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65-451: Peridotite is the dominant rock of the upper part of Earth's mantle . The compositions of peridotite nodules found in certain basalts are of special interest along with diamond pipes ( kimberlite ), because they provide samples of Earth's mantle brought up from depths ranging from about 30 km to 200 km or more. Some of the nodules preserve isotope ratios of osmium and other elements that record processes that occurred when Earth

130-475: A cobalt-60 interior acting as a radioactive heat source. This should take half a year to reach the oceanic Moho . Exploration can also be aided through computer simulations of the evolution of the mantle. In 2009, a supercomputer application provided new insight into the distribution of mineral deposits, especially isotopes of iron, from when the mantle developed 4.5 billion years ago. In 2023 JOIDES Resolution recovered cores of what appeared to be rock from

195-452: A metamorphic mineral in metamorphosed limestones and silica-poor mudstones . It also occurs as a primary mineral in rare mafic igneous rocks ; in these igneous rocks, the magmas are relatively deficient in alkalis relative to aluminium , and aluminium oxide may form as the mineral corundum or may combine with magnesia to form spinel. This is why spinel and ruby are often found together. The spinel petrogenesis in mafic magmatic rocks

260-466: A deeper discontinuity in colder regions and a shallower discontinuity in hotter regions. This discontinuity is generally linked to the transition from ringwoodite to bridgmanite and periclase . This is thermodynamically an endothermic reaction and creates a viscosity jump. Both characteristics cause this phase transition to play an important role in geodynamical models. There is another major phase transition predicted at 520 km (320 mi) for

325-414: A density of about 3.33 g/cm (0.120 lb/cu in) Upper mantle material that has come up onto the surface comprises about 55% olivine and 35% pyroxene, and 5 to 10% of calcium oxide and aluminum oxide . The upper mantle is dominantly peridotite , composed primarily of variable proportions of the minerals olivine, clinopyroxene , orthopyroxene , and an aluminous phase. The aluminous phase

390-486: A double chain structure incorporating water. Hornblende itself has a highly variable composition, ranging from tschermakite ( Ca 2 (Mg,Fe) 3 Al 2 Si 6 Al 2 O 22 (OH) 2 ) to pargasite ( NaCa 2 (Mg,Fe) 4 AlSi 6 Al 2 O 22 (OH) 2 ) with many other variations in composition. It is present in peridotite mostly as a consequence of alteration by hydrous fluids. Although peridotites are classified by their content of olivine, pyroxenes, and hornblende,

455-432: A large group of different minerals. These are divided into orthopyroxenes (with an orthorhombic crystal structure) and clinopyroxenes (with a monoclinic crystal structure). This distinction is important in the classification of pyroxene peridotites since clinopyroxene melts more easily than orthopyroxene or olivine. The most common orthopyroxene is enstatite , Mg 2 Si 2 O 6 , in which iron substitutes for some of

520-504: A lhertzolite contains up to 5% spinel, it is a spinel-bearing lhertzolite , while for amounts up to 50%, it would be classified as a spinel lhertzolite . The accessory minerals can be useful for estimating the depth of formation of the peridotite. For example, the aluminium in lhertzolite is present as plagioclase at depths shallower than about 20 kilometers (12 mi), while it is present as spinel between 20 km and 60 kilometers (37 mi) and as garnet below 60 km. Peridotite

585-460: A low-cost, safe and permanent method of capturing and storing atmospheric CO 2 as part of climate change -related greenhouse gas sequestration . It was already known that peridotite reacts with CO 2 to form a solid carbonate -like limestone or marble mineral; and this process can be sped up a million times or more by simple drilling and hydraulic fracturing to allow injection of the CO 2 into

650-518: A number of other mineral families are characteristically present in peridotites and may make up a significant fraction of their composition. For example, chromite is sometimes present in amounts of up to 50%. (A chromite composition above 50% reclassifies the rock as a peridotitic chromitite .) Other common accessory minerals include spinel , garnet , biotite , or magnetite . A peridotite containing significant amounts of one of these minerals may have its classification refined accordingly; for example, if

715-420: A process in which the pyroxenes and olivines are converted to green serpentine . This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures. Serpentinites are mechanically weak and so flow readily within the earth. Distinctive plant communities grow in soils developed on serpentinite, because of the unusual composition of the underlying rock. One mineral in

