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90-523: Volcanic rocks are among the most common rock types on Earth's surface, particularly in the oceans. On land, they are very common at plate boundaries and in flood basalt provinces . It has been estimated that volcanic rocks cover about 8% of the Earth's current land surface. Volcanic rocks are usually fine-grained or aphanitic to glass in texture. They often contain clasts of other rocks and phenocrysts . Phenocrysts are crystals that are larger than

180-429: A black glassy substance called obsidian . When filled with bubbles of gas, the same lava may form the spongy appearing pumice . Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite. The lavas, having cooled rapidly in contact with the air or water, are mostly finely crystalline or have at least fine-grained ground-mass representing that part of the viscous semi-crystalline lava flow that

270-582: A body of mantle rocks on Zabargad Island in the Red Sea . Olivine occurs in both mafic and ultramafic igneous rocks and as a primary mineral in certain metamorphic rocks . Mg-rich olivine crystallizes from magma that is rich in magnesium and low in silica . That magma crystallizes to mafic rocks such as gabbro and basalt . Ultramafic rocks usually contain substantial olivine, and those with an olivine content of over 40% are described as peridotites . Dunite has an olivine content of over 90% and

360-495: A cheap way to sequester CO 2 . The presence of iddingsite on Mars would suggest that liquid water once existed there, and might enable scientists to determine when there was last liquid water on the planet. Because of its rapid weathering, olivine is rarely found in sedimentary rock . Norway is the main source of olivine in Europe, particularly in an area stretching from Åheim to Tafjord , and from Hornindal to Flemsøy in

450-409: A close analogy to hot solutions of salts in water, which, when they approach the saturation temperature, first deposit a crop of large, well-formed crystals (labile stage) and subsequently precipitate clouds of smaller less perfect crystalline particles (metastable stage). In igneous rocks the first generation of crystals generally forms before the lava has emerged to the surface, that is to say, during

540-454: A cubic km per day per km of fissure length ) and the relatively low viscosity of basaltic lava. However, the lateral extent of individual flood basalt flows is astonishing even for so fluid a lava in such quantities. It is likely that the lava spreads by a process of inflation in which the lava moves beneath a solid insulating crust, which keeps it hot and mobile. Studies of the Ginkgo flow of

630-476: A dead zone. However, not all large igneous provinces are connected with extinction events. The formation and effects of a flood basalt depend on a range of factors, such as continental configuration, latitude, volume, rate, duration of eruption, style and setting (continental vs. oceanic), the preexisting climate , and the biota resilience to change. Representative continental flood basalts and oceanic plateaus, arranged by chronological order, together forming

720-566: A depth of about 410 km (250 mi) within Earth. Because it is thought to be the most abundant mineral in Earth's mantle at shallower depths, the properties of olivine have a dominant influence upon the rheology of that part of Earth and hence upon the solid flow that drives plate tectonics . Experiments have documented that olivine at high pressures (12  GPa , the pressure at depths of about 360 km (220 mi)) can contain at least as much as about 8900 parts per million (weight) of water, and that such water content drastically reduces

810-446: A depth of about 660 km (410 mi), ringwoodite decomposes into silicate perovskite ( (Mg,Fe)SiO 3 ) and ferropericlase ( (Mg,Fe)O ) in an endothermic reaction. These phase transitions lead to a discontinuous increase in the density of the Earth's mantle that can be observed by seismic methods. They are also thought to influence the dynamics of mantle convection in that the exothermic transitions reinforce flow across

900-411: A general position. At the high temperatures and pressures found at depth within the Earth the olivine structure is no longer stable. Below depths of about 410 km (250 mi) olivine undergoes an exothermic phase transition to the sorosilicate , wadsleyite and, at about 520 km (320 mi) depth, wadsleyite transforms exothermically into ringwoodite , which has the spinel structure. At

990-458: A good supplier will know what sort of volcanic rock they are selling. The sub-family of rocks that form from volcanic lava are called igneous volcanic rocks (to differentiate them from igneous rocks that form from magma below the surface, called igneous plutonic rocks ). The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava-stream cools quickly, it can quickly freeze into

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1080-431: A hexagonal, close-packed array of oxygen ions with half of the octahedral sites occupied with magnesium or iron ions and one-eighth of the tetrahedral sites occupied by silicon ions. There are three distinct oxygen sites (marked O1, O2 and O3 in figure 1), two distinct metal sites (M1 and M2) and only one distinct silicon site. O1, O2, M2 and Si all lie on mirror planes , while M1 exists on an inversion center. O3 lies in

