Rocks and Rock Formations

B ◘ basalt, basaltic ◘ breccia F ◘ felsic G ◘ gabbro ◘ granite H ◘ hornfels I ◘ igneous rocks L ◘ lava M ◘ mafic ◘ magma ◘ metamorphic rocks O ◘ ophiolite complexes P
◘ peridotite R ◘ rocks S ◘ sedimentary rocks

◙◙ Rock Index: alphabetic ◙◙ Rock Index: Igneous Rocks ◙◙ Rock Index: Metamorphic Rocks ◙◙ Rock Index: Sedimentary Rocks ◙◙

basalt

Basalt is a hard gray or black, mafic igneous volcanic rock that is usually fine-grained due to rapid cooling of lava, though it contain larger crystals in a fine matrix (porphyritic), be vesicular, or be a frothy scoria.

Basalt magmas form by decompression melting of peridotite in the mantle. The crustal portions of oceanic tectonic plates comprised predominantly basalt, derived from upwelling peridotite in the mantle below ocean ridges. The basalt shield volcanoes of the Hawaiian island chain sit above a mantle plume, or 'hot spot'. (left - click to enlarge - aa flows over ropey pahoehoe in Hawaii - image courtesy of USGS.)

Basalt is TAS classified according to the relationships between the combined alkali content and the silica content. Basalt typically containts a preponderance of calcic plagioclase feldspar and pyroxene; olivine can also be a significant constituent. Accessory minerals include iron oxides and iron-titanium oxides, providing basalt with a paleomagnetic signature.

Phaneritic, shallow intrusive igneous rocks with a basaltic composition are generally referred to as dolerite (also called diabase) or gabbro.

(image left - click to enlarge - courtesy USGS - top-down: basaltic lava; lava field; flow-lines in basalt formation; close-up of vesicular basalt with olivine crystals; surface of basalt hand specimen; basalt columns.)

[images - roll-over link for preview (where available); large images of hand-speciments (well worth a visit) show only as a corner on preview : water-sculpted basalt at Fossil Falls in Yosemite : Basalt Fall unterhalb des Hengifoss, basalt columns, Dverghamrar basaltic columns in Iceland, 2 : cliff of basalt columns : Columbia River basalts, Catherine Creek arch in Miocene columnar basalts : flowing curves of basalt entablature in Yellowstone : basalt columns Armenia : basalt field : basalt and sandstone : 3.7 Ga moon-rock basalt : hand-specimen : hand-specimen vesicular basalt, vesicular basalt with olivine phenocrysts, 2 : hand-specimen diabase : hand-specimen diabase porphyry : hand-specimen diorite : hand-specimen gabbro : hand-specimen scoria : thin-section basalt, 2, 3; thin-sections moon basalts ]

breccia

Breccias are clastic, sedimentary rocks comprising angular fragments from a previous rock structure that have been cemented in a matrix.
image: basalt breccia with epidote groundmass, courtesy of Siim Sepp

Classification of breccias relates to their constituents, mode of occurrence, constituent fragment size, types of clasts, and source of clasts.

Many breccias comprise consolidated talus or scree material, and are made up of accumulations of rock fragments that have fallen down steep hill slopes or cliffs. Breccia are often found above uncomformities, and so are associated with conglomerate, arkose and sandstone. (Conglomerates have rounded fragments.) Other breccias are produced by the fragmentation of rocks during faulting (tectonic or monomictic breccia), or during volcanic eruptions (eruption breccia, vent breccia), or collapses such as in karst areas, or form upon meteorite impacts (often in breccia dikes). Monomictic breccias result from rock deformation by shearing and granulation (cataclasis) in the process of tectonism or dislocation metamorphism, while impact breccias have been called "monomictic movement breccias".