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780-583: A region in central Asia situated in the upper valley of the Panj River , one of the principal tributaries of the Oxus River . However, "Balascia" itself may be derived from Sanskrit bālasūryaka , which translates as "crimson-coloured morning sun". Mines in the Gorno Badakhshan region of Tajikistan constituted for centuries the main source for red and pink spinels. Spinel is found as

845-437: A variety of size scales. Layered peridotites may form the base layers of layered intrusions. These are characterized by cumulate textures , characterized by a fabric of coarse (>5mm) interlocking euhedral (well-formed) crystals in a groundmass of finer crystals formed from liquid magma trapped in the cumulate. Many show poikilitic texture in which crystallization of this liquid has produced crystals that overgrow and enclose

910-513: A world record for total length for a vertical drilling string of 10,062 m (33,011 ft). The previous record was held by the U.S. vessel Glomar Challenger , which in 1978 drilled to 7,049.5 meters (23,130 feet) below sea level in the Mariana Trench . On 6 September 2012, Scientific deep-sea drilling vessel Chikyū set a new world record by drilling down and obtaining rock samples from deeper than 2,111 metres (6,926 ft) below

975-412: Is a conversion to a more dense mineral structure, the seismic velocity rises abruptly and creates a discontinuity. At the top of the transition zone, olivine undergoes isochemical phase transitions to wadsleyite and ringwoodite . Unlike nominally anhydrous olivine, these high-pressure olivine polymorphs have a large capacity to store water in their crystal structure. This has led to the hypothesis that

1040-441: Is a very thick layer of rock inside the planet, which begins just beneath the crust (at about 10 km (6.2 mi) under the oceans and about 35 km (22 mi) under the continents) and ends at the top of the lower mantle at 670 km (420 mi). Temperatures range from approximately 500 K (227 °C; 440 °F) at the upper boundary with the crust to approximately 1,200 K (930 °C; 1,700 °F) at

1105-466: Is an abrupt increase of P -wave and S -wave velocities at a depth of 220 km (140 mi) (Note that this is a different "Lehmann discontinuity" than the one between the Earth's inner and outer cores labeled in the image on the right.) The transition zone is located between the upper mantle and the lower mantle between a depth of 410 km (250 mi) and 670 km (420 mi). This

1170-416: Is an older term for an ophiolite emplaced in a mountain belt during a continent-continent plate collision. Peridotites also occur as fragments ( xenoliths ) carried up by magmas from the mantle. Among the rocks that commonly include peridotite xenoliths are basalt and kimberlite . Although kimberlite is a variant of peridotite, kimberlite is also considered as brecciated volcanic material as well, which

1235-568: Is because olivine is easily weathered to iddingsite . While green and yellow are the most common colors, peridotitic rocks may exhibit a wide range of colors including blue, brown, and red. Coarse-grained igneous rocks in which mafic minerals (minerals rich in magnesium and iron ) make up over 90% of the volume of the rock are classified as ultramafic rocks . Such rocks typically contain less than 45% silica. Ultramafic rocks are further classified by their relative proportions of olivine , orthopyroxene , clinopyroxene , and hornblende , which are

1300-705: Is found on the walls of rifts in the deep sea floor. Oceanic plates are usually subducted back into the mantle in subduction zones . However, pieces can be emplaced into or overthrust on continental crust by a process called obduction , rather than carried down into the mantle. The emplacement may occur during orogenies , as during collisions of one continent with another or with an island arc . The pieces of oceanic plates emplaced within continental crust are referred to as ophiolites . Typical ophiolites consist mostly of peridotite plus associated rocks such as gabbro , pillow basalt , diabase sill-and-dike complexes, and red chert. Alpine peridotite or orogenic peridotite massif

1365-467: Is generally less than 10 km (6.2 mi) thick. Continental crust is about 35 km (22 mi) thick, but the large crustal root under the Tibetan Plateau is approximately 70 km (43 mi) thick. The thickness of the upper mantle is about 640 km (400 mi). The entire mantle is about 2,900 km (1,800 mi) thick, which means the upper mantle is only about 20% of