1170-602: A listing of large igneous provinces : Flood basalts are the dominant form of magmatism on the other planets and moons of the Solar System. The maria on the Moon have been described as flood basalts composed of picritic basalt. Individual eruptive episodes were likely similar in volume to flood basalts of Earth, but were separated by much longer quiescent intervals and were likely produced by different mechanisms. Extensive flood basalts are present on Mars. Trap rock

1260-459: A paramorph of augite and magnetite, which may partially or completely substitute for the original crystal but still retains its characteristic outlines. The mechanical behaviour of volcanic rocks is complicated by their complex microstructure. For example, attributes such as the partitioning of the void space (pores and microcracks), pore and crystal size and shape, and hydrothermal alteration can all vary widely in volcanic rocks and can all influence

1350-592: A result of traces of nickel , though it may alter to a reddish color from the oxidation of iron. Translucent olivine is sometimes used as a gemstone called peridot ( péridot , the French word for olivine). It is also called chrysolite (or chrysolithe , from the Greek words for gold and stone), though this name is now rarely used in the English language. Some of the finest gem-quality olivine has been obtained from

1440-563: A silica content of around 52%. The magnesium number (the mol% of magnesium out of the total iron and magnesium content) is around 55, versus 60 for a typical MORB. The rare earth elements show abundance patterns suggesting that the original (primitive) magma formed from rock of the Earth's mantle that was nearly undepleted ; that is, it was mantle rock rich in garnet and from which little magma had previously been extracted. The chemistry of plagioclase and olivine in flood basalts suggests that

1530-520: A single province. For example, the flood basalts of the Parana Basin can be divided into a low phosphorus and titanium group (LPT) and a high phosphorus and titanium group (HPT). The difference has been attributed to inhomogeneity in the upper mantle, but strontium isotope ratios suggest the difference may arise from the LPT magma being contaminated with a greater amount of melted crust. Theories of

1620-473: A term particularly used in the quarry industry. The great thickness of the basalt accumulations, often in excess of 1,000 meters (3,000 ft), usually reflects a very large number of thin flows, varying in thickness from meters to tens of meters, or more rarely to 100 meters (330 ft). There are occasionally very thick individual flows. The world's thickest basalt flow may be the Greenstone flow of

1710-656: Is a common mineral in Earth's subsurface, but weathers quickly on the surface. Olivine has many uses, such as the gemstone peridot (or chrysolite), as well as industrial applications like metalworking processes. The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember: Mg 2 Si O 4 ) and fayalite (Fe-endmember: Fe 2 Si O 4 ). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and/or fayalite (Fa) ( e.g. , Fo 70 Fa 30 , or just Fo 70 with Fa 30 implied). Forsterite's melting temperature

1800-401: Is a molten liquid and rock is solid). "Lava stone" may describe anything from a friable silicic pumice to solid mafic flow basalt, and is sometimes used to describe rocks that were never lava , but look as if they were (such as sedimentary limestone with dissolution pitting ). To convey anything about the physical or chemical properties of the rock, a more specific term should be used;

1890-496: Is comparable with the rate of extrusion of lava at mid-ocean ridges and much higher than the rate of extrusion by hotspots. However, extrusion at mid-ocean ridges is relatively steady, while extrusion of flood basalts is highly episodic. Flood basalts create new continental crust at a rate of 0.1 to 8 cubic kilometers (0.02 to 2 cu mi) per year, while the eruptions that form oceanic plateaus produce 2 to 20 cubic kilometers (0.5 to 5 cu mi) of crust per year. Much of

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1980-411: Is dependent on two things: the initial composition of the primary magma and the subsequent differentiation. Differentiation of most magmas tends to increase the silica ( SiO 2 ) content, mainly by crystal fractionation . The initial composition of most magmas is basaltic , albeit small differences in initial compositions may result in multiple differentiation series. The most common of these series are

2070-561: Is episodic, and each episode has its own chemical signature. There is some tendency for lava within a single eruptive episode to become more silica-rich with time, but there is no consistent trend across episodes. Large Igneous Provinces (LIPs) were originally defined as voluminous outpourings, predominantly of basalt, over geologically very short durations. This definition did not specify minimum size, duration, petrogenesis, or setting. A new attempt to refine classification focuses on size and setting. LIPs characteristically cover large areas, and