[images: hand-specimens: breccia, 2, 3, 4, chert breccia; breccia in copper, 2; impact breccia; thin-sections : volcanic breccia; fault breccia in the Antietam Formation; Breccia Pernice marble, Breccia Aurora marble; formations: cliffs of volcanic breccias formed by lahars; Wawa xenolithic breccia, Wawa Heterolithic Breccia; Ries impact structure, Iggenhausen quarry, monomictic movement breccia, close-up; Azuara impact structure close-up; grit brecciation and mortar texture, Rubielos de la Cérida impact basin, and heavily brecciated and polished scour surface; strongly brecciated chert nodule, Malmian limestone, Ries impact structure; breccia dikes in Liassic limestones south of Belchite (Deutsche)]

felsic

Felsic (feldspar-silica) refers to silicate minerals, magmas, and rocks relatively enriched in the lighter elements such as silica, oxygen, aluminium, sodium, and potassium.

The sial is the upper layer of Earth's crust – the continental crust – and is rich in aluminum/silica minerals (granitic layer). Because felsic-sial rocks are lower in density than mafic-sima rocks, the continental crust 'floats' atop the deeper, denser sima crust. The Conrad discontinuity, arbitrarily set at 2800kg/m^3 marks the base of the sial where it grades into the basalts of the sima.

Felsic minerals are usually light in color, with specific gravities less than 3 (2700 - 2800 kg/m^3). Common felsic minerals include quartz, muscovite, hornblende, orthoclase, and the sodium rich plagioclase feldspars. Felsic rocks contain >75% felsic minerals. Granite is the most common felsic rock. Rocks with greater than 90% felsic minerals are also termed leucocratic, meaning 'light-coloured'.

The term acid rock, although sometimes used as a synonym for felsic rocks, refers to volcanic rocks with high silica content (greater than 63% SiO2 by weight) such as rhyolite.

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gabbro

Gabbro is a coarse-grained, mafic, plutonic igneous rock that forms at spreading centers in rift zones and mid-ocean ridges (so underlies oceanic crust). Gabbros can form as massive uniform intrusions or as layered ultramafic intrusions formed by settling of pyroxene and plagioclase (pyroxene-plagioclase cumulate).

As an essential component of the oceanic crust, gabbros are found in many ophiolite complexes in the sheeted dyke zone to massive gabbro zone (zones III and IV). Long belts of gabbroic intrusions are typical at proto-rift zones and around ancient rift zone margins, where they intrude into the rift flanks.

Gabbro is a dense rock that is greenish or dark-colored and comprises varied percentages of pyroxene, plagioclase feldspar, amphiboles, and olivine. Where olivine is present in large quantities, the rock is termed olivine gabbro.

A finer grained rock with the same composition as gabbro is termed diabase.

[images : layered gabbro, North Cascades : Salem gabbro-diorite cut by a a composite dike with felsic margins and a central core of basaltic rock : White Mountain Magma Series : pegmatitic gabbro : oceanic crust exposed on Cyprus : oceanic crust gabbro, 2 : thin section Oman Ophiolite gabbro : thin section of olivine gabbro - pyroxene and olivine show bright colours, striped grey rectangular crystals are plagioclase feldspar : thin section of gabbro with plagioclase and hypersthene (orthopyroxene) : hypersthene gabbro : thin section with pyroxene and (striped) plagioclases : thin section orthopyroxenes crystals surrounded by alteration (uralite) : thin section : thin section with twinned plagioclases :

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granite

Granite is typically a medium to coarse grained felsic, intrusive igneous rock (plutonic) that is usually pink to dark gray, sometimes black, depending on its chemistry and mineralogy. Granites are the commonest basement rocks of the continental crust, many dating from the Precambrian.

In some granites, individual crystals are larger than the groundmass (porphyrys). Granites primarily comprises orthoclase and plagioclase feldspars, quartz, hornblende, muscovite and/or biotite micas, with minor accessory minerals such as magnetite, garnets, zircon and apatite. Rarely, a pyroxene is present. Very rarely, iron-rich olivine, fayalite, occurs.

Granites are classified according to the QAPF diagram for granitoids and phaneritic foidolites (plutonic rocks) that compares the percentages of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar.

As a plutonic rock, granite is often exposed in weathered tors, dykes and as massive batholiths.

hornfels

Hornfels are hard, mostly fine-grained metamorphic rocks that results from contact metamorphism.