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1430-465: Is magnesium. This is reflected in the composition of the mafic minerals making up the peridotite. Olivine is the essential mineral found in all peridotites. It is a magnesium orthosilicate containing some iron with the variable formula (Mg,Fe) 2 SiO 4 . The magnesium-rich olivine of peridotites is typically olive-green in color. Pyroxenes are chain silicates having the variable formula (Ca,Na,Fe,Mg)(Cr,Al,Fe,Mg,Mn,Ti,V)Si 2 O 6 comprising

1495-415: Is plagioclase in the uppermost mantle, then spinel, and then garnet below about 100 kilometres (62 mi). Gradually through the upper mantle, pyroxenes become less stable and transform into majoritic garnet . Experiments on olivines and pyroxenes show that these minerals change the structure as pressure increases at greater depth, which explains why the density curves are not perfectly smooth. When there

1560-401: Is strongly debated, but certainly results from mafic magma interaction with more evolved magma or rock (e.g. gabbro, troctolite). Spinel, (Mg,Fe)(Al,Cr) 2 O 4 , is common in peridotite in the uppermost Earth's mantle , between approximately 20 km to approximately 120 km, possibly to lower depths depending on the chromium content. At significantly shallower depths, above

1625-405: Is the dominant rock of the Earth's mantle above a depth of about 400 km; below that depth, olivine is converted to the higher-pressure mineral wadsleyite . Oceanic plates consist of up to about 100 km of peridotite covered by a thin crust. The crust, commonly about 6 km thick, consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssal peridotite,"

1690-518: Is the most complex discontinuity and marks the boundary between the upper and lower mantle. It appears in PP precursors (a wave that reflects off the discontinuity once) only in certain regions but is always apparent in SS precursors. It is seen as single and double reflections in receiver functions for P to S conversions over a broad range of depths (640–720 km, or 397–447 mi). The Clapeyron slope predicts

1755-429: Is thought to occur as a result of the rearrangement of grains in olivine to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of 670 km (420 mi), due to pressure changes, ringwoodite minerals change into two new denser phases, bridgmanite and periclase. This can be seen using body waves from earthquakes , which are converted, reflected, or refracted at

1820-602: Is usually various shades of red, lavender , blue, green, brown, black, or yellow. Chromium(III) causes the red color in spinel from Burma. Some spinels are among the most famous gemstones; among them are the Black Prince's Ruby and the " Timur ruby " in the British Crown Jewels , and the "Côte de Bretagne", formerly from the French Crown jewels. The Samarian Spinel is the largest known spinel in

1885-399: Is why it is referred to as a source of peridotite xenoliths. Peridotite xenoliths contain osmium and other elements whose stable isotope ratios provide clues on the formation and evolution of the Earth's mantle. Such xenoliths originate from depths of up to nearly 200 kilometers (120 mi) or more. The volcanic equivalent of peridotites are komatiites , which were mostly erupted early in

1950-511: The Moho , calcic plagioclase is the more stable aluminous mineral in peridotite while garnet is the stable phase deeper in the mantle below the spinel stability region. Spinel, (Mg,Fe)Al 2 O 4 , is a common mineral in the Ca-Al-rich inclusions (CAIs) in some chondritic meteorites . Spinel has long been found in the gemstone -bearing gravel of Sri Lanka and in limestones of

2015-832: The platinum used in the world today is mined from the Bushveld Igneous Complex in South Africa and the Great Dyke of Zimbabwe . The chromite bands found in peridotites are the world's major source of chromium . [REDACTED] Media related to Peridotite at Wikimedia Commons Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Upper mantle (Earth) The upper mantle of Earth

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2080-762: The Badakshan Province in modern-day Afghanistan and Tajikistan ; and of Mogok in Myanmar . Over the last decades gem quality spinels are found in the marbles of Lục Yên District ( Vietnam ), Mahenge and Matombo ( Tanzania ), Tsavo ( Kenya ) and in the gravels of Tunduru ( Tanzania ) and Ilakaka ( Madagascar ). Since 2000, in several locations around the world, spinels have been discovered with unusual vivid pink or blue colors. Such "glowing" spinels are known from Mogok (Myanmar), Mahenge plateau (Tanzania), Lục Yên District (Vietnam) and some more localities. In 2018 bright blue spinels have been reported also in