2160-474: Is likely a cumulate formed by olivine crystallizing and settling out of magma or a vein mineral lining magma conduits. Olivine and high pressure structural variants constitute over 50% of the Earth's upper mantle, and olivine is one of the Earth's most common minerals by volume. The metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content also produces Mg-rich olivine, or forsterite . Fe-rich olivine fayalite

2250-477: Is reflected in several local names with "red" such as Raudbergvik (Red rock bay) or Raudnakken (Red ridge). Hans Strøm in 1766 described the olivine's typical red color on the surface and the blue color within. Strøm wrote that in Norddal district large quantities of olivine were broken from the bedrock and used as sharpening stones . Kallskaret near Tafjord is a nature reserve with olivine. Olivine

2340-410: Is relatively much less common, but it occurs in igneous rocks in small amounts in rare granites and rhyolites , and extremely Fe-rich olivine can exist stably with quartz and tridymite . In contrast, Mg-rich olivine does not occur stably with silica minerals, as it would react with them to form orthopyroxene ( (Mg,Fe) 2 Si 2 O 6 ). Mg-rich olivine is stable to pressures equivalent to

2430-419: Is required for so much magma to be generated in so short a time. This is widely believed to have been supplied by a mantle plume impinging on the base of the Earth's lithosphere , its rigid outermost shell. The plume consists of unusually hot mantle rock of the asthenosphere , the ductile layer just below the lithosphere, that creeps upwards from deeper in the Earth's interior. The hot asthenosphere rifts

2520-405: Is the most durable construction aggregate of all rock types, because the interlocking crystals are oriented at random. Olivine The mineral olivine ( / ˈ ɒ l . ɪ ˌ v iː n / ) is a magnesium iron silicate with the chemical formula ( Mg , Fe ) 2 Si O 4 . It is a type of nesosilicate or orthosilicate . The primary component of the Earth's upper mantle , it

2610-539: Is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava . Many flood basalts have been attributed to the onset of a hotspot reaching the surface of the Earth via a mantle plume . Flood basalt provinces such as the Deccan Traps of India are often called traps , after the Swedish word trappa (meaning "staircase"), due to

2700-473: Is uncommon in flood basalt provinces. One possibility is that a primitive melt stagnates when it reaches the mantle-crust boundary, where it is not buoyant enough to penetrate the lower-density crust rock. As a tholeiitic magma differentiates (changes in composition as high-temperature minerals crystallize and settle out of the magma) its density reaches a minimum at a magnesium number of about 60, similar to that of flood basalts. This restores buoyancy and permits

2790-418: Is unusually high at atmospheric pressure, almost 1,900 °C (3,450 °F), while fayalite's is much lower – about 1,200 °C (2,190 °F). Melting temperature varies smoothly between the two endmembers, as do other properties. Olivine incorporates only minor amounts of elements other than oxygen (O), silicon (Si), magnesium (Mg) and iron (Fe). Manganese (Mn) and nickel (Ni) commonly are

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2880-558: Is used as a substitute for dolomite in steel works. The aluminium foundry industry uses olivine sand to cast objects in aluminium. Olivine sand requires less water than silica sands while still holding the mold together during handling and pouring of the metal. Less water means less gas (steam) to vent from the mold as metal is poured into the mold. In Finland, olivine is marketed as an ideal rock for sauna stoves because of its comparatively high density and resistance to weathering under repeated heating and cooling. Gem-quality olivine

2970-653: The Keweenaw Peninsula of Michigan , US, which is 600 meters (2,000 ft) thick. This flow may have been part of a lava lake the size of Lake Superior . Deep erosion of flood basalts exposes vast numbers of parallel dikes that fed the eruptions. Some individual dikes in the Columbia River Plateau are over 100 kilometers (60 mi) long. In some cases, erosion exposes radial sets of dikes with diameters of several thousand kilometers. Sills may also be present beneath flood basalts, such as

3060-478: The Palisades Sill of New Jersey , US. The sheet intrusions (dikes and sills) beneath flood basalts are typically diabase that closely matches the composition of the overlying flood basalts. In some cases, the chemical signature allows individual dikes to be connected with individual flows. Flood basalt commonly displays columnar jointing , formed as the rock cooled and contracted after solidifying from

3150-861: The Permian-Triassic boundary, the Triassic-Jurassic boundary, and in the Toarcian Age of the Jurassic correspond to the ages of large igneous provinces in Siberia, the Central Atlantic Magmatic Province, and the Karoo-Ferrar flood basalt. Some idea of the impact of flood basalts can be given by comparison with historical large eruptions. The 1783 eruption of Lakagígar was the largest in