Banding from the country rock can be retained, but because original bedding planes have typically been lost from the country rock upon baking by intruding magma, hornfels tend to separate into cubical fragments rather than into thin plates. Hornfels display characteristic mosaic-like interlocking of minerals, sometimes with enclosed particles of the other minerals [3]. The minerals rarely show crystalline form and are typically of nearly equal dimensions. The interlocking, equidimensional texture has been called pfiaster or pavement structure [photomicrographs 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]


Hornfels types and mineral assemblages depend upon the country rock that was baked during contact metamorphism:
● biotite hornfelses (the commonest) derive from slates, shales and clays
● calcite-silicate-hornfelses from impure limestone (purer limestones recrystallize as marbles)
● hornfelses with eldspar with hornblende (generally of brown color) and pale pyroxene arise from diabases, basalts, and andesites

Whole specimens range are dense, dull (not shiny), may be spotted, and vary in color : biotite hornfelses from slates, shales and clays are dark-brown to black; lime hornfelses are often white, yellow, pale-green, or brown; andalusite may be pink and pleochroic.

Some hornfels do retain layering [1]
hand specimens : andalusite hornfels : graywacke : hornfels : hornfels (Løkken Verk) : hornfelse :
by location : St Sjöfallet : Triangle Lake : Svenådalen :
rock outcrops : Tonschiefer-Hornfels : New Zealand : Japan :
photomicrographs : andalusite hornfels : biotite hornfels : biotite hornfels 2 : cordierite hornfels : garnet hornfels : pyroxene hornfels : sillimanite hornfels : spotted hornfels

igneous rocks

Igneous rocks result when molten rock cools (± crystallization) from plutonic (intruded) magma or from volcanic (extruded) lava.

Left - click to enlarge image - texture and mineral composition of common igneous rocks:

Increasing to the left: SiO2 content, viscosity; Increasing to the right: darkness, mafic : felsic composition, (Fe, Mg, Ca) : (K, Na) ratio, temperature of melting:

◘ a/p - aphanitic or porphyritic texture, derived from extruded magma (lava),
◘ 1. rhyolite ◘ 3. dacite ◘ 5. andesite ◘ 7. basalt
phaneritic texture, emplaced as magma (plutonic)
◘ 2. granite ◘ 4. granodiorite ◘ 6. diorite ◘ 8. gabbro to peridotite
0-100: percentage mineral content:
◊ a. quartz ◊ b. K-feldspar ◊ c. Na-feldspar ◊ pl. plagioclase to Ca-rich plagioclase ◊ d. muscovite
◊ e. biotite ◊ f. amphiboles ◊ g. pyroxenes ◊ h. olivine

Igneous rocks, predominantly plutonic, comprise about 95% of the Earth's crust, though their burial by sedimentary rocks and association with metamorphic rocks disguises their true extent.
The crystalline basement rock (shield) at the core of most continents is ancient, having arisen predominantly during a period from 3.0 to 2.5 billion years ago, which was the period of maximum continent formation. Most of the earliest rocks have been greatly altered through regional metamorphic processes, but later rocks (3.2-2.5 Ga) are mostly pillow-basalts that formed beneath the vast oceans. Archean sedimentary rocks are mostly coarse and poorly sorted sandstones and conglomerates.

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lava

The silica content of magma, along with amount of dissolved gas, affects the nature of volcanic eruptions as wells as the amount and composition of lava extruded during an eruption.

Volcanic rocks ◙ andesites ◙ basalts ◙ dacites ◙ rhyolites

Lava composition

_ ● felsic > 63% silica = viscous
___ ● dacitic
___ ● rhyolitic
– Rhyolite is named from the Greek for 'steam', and rhyolitic magma often erupts explosively because its high silica content results in extremely high viscosity, which hinders degassing. Effusive eruptions of rhyolite often produce obsidian, which is bubble-free and black. When bubbles form, they can cause the magma to explode, fragmenting the rock into pumice and tiny particles of volcanic ash. Some of the United States' largest and most active calderas formed during eruption of rhyolitic magmas (for example, Yellowstone in Wyoming, Long Valley in California and Valles in New Mexico). Rhyolite is the volcanic equivalent of plutonic granite.
– Even though dacite contains less silica than rhyolite, dacite can be even more viscous, and so just as dangerous as rhyolites. Dacite was erupted from Mount St. Helens 1980-86, Mount Pinatubo in 1991, and Mount Unzen 1991-1996.