2145-428: The Earth's history and are rare in rocks younger than Archean in age. Small pieces of peridotite have been found in lunar breccias. The rocks of the peridotite family are uncommon at the surface and are highly unstable, because olivine reacts quickly with water at typical temperatures of the upper crust and at the Earth's surface. Many, if not most, surface outcrops have been at least partly altered to serpentinite ,

2210-431: The Earth's surface and outer core and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle. Hot material upwells , while cooler (and heavier) material sinks downward. Downward motion of material occurs at convergent plate boundaries called subduction zones . Locations on

2275-644: The Japanese vessel Chikyū to drill up to 7,000 m (23,000 ft) below the seabed. On 27 April 2012, Chikyū drilled to a depth of 7,740 metres (25,390 ft) below sea level, setting a new world record for deep-sea drilling. This record has since been surpassed by the ill-fated Deepwater Horizon mobile offshore drilling unit, operating on the Tiber prospect in the Mississippi Canyon Field, United States Gulf of Mexico, when it achieved

2340-492: The Latin word spinella , a diminutive form of spine, in reference to its pointed crystals. Spinel crystallizes in the isometric system; common crystal forms are octahedra , usually twinned . It has no true cleavage , but shows an octahedral parting and a conchoidal fracture . Its hardness is 8, its specific gravity is 3.5–4.1, and it is transparent to opaque with a vitreous to dull luster . It may be colorless, but

2405-417: The base of the transition zone, ringwoodite decomposes into bridgmanite (formerly called magnesium silicate perovskite), and ferropericlase . Garnet also becomes unstable at or slightly below the base of the transition zone. Kimberlites explode from the earth's interior and sometimes carry rock fragments. Some of these xenolithic fragments are diamonds that can only come from the higher pressures below

2470-429: The boundary with the lower mantle. Upper mantle material that has come up onto the surface comprises about 55% olivine , 35% pyroxene , and 5 to 10% of calcium oxide and aluminum oxide minerals such as plagioclase , spinel , or garnet , depending upon depth. The density profile through Earth is determined by the velocity of seismic waves. Density increases progressively in each layer, largely due to compression of

2535-557: The boundary, and predicted from mineral physics , as the phase changes are temperature and density-dependent and hence depth-dependent. A single peak is seen in all seismological data at 410 km (250 mi), which is predicted by the single transition from α- to β- Mg 2 SiO 4 (olivine to wadsleyite ). From the Clapeyron slope this discontinuity is expected to be shallower in cold regions, such as subducting slabs, and deeper in warmer regions, such as mantle plumes . This

2600-474: The core-mantle boundary. The highest temperature of the upper mantle is 1,200 K (930 °C; 1,700 °F). Although the high temperature far exceeds the melting points of the mantle rocks at the surface, the mantle is almost exclusively solid. The enormous lithostatic pressure exerted on the mantle prevents melting because the temperature at which melting begins (the solidus ) increases with pressure. Pressure increases as depth increases since

2665-423: The crust. The rocks that come with this are ultramafic nodules and peridotite. The composition seems to be very similar to the crust. One difference is that rocks and minerals of the mantle tend to have more magnesium and less silicon and aluminum than the crust. The first four most abundant elements in the upper mantle are oxygen, magnesium, silicon, and iron. Exploration of the mantle is generally conducted at

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2730-715: The deformation associated with their tectonic mode of emplacement. Peridotites have two primary modes of origin: as mantle rocks formed during the accretion and differentiation of the Earth, or as cumulate rocks formed by precipitation of olivine ± pyroxenes from basaltic or ultramafic magmas. These magmas are ultimately derived from the upper mantle by partial melting of mantle peridotites. Mantle peridotites are sampled as ophiolites in collisional mountain ranges, as xenoliths in basalt or kimberlite, or as abyssal peridotites (sampled from ocean floor). These rocks represent either fertile mantle (lherzolite) or partially depleted mantle (harzburgite, dunite). Alpine peridotites may be either of

2795-418: The magnesium. The most important clinopyroxene is diopside , CaMgSi 2 O 6 , again with some substitution of iron for magnesium ( hedenbergite , FeCaSi 2 O 6 ). Ultramafic rock in which the fraction of pyroxenes exceeds 60% are classified as pyroxenites rather than peridotites. Pyroxenes are typically dark in color. Hornblende is an amphibole , a group of minerals resembling pyroxenes but with