3240-737: The Sunnmøre district. There is also olivine in Eid municipality. About 50% of the world's olivine for industrial use is produced in Norway. At Svarthammaren in Norddal olivine was mined from around 1920 to 1979, with a daily output up to 600 metric tons. Olivine was also obtained from the construction site of the hydro power stations in Tafjord. At Robbervika in Norddal municipality an open-pit mine has been in operation since 1984. The characteristic red color

3330-566: The matrix and are identifiable with the unaided eye . Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a very fine grained matrix. Volcanic rocks often have a vesicular texture caused by voids left by volatiles trapped in the molten lava . Pumice is a highly vesicular rock produced in explosive volcanic eruptions . Most modern petrologists classify igneous rocks, including volcanic rocks, by their chemistry when dealing with their origin. The fact that different mineralogies and textures may be developed from

3420-473: The orthorhombic P Bravais lattice , which arise from each silica (SiO 4 ) unit being joined by metal divalent cations with each oxygen in SiO 4 bound to three metal ions. It has a spinel -like structure similar to magnetite but uses one quadrivalent and two divalent cations M 2 M O 4 instead of two trivalent and one divalent cations. Olivine is named for its typically olive-green color, thought to be

3510-407: The pyroxenes . Bowen's reaction series correctly predicts the order of formation of the most common minerals in volcanic rocks. Occasionally, a magma may pick up crystals that crystallized from another magma; these crystals are called xenocrysts . Diamonds found in kimberlites are rare but well-known xenocrysts; the kimberlites do not create the diamonds, but pick them up and transport them to

3600-560: The tholeiitic , calc-alkaline , and alkaline . Most volcanic rocks share a number of common minerals . Differentiation of volcanic rocks tends to increase the silica (SiO 2 ) content mainly by fractional crystallization . Thus, more evolved volcanic rocks tend to be richer in minerals with a higher amount of silica such as phyllo and tectosilicates including the feldspars, quartz polymorphs and muscovite . While still dominated by silicates, more primitive volcanic rocks have mineral assemblages with less silica, such as olivine and

3690-772: The Atlantic Ocean, formed around 125 million years ago as the South Atlantic opened, while a second set of smaller flood basalts formed near the Triassic-Jurassic boundary in eastern North America as the North Atlantic opened. However, the North Atlantic flood basalts are not connected with any hot spot traces, but seem to have been evenly distributed along the entire divergent boundary. Flood basalts are often interbedded with sediments, typically red beds . The deposition of sediments begins before

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3780-472: The Caribbean, Nauru, East Mariana, and Pigafetta provinces. Continental flood basalts (CFBs) or plateau basalts are the continental expressions of large igneous provinces. Flood basalts contribute significantly to the growth of continental crust. They are also catastrophic events, which likely contributed to many mass extinctions in the geologic record. The extrusion of flood basalts, averaged over time,

3870-485: The Columbia River Plateau, which is 30 to 70 meters (98 to 230 ft) thick, show that the temperature of the lava dropped by just 20 °C (68 °F) over a distance of 500 kilometers (310 mi). This demonstrates that the lava must have been insulated by a surface crust and that the flow was laminar , reducing heat exchange with the upper crust and base of the flow. It has been estimated that

3960-599: The Earth's surface with lava on a regional scale. These vast accumulations of flood basalt constitute large igneous provinces . These are characterized by plateau landforms, so that flood basalts are also described as plateau basalts . Canyons cut into the flood basalts by erosion display stair-like slopes, with the lower parts of flows forming cliffs and the upper part of flows or interbedded layers of sediments forming slopes. These are known in Dutch as trap or in Swedish as trappa , which has come into English as trap rock ,

4050-452: The Ginkgo flow advanced 500 km in six days (a rate of advance of about 3.5 km per hour). The lateral extent of a flood basalt flow is roughly proportional to the cube of the thickness of the flow near its source. Thus, a flow that is double in thickness at its source can travel roughly eight times as far. Flood basalt flows are predominantly pāhoehoe flows, with ʻaʻā flows much less common. Eruption in flood basalt provinces