_ ● intermediate 52-63% silica
___ ● andesitic
– Andesite magma commonly erupts from stratovolcanoes as thick lava flows, and can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesite, which is common in the Andes mountains, was the main rock type erupted during the great Krakatau eruption of 1883.

_ ● mafic 45 - 52%
___ ● basaltic
– Basalt's low silica content gives it a low viscosity (resistance to flow), so basaltic lava can flow quickly and can easily move >20 km from a vent. The low viscosity of basaltic magma usually allows volcanic gases to escape without generating enormous eruption columns. However, basaltic lava fountains and fissure eruptions still form explosive fountains hundreds of meters tall. Shield volcanoes, such as those in the Hawaiian islands (movies), are composed almost entirely of basalt, which is the commonest rock type in the Earth's crust; huge, ancient outpourings of lava called 'flood basalts' make up large igneous provinces on many continents; and, most of the ocean floor is basalt that has been extruded at spreading mid-ocean ridges. aa, aa active photo, hornito, lava tube, lava lake, pahoehoe, pahoehoe photo, pahoehoe active photo, Pele's hair, Pele's tears, tumulus

_ ● ultramafic ≤ 45%
___ ● komatiites

mafic

Mafic (magnesium-ferric) minerals and rocks are silicate minerals, magmas, volcanic, and intrusive igneous rocks with relatively high concentrations of heavier elements.

The term sima designates the lowest layer of the Earth's crust and the ocean floors (basal crust, basal layer, basalt layer). Sima comprises magnesium rich silicon minerals and when it rises to the surface as lava sima forms mafic rocks (usually basalts) or rocks with mafic minerals. The densest sima forms ultramafic rocks.

Constituent elements include Mg, Fe, Ca, and Na. Mafic minerals are usually dark in color, with a specific gravity greater than 3 (2800 to 3300 kg/m^3). Common rock-forming mafic minerals include amphibolte, biotite, olivine, pyroxene, and other micas, augite and the calcium-rich plagioclase feldspars. Common mafic rocks include basalt and gabbro.

Because of its lower silica content, molten mafic lava has a lower viscosity than felsic lava. Mafic volcanoes are less explosive than felsic lava eruptions because water and other volatiles more easily and gradually escape from mafic lava. Most mafic lava volcanoes are oceanic volcanoes, like Hawaii.

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magma

Magma is molten rock, which forms igneous rock when it crystallizes on cooling as igneous intrusions or extruded lava.

Most magma comprises solutions of silicates melted at temperatures from 700-1600 °C depending upon the environment in which the parent rock melted. Unusual black lava carbonates (natrocarbonatite) of the intracontinental volcano Oldoinyo Lengai are molten at temperatures as low as 600 °C.

The melting of rock is determined by temperature, pressure, and composition and occurs in association with mantle plumes or tectonic processes. The composition of magma can alter after melting of the parent rock by processed that include contamination, fractional crystallization, and mixing of magma with other molten rock.

Rocks melt at a range of temperatures, depending upon pressure and the presence of water and gases. Greater temperatures are required to melt a given dry rock at greater pressures, whereas wet rocks initially melt at decreasing temperatures with increasing pressure and then transition to requiring greater temperatures with further increase in pressure. (above left -dry rock with partial melts of liquid and crystals; above right - wet rock with partial melts comprising liquid, crystals, and vapor).

Burial of rock exposes the minerals to heating along the geothermal gradient, which is elevated by convection within the asthenosphere, bringing the rock to temperatures high enough for partial melting.

The geothermal gradient is defined as the rate of change of temperature (ΔT) with depth (ΔZ), in the Earth. At depths down to about 60 m, temperature is constant at about 11°C. Between 60 and 120 m, the geothermal gradient is variable because it is affected by atmospheric changes and circulating ground water. Below 120 m, temperature almost invariably increases with depth, though the rate of increase with depth varies with both tectonic setting and the thermal properties of the rock.