2860-451: The mantle is defined by a sudden increase in the speed of seismic waves, which Andrija Mohorovičić first noted in 1909; this boundary is now referred to as the Mohorovičić discontinuity or "Moho." The Moho defines the base of the crust and varies from 10 km (6.2 mi) to 70 km (43 mi) below the surface of the Earth. Oceanic crust is thinner than continental crust and

2925-611: The material beneath has to support the weight of all the material above it. The entire mantle is thought to deform like a fluid on long timescales, with permanent plastic deformation. The highest pressure of the upper mantle is 24.0 GPa (237,000 atm) compared to the bottom of the mantle, which is 136 GPa (1,340,000 atm). Estimates for the viscosity of the upper mantle range between 10 and 10 Pa·s , depending on depth, temperature, composition, state of stress, and numerous other factors. The upper mantle can only flow very slowly. However, when large forces are applied to

2990-444: The most abundant families of mafic minerals in most ultramafic rocks. Peridotite is then defined as coarse-grained ultramafic rock in which olivine makes up 40% or more of the total volume of these four mineral families in the rock. Peridotites are further classified as follows: Mantle peridotite is highly enriched in magnesium, with a typical magnesium number of 89. In other words, of the total content of iron plus magnesium, 89 mol%

3055-448: The ophiolite association and representing the uppermost mantle below ocean basins, or masses of subcontinental mantle emplaced along thrust faults in mountain belts. Layered peridotites are igneous sediments and form by mechanical accumulation of dense olivine crystals. They form from mantle-derived magmas, such as those of basalt composition. Peridotites associated with Alaskan-type ultramafic complexes are cumulates that probably formed in

3120-406: The original cumulus crystals (called chadrocrysts ). Another texture is a well-annealed texture of equal sized anhedral crystals with straight grain boundaries intersecting at 120°. This may result when slow cooling allowed recrystallization to minimize surface energy. Cataclastic texture, showing irregular fractures and deformation twinning of olivine grains, is common in peridotites because of

3185-408: The rock at increased depths. Abrupt changes in density occur where the material composition changes. The upper mantle begins just beneath the crust and ends at the top of the lower mantle. The upper mantle causes the tectonic plates to move. Crust and mantle are distinguished by composition, while the lithosphere and asthenosphere are defined by a change in mechanical properties. The top of

3250-593: The root zones of volcanoes. Cumulate peridotites are also formed in komatiite lava flows. Komatiites are high temperature partial melts of peridotite characterized by a high degree of partial melting deep below the surface. Eclogite , a rock similar to basalt in composition, is composed primarily of omphacite (sodic clinopyroxene) and pyrope -rich garnet. Eclogite is associated with peridotite in some xenolith occurrences; it also occurs with peridotite in rocks metamorphosed at high pressures during processes related to subduction. Peridotite may potentially be used in

3315-402: The samples offer a much closer analogue to mantle rock than magmatic xenoliths as the sampled rock never melted into magma or recrystallized. Spinel Spinel ( / s p ɪ ˈ n ɛ l , ˈ s p ɪ n əl / ) is the magnesium / aluminium member of the larger spinel group of minerals. It has the formula MgAl 2 O 4 in the cubic crystal system . Its name comes from

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3380-413: The seabed rather than on land because of the oceanic crust's relative thinness as compared to the significantly thicker continental crust. The first attempt at mantle exploration, known as Project Mohole , was abandoned in 1966 after repeated failures and cost overruns. The deepest penetration was approximately 180 m (590 ft). In 2005 an oceanic borehole reached 1,416 metres (4,646 ft) below

3445-747: The seafloor from the ocean drilling vessel JOIDES Resolution . On 5 March 2007, a team of scientists on board the RRS James Cook embarked on a voyage to an area of the Atlantic seafloor where the mantle lies exposed without any crust covering, midway between the Cape Verde Islands and the Caribbean Sea . The exposed site lies approximately 3 kilometres (1.9 mi) beneath the ocean surface and covers thousands of square kilometers. The Chikyu Hakken mission attempted to use

3510-553: The seafloor off the Shimokita Peninsula of Japan in the northwest Pacific Ocean. A novel method of exploring the uppermost few hundred kilometers of the Earth was proposed in 2005, consisting of a small, dense, heat-generating probe that melts its way down through the crust and mantle while its position and progress are tracked by acoustic signals generated in the rocks. The probe consists of an outer sphere of tungsten about 1 metre (3 ft 3 in) in diameter with