4140-611: The TAS diagram are further subdivided by the ratio of potassium oxide to sodium oxide. Additional classifications may be made on the basis of other components, such as aluminum or iron content. Volcanic rocks are also broadly divided into subalkaline, alkaline, and peralkaline volcanic rocks. Subalkaline rocks are defined as rocks in which SiO 2 < -3.3539 × 10 × A + 1.2030 × 10 × A - 1.5188 × 10 × A + 8.6096 × 10 × A - 2.1111 × A + 3.9492 × A + 39.0 where both silica and total alkali oxide content (A) are expressed as molar fraction . Because

4230-434: The TAS diagram uses weight fraction and the boundary between alkaline and subalkaline rock is defined in terms of molar fraction, the position of this curve on the TAS diagram is only approximate. Peralkaline volcanic rocks are defined as rocks having Na 2 O + K 2 O > Al 2 O 3 , so that some of the alkali oxides must be present as aegirine or sodic amphibole rather than feldspar . The chemistry of volcanic rocks

4320-401: The additional elements present in highest concentrations. Olivine gives its name to the group of minerals with a related structure (the olivine group ) – which includes tephroite ( Mn 2 SiO 4 ), monticellite ( Ca MgSiO 4 ), larnite (Ca 2 SiO 4 ) and kirschsteinite (CaFeSiO 4 ) (commonly also spelled kirschteinite ). Olivine's crystal structure incorporates aspects of

4410-428: The ascent from the subterranean depths to the crater of the volcano. It has frequently been verified by observation that freshly emitted lavas contain large crystals borne along in a molten, liquid mass. The large, well-formed, early crystals ( phenocrysts ) are said to be porphyritic ; the smaller crystals of the surrounding matrix or ground-mass belong to the post-effusion stage. More rarely lavas are completely fused at

4500-404: The bottom third of the flow. The greater hydrostatic pressure, due to the weight of overlying rock, also contributes to making the lower columns larger. By analogy with Greek temple architecture, the more regular lower columns are described as the colonnade and the more irregular upper fractures as the entablature of the individual flow. Columns tend to be larger in thicker flows, with columns of

4590-465: The characteristic stairstep geomorphology of many associated landscapes. Michael R. Rampino and Richard Stothers (1988) cited eleven distinct flood basalt episodes occurring in the past 250 million years, creating large igneous provinces , lava plateaus , and mountain ranges . However, more have been recognized such as the large Ontong Java Plateau , and the Chilcotin Group , though

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4680-794: The crust, covering an area of 5 million square kilometres (1.9 million square miles), equal to 62% of the area of the contiguous states of the United States. The hot magma contained vast quantities of carbon dioxide and sulfur oxides , and released additional carbon dioxide and methane from deep petroleum reservoirs and younger coal beds in the region. The released gases created over 6400 diatreme -like pipes , each typically over 1.6 kilometres (1 mi) in diameter. The pipes emitted up to 160 trillion tons of carbon dioxide and 46 trillion tons of methane. Coal ash from burning coal beds spread toxic chromium , arsenic , mercury , and lead across northern Canada. Evaporite beds heated by

4770-432: The crystal. It is clear that after the mineral had crystallized it was partly again dissolved or corroded at some period before the matrix solidified. Corroded phenocrysts of biotite and hornblende are very common in some lavas; they are surrounded by black rims of magnetite mixed with pale green augite. The hornblende or biotite substance has proved unstable at a certain stage of consolidation, and has been replaced by

4860-444: The development of a large igneous province and marks a shift to more centralized volcanism. Flood basalts show a considerable degree of chemical uniformity across geologic time, being mostly iron-rich tholeiitic basalts. Their major element chemistry is similar to mid-ocean ridge basalts (MORBs), while their trace element chemistry, particularly of the rare earth elements , resembles that of ocean island basalt . They typically have

4950-401: The early Solar System ; and pallasites , mixes of iron-nickel and olivine. The rare A-type asteroids are suspected to have a surface dominated by olivine. The spectral signature of olivine has been seen in the dust disks around young stars. The tails of comets (which formed from the dust disk around the young Sun ) often have the spectral signature of olivine, and the presence of olivine

5040-418: The first flood basalt eruptions, so that subsidence and crustal thinning are precursors to flood basalt activity. The surface continues to subside as basalt erupt, so that the older beds are often found below sea level. Basalt strata at depth ( dipping reflectors ) have been found by reflection seismology along passive continental margins. The composition of flood basalts may reflect the mechanisms by which

5130-487: The flows are very homogeneous and rarely contain xenoliths , fragments of the surrounding rock ( country rock ) that have been entrained in the lava. Because the lavas are low in dissolved gases, pyroclastic rock is extremely rare. Except where the flows entered lakes and became pillow lava , the flows are massive (featureless). Occasionally, flood basalts are associated with very small volumes of dacite or rhyolite (much more silica-rich volcanic rock), which forms late in