High gradients (up to 200°C/km) are observed along oceanic spreading centers and along island arcs due to magma rising to the surface. Low gradients are observed in tectonic subduction zones because of cold, water-filled sediments thrusting beneath the existing crust. Tectonically stable shield areas and sedimentary basins have average gradients that typically vary from 15–30°C/km.

◙ subduction zone magmas ◙

metamorphic rocks

Gneiss (pron. 'niece') is a common rock resulting from high-grade regional metamorphism of igneous rocks (orthogneiss) or sedimentary rocks (paragneiss).

Gneisses typically occur in gneiss belts comprising large areas within the high-grade cores of regional metamorphic belts. The high temperatures and shear stresses of high grade metamorphism are probably due to deep tectonic burial and major regional compression, so gneissic terranes can form in areas of convergent plate tectonics.

Gneissic rocks are coarsely foliated rocks with alternating light (quartz and feldspar) and dark (hornblende and biotite) bands. Micas are absent of present only in small amounts in gneissic rocks, but predominate in the often finer grained schists. Individual bands in gneisses are 1 mm to 1 cm in thickness and result from recrystalization of component minerals during subjection to formative high temperature and pressure (shear stress). Those rocks without obvious banding are termed leptites. Individual mineral grains are often flattened parallel to banding, and gneiss is defined by this texture although the term gneiss often indicates mineral composition of granitic type, dominated by quartz and feldspar.

Where not of granitic origin, gneisses are named for their parent rock such as diorite and amphibolite, or for the presence of index minerals such as albite, biotite, biotite-plagioclase, chlorite, and garnet, hornblende-plagioclase.

(above left - click to enlarge image : top, banded gneiss; center, close-up of augen gneiss showing characteristic elliptic or lenticular feldspaths (normally microcline); bottom kinked banding in gneiss)

[images, roll-over for preview : biotite gneiss : pyroxene gneiss : Passagassawakeag gneiss 1 : Passagassawakeag gneiss 2 : Passagassawakeag gneiss 3 : augen gneiss with gneissic banding : augen gneiss close-up : gneiss sample : alternating pink K-spar and black amphibole and white plagioclase layers : chloritoid gneiss 1 : chloritoid gneiss 2 : chloritoid gneiss 3 : migmatitic gneiss : transposed gneiss : diagram of banding in gneiss : gneiss boulders : Archean banded gneiss, Black River, Wi : ]

Schist (pron. shist) is a medium- to coarse-grained, often shiny, mica-laden rock. Medium-grade metamorphism causes recrystallization, rotation, and new growth of micas (predominantly muscovite, biotite, and chlorites) from fine-grained, mica-bearing rocks such as shales and slates, which results in the well-developed planar to wavy foliation (schistosity) characteristic of schists. Schists, such as garnet-biotite schists containing porphyroblasts of garnet and a schistosity dominated by biotite, are named for their assemblage of minerals.

(right - click to enlarge : top, schist bed at Corea Ck.; center, surface of garnet schist (left) and biotite-mica schist (right); bottom, photomicrograph of garnet-mica schist).

[images, roll-over for preview : gallery of rock photomicrographs : amphibolite : garnet-mica schist : mica schist : talc-tremolite schist : muscovite-foliation between quartz grains : garnet-kyonite-quartz schist : garnet-staurolite-muscovite schist : garnet-staurolite schist : kyanite schist : muscovite mica schist with crenulations : tourmaline mica schist : Connemara Schists : Schist Wave : schist landslide, NZ]

: simple animation of metamorphism :

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ophiolite complexes

Ophiolite complexes, or ophiolites (for 'snake stones') are rock sequences interpreted as being uplifted sections of oceanic crust and subjacent upper mantle that have become emplaced within rocks of the continental crustal. The age of ophiolite formation is often quite close to the age of their emplacement into the continental crust.

(left - click to route to larger image - Ophiolites (Mohorovičić discontinuity) in Gros Morne National Park, Newfoundland.)