3575-421: The serpentine group, chrysotile , is a type of asbestos. Most peridotite is green in color due to its high olivine content. However, peridotites can range in color from greenish-gray to nearly black to pale yellowish-green. Peridotite weathers to form a distinctive brown crust in subaerial exposures and to a deep orange color in submarine exposures. Peridotites can take on a massive form or may be in layers on

3640-516: The southern part of Baffin Island (Canada). The pure blue coloration of spinel is caused by small additions of cobalt . Synthetic spinel can be produced by similar means to synthetic corundum, including the Verneuil method and the flux method pioneered by Edmond Frémy . It is widely used as an inexpensive cut gem in birthstone jewelry for the month of August . Light blue synthetic spinel

3705-788: The subsurface peridotite formation. Peridotite is named for the gemstone peridot , a glassy green gem originally mined on St. John's Island in the Red Sea and now mined on the San Carlos Apache Indian Reservation in Arizona. Peridotite that has been hydrated at low temperatures is the protolith for serpentinite , which may include chrysotile asbestos (a form of serpentine) and talc . Layered intrusions with cumulate peridotite are typically associated with sulfide or chromite ores. Sulfides associated with peridotites form nickel ores and platinoid metals; most of

3770-403: The surface that lie over plumes are predicted to have high elevation (because of the buoyancy of the hotter, less-dense plume beneath) and to exhibit hot spot volcanism . The seismic data is not sufficient to determine the composition of the mantle. Observations of rocks exposed on the surface and other evidence reveal that the upper mantle is mafic minerals olivine and pyroxene, and it has

3835-472: The total mantle thickness. The boundary between the upper and lower mantle is a 670 km (420 mi) discontinuity. Earthquakes at shallow depths result from strike-slip faulting ; however, below about 50 km (31 mi), the hot, high-pressure conditions inhibit further seismicity. The mantle is viscous and incapable of faulting . However, in subduction zones , earthquakes are observed down to 670 km (420 mi). The Lehmann discontinuity

3900-403: The transition of olivine (β to γ) and garnet in the pyrolite mantle. This one has only sporadically been observed in seismological data. Other non-global phase transitions have been suggested at a range of depths. Temperatures range from approximately 500 K (227 °C; 440 °F) at the upper boundary with the crust to approximately 4,200 K (3,930 °C; 7,100 °F) at

3965-510: The transition zone may host a large quantity of water. In Earth's interior, olivine occurs in the upper mantle at depths less than 410 kilometres (250 mi), and ringwoodite is inferred within the transition zone from about 520 to 670 kilometres (320 to 420 mi) depth. Seismic activity discontinuities at about 410 kilometres (250 mi), 520 kilometres (320 mi), and 670 kilometres (420 mi) depth have been attributed to phase changes involving olivine and its polymorphs . At

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4030-474: The upper mantle after drilling only a few hundred meters into the Atlantis Massif . The borehole reached a maximum depth of 1,268 meters and recovered 886 meters of rock samples consisting of primarily peridotite . There is debate over the extent to which the samples represent the upper mantle with some arguing the effects of seawater on the samples situates them as examples of deep lower crust. However,

4095-420: The uppermost mantle, it can become weaker, and this effect is thought to be important in allowing the formation of tectonic plate boundaries. Although there is a tendency to larger viscosity at greater depth, this relation is far from linear and shows layers with dramatically decreased viscosity, in particular in the upper mantle and at the boundary with the core. Because of the temperature difference between

4160-430: The world, weighing 500 carats (100 g). The transparent red spinels were called spinel-rubies or balas rubies. In the past, before the arrival of modern science, spinels and rubies were equally known as rubies. After the 18th century, the word ruby was only used for the red gem variety of the mineral corundum , and the word spinel came to be used. "Balas" is derived from Balascia, the ancient name for Badakhshan ,

4225-485: Was formed, and so they are of special interest to paleogeologists because they provide clues to the early composition of Earth's mantle and the complexities of the processes that occurred. The word peridotite comes from the gemstone peridot , which consists of pale green olivine. Classic peridotite is bright green with some specks of black, although most hand samples tend to be darker green. Peridotitic outcrops typically range from earthy bright yellow to dark green; this

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