5220-407: The formation of flood basalts must explain how such vast amounts of magma could be generated and erupted as lava in such short intervals of time. They must also explain the similar compositions and tectonic settings of flood basalts erupted across geologic time and the ability of flood basalt lava to travel such great distances from the eruptive fissures before solidifying. A tremendous amount of heat

5310-410: The great bulk of the magmatism occurs in less than 1 Ma. Principal LIPs in the ocean basins include Oceanic Volcanic Plateaus (OPs) and Volcanic Passive Continental Margins . Oceanic flood basalts are LIPs distinguished from oceanic plateaus by some investigators because they do not form morphologic plateaus, being neither flat-topped nor elevated more than 200 m above the seafloor. Examples include

5400-404: The ground-mass . Microscopic examination of the phenocrysts often reveals that they have had a complex history. Very frequently they show layers of different composition, indicated by variations in color or other optical properties; thus augite may be green in the center surrounded by various shades of brown; or they may be pale green centrally and darker green with strong pleochroism (aegirine) at

5490-640: The historical record, killing 75% of the livestock and a quarter of the population of Iceland. However, the eruption produced just 14 cubic kilometers (3.4 cu mi) of lava, which is tiny compared with the Roza Member of the Columbia River Plateau, erupted in the mid- Miocene , which contained at least 1,500 cubic kilometers (360 cu mi) of lava. During the eruption of the Siberian Traps , some 5 to 16 million cubic kilometers (1.2 to 3.8 million cubic miles) of magma penetrated

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5580-572: The huge volumes of lava erupted in geologically short time intervals. A single flood basalt province may contain hundreds of thousands of cubic kilometers of basalt erupted over less than a million years, with individual events each erupting hundreds of cubic kilometers of basalt. This highly fluid basalt lava can spread laterally for hundreds of kilometers from its source vents, covering areas of tens of thousands of square kilometers. Successive eruptions form thick accumulations of nearly horizontal flows, erupted in rapid succession over vast areas, flooding

5670-411: The iron content decreases the pressure of the phase transition and narrows the wadsleyite stability field. At about 0.8 mole fraction fayalite, olivine transforms directly to ringwoodite over the pressure range 10.0 to 11.5 GPa (99,000–113,000 atm). Fayalite transforms to Fe 2 SiO 4 spinel at pressures below 5 GPa (49,000 atm). Increasing the temperature increases

5760-541: The latter may be linked to the Columbia River Basalt Group . Large igneous provinces have been connected to five mass extinction events, and may be associated with bolide impacts. Flood basalts are the most voluminous of all extrusive igneous rocks , forming enormous deposits of basaltic rock found throughout the geologic record. They are a highly distinctive form of intraplate volcanism , set apart from all other forms of volcanism by

5850-403: The lava prior to its being erupted to the surface, which are often present in other extrusive igneous rocks. Phenocrysts are more abundant in the dikes that fed lava to the surface. Flood basalts are most often quartz tholeiites . Olivine tholeiite (the characteristic rock of mid-ocean ridges ) occurs less commonly, and there are rare cases of alkali basalts . Regardless of composition,

5940-419: The lava. The rock fractures into columns, typically with five to six sides, parallel to the direction of heat flow out of the rock. This is generally perpendicular to the upper and lower surfaces, but rainwater infiltrating the rock unevenly can produce "cold fingers" of distorted columns. Because heat flow out of the base of the flow is slower than from its upper surface, the columns are more regular and larger in

6030-784: The lithosphere above the plume, allowing magma produced by decompressional melting of the plume head to find pathways to the surface. The swarms of parallel dikes exposed by deep erosion of flood basalts show that considerable crustal extension has taken place. The dike swarms of west Scotland and Iceland show extension of up to 5%. Many flood basalts are associated with rift valleys, are located on passive continental plate margins, or extend into aulacogens (failed arms of triple junctions where continental rifting begins.) Flood basalts on continents are often aligned with hotspot volcanism in ocean basins. The Paraná and Etendeka traps , located in South America and Africa on opposite sides of

6120-436: The magma reaches the surface. The original melt formed in the upper mantle (the primitive melt ) cannot have the composition of quartz tholeiite, the most common and typically least evolved volcanic rock of flood basalts, because quartz tholeiites are too rich in iron relative to magnesium to have formed in equilibrium with typical mantle rock. The primitive melt may have had the composition of picrite basalt , but picrite basalt

