, viscosity (resistance to flow), composition, explosiveness, and types, amounts, and sizes of minerals."[70]

"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").[70]

"Components of Igneous Rocks":[70]

  1. "rhyolite consists of more than 68% silica"[70]
  2. "dacite consists of about 63-68% silica"[70]
  3. "andesite consists of about 52-63% silica"[70]
  4. "basalt consists of about 48-52% silica".[70]

"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."[70]

Hypotheses[edit | edit source]

  1. Variations in rocks from one mineral composition into another can occur with each rock type.
  2. The temperatures necessary to melt rocks and produce volcanic rocks can come from high resistance locally to electrical flow.

See also[edit | edit source]

References[edit | edit source]

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