Similar ophiolite sequences are also associated with spreading centers at mid-oceanic ridges in the oceanic crust proper.

Stratigraphic components of ophiolite complexes from uppermost layer to base:
● frequently, oceanic sedimentary rocks (pelagic, Flysch sediments) such as bedded chert, mudstone, limestone, and graywacke sandstone
● pillow basalts – formed when hot magma is extruded onto the ocean floor. These basalts are subject to extensive low temperature/low pressure alteration (prehnite-pumpellyite facies metamorphism) of existing minerals and precipitation of new minerals when lavas and crustal and mantle fluids interact with seawater. This circulation reaches depths of 3 km and provides for chemical interactions that release metal sulphides, water, methane, and carbon dioxide into the ocean, and entrain water and metal oxides into the crust. The circulation causes metamorphism and serpentinization of the basalts and gabbros.
● sheeted mafic dikes and sills – feeders to the subaqueous pillow basalts that typically intruded consecutively into one another before cooling was complete
● gabbro (frequently layered) – the upper layer of the gabbro is typically not stratified; the basal gabbro layer often comprises cumulate layers, which were the first formed crystals that sank to the base of the chamber.
__● for ophiolite sequences at spreading oceanic ridges, the base of the gabbro, where the cumulate gabbro passes into an ultramafic cumulate, marks the geophysical base of the crust (the Moho, where the density contrast between causes a marked attenuation in seismic velocity)
● peridotite layer, or
__● for oceanic crust, where this layer marks the petrological contact between the Earth's crust and the mantle, the 'depleted' harzburgite layer at the top of the mantle is composed only of orthopyroxene and olivine, and lacks the typical clinopyroxene and spinel of the underlying fertile mantle rocks (the lherzolite).

Ophiolite complexes are remnants of ocean crust and underlying oceanic mantle, which have been embedded in continental crust. As such, they represent the relicts of earlier ocean basins with spreading zones, which closed up following a reversal of the plate’s movement from rifting to accreting, with mountain building in the collision zone. Depending on the relative geotectonic positions of basins to oceanic crusts prior to orogenies, geologist can differentiate between “mid-ocean ridge”, “supra-subduction”, and “back arc basin” types of ophiolites.

Most continental crust ophiolites are assigned to either the Tethyan or Cordilleran groups, which have different modes of emplacement yet are both SSZ in origin. The episodic emplacement of ophiolites throughout geological history suggests that the complexes formed and emplaced at times of super-continent break-up and dispersal when large ocean basins adjacent to super-continents subducted as rifting progressed.

Tethyan ophiolites are named for the ancient Tethys ocean, and are found in the eastern Mediterranean areas ( Troodos in Cyprus, Semail in Oman). These are relatively complete classic ophiolite assemblages that were emplaced intact onto a passive continental margin.

Cordilleran ophiolites are named for the Cordillera mountain belts of western North America. These ophiolites are not associated with a passive continental margin and sit on subduction zone accretionary complexes (subduction complexes). Cordilleran ophiolites include the Coast Range ophiolite of California, the Josephine ophiolite of the Klamath Mountains (California, Oregon), and ophiolites in the southern Andes of South America.

Ophiolite assemblages in collisional mountain belts, such as the Alps, represent incipient ocean crust at thinned continental margins (formed during rifting and continental drift) that has been emplaced into the collision zone.

The stratigraphic sequences observed in some ophiolites suggest origins in lithosphere-forming, spreading centers at mid-oceanic ridges. However, supra-subduction zone (SSZ) ophiolites are more closely related to island arcs than to ocean ridges, and are formed by rapid extension of fore-arc crust during subduction initiation followed by rebound of the continental crust carrying forearc lithosphere (ophiolite) atop it. The occurrence of ophiolite complexes within orogenic belts documents the former existence of ocean basins now consumed by subduction, so providing supporting evidence for plate tectonics.