6210-694: The magma released hydrochloric acid , methyl chloride , methyl bromide , which damaged the ozone layer and reduced ultraviolet shielding by as much as 85%. Over 5 trillion tons of sulfur dioxide was also released. The carbon dioxide produced extreme greenhouse conditions, with global average sea water temperatures peaking at 38 °C (100 °F), the highest ever seen in the geologic record. Temperatures did not drop to 32 °C (90 °F) for another 5.1 million years. Temperatures this high are lethal to most marine organisms, and land plants have difficulty continuing to photosynthesize at temperatures above 35 °C (95 °F). The Earth's equatorial zone became

6300-460: The magma to complete its journey to the surface, and also explains why flood basalts are predominantly quartz tholeiites. Over half the original magma remains in the lower crust as cumulates in a system of dikes and sills. As the magma rises, the drop in pressure also lowers the liquidus , the temperature at which the magma is fully liquid. This likely explains the lack of phenocrysts in erupted flood basalt. The resorption (dissolution back into

6390-471: The magma was only slightly contaminated with melted rock of the Earth's crust , but some high-temperature minerals had already crystallized out of the rock before it reached the surface. In other words, the flood basalt is moderately evolved . However, only small amounts of plagioclase appear to have crystallized out of the melt. Though regarded as forming a chemically homogeneous group, flood basalts sometimes show significant chemical diversity even with in

6480-465: The mechanical behaviour of volcanic rocks can help us better understand volcanic hazards, such as flank collapse. Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Flood basalt A flood basalt (or plateau basalt )

6570-408: The melt) of a mixture of solid olivine, augite, and plagioclase—the high-temperature minerals likely to form as phenocrysts—may also tend to drive the composition closer to quartz tholeiite and help maintain buoyancy. Once the magma reaches the surface, it flows rapidly across the landscape, literally flooding the local topography. This is possible in part because of the rapid rate of extrusion (over

6660-713: The moment of ejection; they may then cool to form a non-porphyritic, finely crystalline rock, or if more rapidly chilled may in large part be non-crystalline or glassy (vitreous rocks such as obsidian, tachylyte , pitchstone ). A common feature of glassy rocks is the presence of rounded bodies ( spherulites ), consisting of fine divergent fibres radiating from a center; they consist of imperfect crystals of feldspar, mixed with quartz or tridymite ; similar bodies are often produced artificially in glasses that are allowed to cool slowly. Rarely these spherulites are hollow or consist of concentric shells with spaces between ( lithophysae ). Perlitic structure, also common in glasses, consists of

6750-438: The more rapidly cooling lava close to the base of the flow forms a thin chilled margin of glassy rock, and the more rapidly crystallized rock just above the glassy margin contains vesicles trapped as the rock was rapidly crystallizing. These have a distinctive appearance likened to a clay tobacco pipe stem, particularly as the vesicle is usually subsequently filled with calcite or other light-colored minerals that contrast with

6840-505: The new crust formed during flood basalt episodes takes the form of underplating , with over half the original magma crystallizing out as cumulates in sills at the base of the crust. The eruption of flood basalts has been linked with mass extinctions. For example, the Deccan Traps , erupted at the Cretaceous-Paleogene boundary , may have contributed to the extinction of the non-avian dinosaurs. Likewise, mass extinctions at

6930-408: The periphery. In the feldspars the center is usually richer in calcium than the surrounding layers, and successive zones may often be noted, each less calcic than those within it. Phenocrysts of quartz (and of other minerals), instead of sharp, perfect crystalline faces, may show rounded corroded surfaces, with the points blunted and irregular tongue-like projections of the matrix into the substance of

7020-399: The phase boundary, whereas the endothermic reaction hampers it. The pressure at which these phase transitions occur depends on temperature and iron content. At 800 °C (1,070 K; 1,470 °F), the pure magnesium end member, forsterite, transforms to wadsleyite at 11.8 gigapascals (116,000  atm ) and to ringwoodite at pressures above 14 GPa (138,000 atm). Increasing

7110-453: The presence of concentric rounded cracks owing to contraction on cooling. The phenocrysts or porphyritic minerals are not only larger than those of the ground-mass; as the matrix was still liquid when they formed they were free to take perfect crystalline shapes, without interference by the pressure of adjacent crystals. They seem to have grown rapidly, as they are often filled with enclosures of glassy or finely crystalline material like that of

7200-465: The pressure of these phase transitions. Olivine is one of the less stable common minerals on the surface according to the Goldich dissolution series . It alters into iddingsite (a combination of clay minerals, iron oxides and ferrihydrite ) readily in the presence of water. Artificially increasing the weathering rate of olivine, e.g. by dispersing fine-grained olivine on beaches, has been proposed as

7290-559: The product of explosive volcanism. They are often felsic (high in silica). Pyroclastic rocks are often the result of volcanic debris, such as ash , bombs and tephra , and other volcanic ejecta . Examples of pyroclastic rocks are tuff and ignimbrite . Shallow intrusions , which possess structure similar to volcanic rather than plutonic rocks, are also considered to be volcanic, shading into subvolcanic . The terms lava stone and lava rock are more used by marketers than geologists, who would likely say "volcanic rock" (because lava

7380-556: The resistance of olivine to solid flow. Moreover, because olivine is so abundant, more water may be dissolved in olivine of the mantle than is contained in Earth's oceans. Olivine pine forest (a plant community ) is unique to Norway. It is rare and found on dry olivine ridges in the fjord districts of Sunnmøre and Nordfjord. Mg-rich olivine has also been discovered in meteorites , on the Moon and Mars , falling into infant stars, as well as on asteroid 25143 Itokawa . Such meteorites include chondrites , collections of debris from

7470-476: The resultant mechanical behaviour (e.g., Young's modulus, compressive and tensile strength, and the pressure at which they transition from brittle to ductile behaviour). As for other crustal rocks, volcanic rocks are brittle and ductile at low and high effective confining pressures, respectively. Brittle behaviour is manifest as faults and fractures, and ductile behaviour can either be distributed (cataclastic pore collapse) or localised (compaction bands). Understanding

7560-588: The same initial magmas has led petrologists to rely heavily on chemistry to look at a volcanic rock's origin. The chemical classification of igneous rocks is based first on the total content of silicon and alkali metals ( sodium and potassium ) expressed as weight fraction of silica and alkali oxides ( K 2 O plus Na 2 O ). These place the rock in one of the fields of the TAS diagram . Ultramafic rock and carbonatites have their own specialized classification, but these rarely occur as volcanic rocks. Some fields of

7650-453: The surface of the Earth. Volcanic rocks are named according to both their chemical composition and texture. Basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has silica content similar to that of granite while basalt is compositionally equal to gabbro . Intermediate volcanic rocks include andesite , dacite , trachyte , and latite . Pyroclastic rocks are

7740-534: The surrounding dark basalt. At still smaller scales, the texture of flood basalts is aphanitic , consisting of tiny interlocking crystals. These interlocking crystals give trap rock its tremendous toughness and durability. Crystals of plagioclase are embedded in or wrapped around crystals of pyroxene and are randomly oriented. This indicates rapid emplacement so that the lava is no longer flowing rapidly when it begins to crystallize. Flood basalts are almost devoid of large phenocrysts , larger crystals present in

7830-409: The very thick Greenstone flow, mentioned earlier, being around 10 meters (30 ft) thick. Another common small-scale feature of flood basalts is pipe-stem vesicles . Flood basalt lava cools quite slowly, so that dissolved gases in the lava have time to come out of solution as bubbles (vesicles) that float to the top of the flow. Most of the rest of the flow is massive and free of vesicles. However,

7920-453: Was going on while the mass was still creeping forward under the surface of the Earth, the latest formed minerals (in the ground-mass ) are commonly arranged in subparallel winding lines that follow the direction of movement (fluxion or fluidal structure)—and larger early minerals that previously crystallized may show the same arrangement. Most lavas fall considerably below their original temperatures before emitted. In their behavior, they present

8010-476: Was still liquid at the moment of eruption. At this time they were exposed only to atmospheric pressure, and the steam and other gases, which they contained in great quantity were free to escape; many important modifications arise from this, the most striking being the frequent presence of numerous steam cavities ( vesicular structure) often drawn out to elongated shapes subsequently filled up with minerals by infiltration ( amygdaloidal structure). As crystallization

8100-458: Was verified in samples of a comet from the Stardust spacecraft in 2006. Comet-like (magnesium-rich) olivine has also been detected in the planetesimal belt around the star Beta Pictoris . Minerals in the olivine group crystallize in the orthorhombic system ( space group P bnm ) with isolated silicate tetrahedra, meaning that olivine is a nesosilicate . The structure can be described as

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