◙ subduction zone magmas ◙

[images: maps: global distribution of ophiolite complexes, Newfoundland, location of Bay of Islands ophiolite, Geological map, Bay of Islands ophiolite; Josephine Ophiolate; Cyprus; photos: red jasper cherts and silicified turbidites overlying ophiolite at Nippers Harbour, Newfoundland, associated gallery, pillow lavas of ophiolite complex at Green Gardens Trail, pillow lavas Bottle Cove, wp ; Troodos Ophiolite, 92 Ma section of oceanic crust created in Tethys Ocean, wp, Troodos Ophiolite; Oman ophiolite and the Hawasina sedimentary units, Oman Ophiolite, Oman LandSat, hi-res, 3D; Semail ASTER, wp#16; Troodos Ophiolite, ASTER, wp#17; Unst ophiolite, Scotland, wp; lithography: Felsic intrusion of "Kennack Gneiss" into mafic oceanic crust at the base of the Lizard Ophiolite, injections of felsic magma into ultramafic mantle; chevron folds in radiolarian chert, folded bedding in radiolarian chert, ultramafic rocks, intensely sheared serpentinite, hanzburgite, dunite and pyroxenite, serpentinite and alteration to asbestos Del Puerto Canyon; model, Semail Ophiolite pillow lavas, Oman, wp, Ophiolite d'Oman, pillow, pillows, dike, layered gabbro, layered gabbro, gabbro lens in dunites, Moho, harzburgites, gallery; close-ups; Troodos; veinlets with fibrous serpentine at Amiandos, Cyprus ; Leka, Norway; plagiogranite intrusive in serpentinites, Le Chenaillet, Alps, wp, red radiolarites from the Gondran cirque, Chenaillet ophiolite; ophiolite complex, Sierra del Cenvento ophiolite mountain interpreted as an allocthonous serpentinite body thrusting over Cretaceous metavolcanite of Purial complex: ophiolite mélange, eastern Cuba; websites: L'Ophiolite de Chamrousse (English); Troodos Ophiolite; Cyprus Rocks - Ophiolite; Oman ophiolites, Oman Mountains; Mantle - Ocean crust rocks; Josephine ophiolite; radiolarian-bearing strata; diagrams: SSZ, ophiolite sequence]

peridotite

Peridotite is an ultramafic, ultrabasic (less than 45% silica), dense, plutonic igneous rock comprising mostly olivine and pyroxene. Most of the Earth's upper mantle (asthenosphere) is composed of peridotite that originated during the accretion and differentiation of the Earth, or that has differentiated, by precipitation of olivine ± pyroxenes, from basaltic or ultramafic magmas in turn derived from partial melting of the upper mantle peridotites. Deeper in the crust, olivine is replaced by a high pressure polymorphs, so peridotites do not occur at depths greater than 400 km.

Peridotite emplaced in the continental crust is typically found in obducted ophiolite complexes, as xenoliths in basalts and kimberlite pipes, and as orogenic peridotite massifs and alpine peridotites. Olivine is unstable at shallow depths and reacts rapidly with water, so that much surface peridotite has been altered to serpentinite by a process in which the pyroxenes and olivines are converted to green serpentine.

rocks

Maps of North American rock types : rock types - metamorphic, plutonic, sedimentary, volcanic; tapestry of time and terrain, terrain ▪ North American Craton

Petrology ◘ basalt ◘ breccia ◘ felsic (sial) ◘ gabbro ◘ granite ▪ greenstone belts ◘ hornfels ◘ igneous rocks ◘ lava ◘ mafic (sima) ◘ magma ◘ metamorphic rocks ◘ peridotite ▪ porphyry ▪ plutonic ◘ rocks ◘ sedimentary rocks ▪ texture ▪ xenoliths ▪

Mineralogy ◘ crystallization ▪ phenocryst

Tables Geology Section  Elemental Composition of Crust  Flood Basalts and Stratigraphic Boundaries  Minerals & Rocks: Carbonates  Minerals & Rocks: Evaporates, Sulphates  Minerals & Rocks: Metamorphic  Minerals & Rocks: Oxides  Radiometric Dating  Volcanoes  Weathering of Minerals 

images : Rock gallery :

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sedimentary rocks

Archean sedimentary rocks are mostly coarse and poorly sorted sandstones and conglomerates.

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