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64-669: The gneisses of the Acasta Gneiss Complex were first regionally mapped during a mapping program carried out by the Geological Survey of Canada. These gneisses were informally named the Acasta gneiss complex for the Acasta River east of Great Bear Lake in 1985 by J. E. King. Later, it was referred to as the Acasta gneisses by S.A. Bowring and others in 1989 In subsequent publications, Acasta Gneiss Complex

128-527: A Dene people, are named after the lake. Grizzly Bear Mountain, or Sahoyue , on the shore of the lake also comes from Chipewyan, meaning "bear-large hill". Sahoyue (Grizzly Bear Mountain), a peninsula on the south side of the lake, and Edacho (Scented Grass Hills), another peninsula on the west side, form the Saoyú-ʔehdacho National Historic Site of Canada. The lake has a surface area of 31,153 km (12,028 sq mi) and

192-423: A mantle wedge above the descending slab. The slab releases water vapor and other volatiles as it descends, which further lowers the melting point, further increasing the amount of decompression melting. Each tectonic setting produces basalt with its own distinctive characteristics. The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene . Olivine can also be

256-493: A basalt is diagnostic of how and where it erupted—for example, whether into the sea, in an explosive cinder eruption or as creeping pāhoehoe lava flows, the classic image of Hawaiian basalt eruptions. Basalt that erupts under open air (that is, subaerially ) forms three distinct types of lava or volcanic deposits: scoria ; ash or cinder ( breccia ); and lava flows. Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated , imparting

320-442: A distinctive pillow shape, through which the hot lava breaks to form another pillow. This "pillow" texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks. Pillows typically consist of a fine-grained core with a glassy crust and have radial jointing. The size of individual pillows varies from 10 cm up to several metres. When pāhoehoe lava enters

384-427: A flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns . These structures, or basalt prisms , are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on

448-465: A high content of plagioclase; these are sometimes described as leucobasalts . It can be difficult to distinguish between lighter-colored basalt and andesite , so field researchers commonly use a rule of thumb for this purpose, classifying it as basalt if it has a color index of 35 or greater. The physical properties of basalt result from its relatively low silica content and typically high iron and magnesium content. The average density of basalt

512-621: A lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidized iron from weathered iron-rich minerals such as pyroxene . ʻAʻā types of blocky cinder and breccia flows of thick, viscous basaltic lava are common in Hawaiʻi. Pāhoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms lava lakes . Lava tubes are common features of pāhoehoe eruptions. Basaltic tuff or pyroclastic rocks are less common than basaltic lava flows. Usually basalt

576-623: A lower total content of alkali oxides than trachybasalt and most basanites and tephrites. Basalt generally has a composition of 45–52 wt% SiO 2 , 2–5 wt% total alkalis, 0.5–2.0 wt% TiO 2 , 5–14 wt% FeO and 14 wt% or more Al 2 O 3 . Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%. High-alumina basalts have aluminium contents of 17–19 wt% Al 2 O 3 ; boninites have magnesium (MgO) contents of up to 15 percent. Rare feldspathoid -rich mafic rocks, akin to alkali basalts, may have Na 2 O + K 2 O contents of 12% or more. The abundances of

640-507: A phenocryst, and when present, may have rims of pigeonite. The groundmass contains interstitial quartz or tridymite or cristobalite . Olivine tholeiitic basalt has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the groundmass . Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine. Feldspar phenocrysts typically are labradorite to andesine in composition. Augite

704-523: A significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite , ulvöspinel , and ilmenite . Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt. In tholeiitic basalt , pyroxene ( augite and orthopyroxene or pigeonite ) and calcium -rich plagioclase are common phenocryst minerals. Olivine may also be

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768-720: A significant volume of 3.37 Ga granitoids. This study also concluded that near its inferred eastern limit, the unmapped Acasta Gneiss Complex contain rocks at least as old as 3.95 Ga. The base of Acasta Gneiss Complex is unexposed and unknown. It is considered to be unconformibly overlain by Central Slave Cover Group. The base of younger Late Mesoarchean greenstone belts that comprise the Central Slave Cover Group consists largely of fuchsite-bearing quartz arenite,. These quartz arenites are associated with chert-magnetite iron formation, rhyolite, and locally some ultrabasic rocks. These strata are intrerpreted to be remnants of

832-432: A volume of 2,234 km (536 cu mi). Its maximum depth is 446 m (1,463 ft) and average depth 71.7 m (235 ft). The shoreline is 2,719 km (1,690 mi) and the catchment area of the lake is 114,717 km (44,292 sq mi). Great Bear Lake is covered with ice from late November to July. The lake is known for its considerable clarity. Explorer John Franklin wrote in 1828 that

896-767: A white rag placed in the water did not disappear until it exceeded a depth of 15 fathoms (90 ft; 27 m). Arms of Great Bear Lake include the Smith Arm (northwest), the Dease Arm (northeast), the McTavish Arm (southeast), the McVicar Arm (south) and the Keith Arm (southwest). The community of Délı̨nę is located on the Keith Arm near the outflow of the Great Bear River that flows west into

960-427: Is 2.9 g/cm , compared, for example, to granite ’s typical density of 2.7 g/cm . The viscosity of basaltic magma is relatively low—around 10 to 10 cP —similar to the viscosity of ketchup , but that is still several orders of magnitude higher than the viscosity of water, which is about 1 cP). Basalt is often porphyritic , containing larger crystals ( phenocrysts ) that formed before the extrusion event that brought

1024-401: Is a relatively homogeneous mafic tonalite that consists primarily of plagioclase , quartz , hornblende , biotite and minor garnet , with small, cross-cutting felsic veins . The Acasta Gneiss Complex is a heterogeneous assemblage of foliated to gneissic tonalites, trondhjemites, granodiorites, and granites which contains minor quartz-diorites, diorites, gabbros, and ultramafic rocks. Of

1088-688: Is also an important rock type on other planetary bodies in the Solar System . For example, the bulk of the plains of Venus , which cover ~80% of the surface, are basaltic; the lunar maria are plains of flood-basaltic lava flows ; and basalt is a common rock on the surface of Mars . Molten basalt lava has a low viscosity due to its relatively low silica content (between 45% and 52%), resulting in rapidly moving lava flows that can spread over great areas before cooling and solidifying. Flood basalts are thick sequences of many such flows that can cover hundreds of thousands of square kilometres and constitute

1152-401: Is also produced by some subglacial volcanic eruptions. Basalt is the most common volcanic rock type on Earth, making up over 90% of all volcanic rock on the planet. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below the ocean ridges . Basalt is also the principal volcanic rock in many oceanic islands , including

1216-761: Is considered a key to understanding plate tectonics , its compositions have been much studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform. For instance, compositions change with position along the Mid-Atlantic Ridge , and the compositions also define different ranges in different ocean basins. Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB). Isotope ratios of elements such as strontium , neodymium , lead , hafnium , and osmium in basalts have been much studied to learn about

1280-481: Is defined as volcanic rock with a content of between 45% and 52% silica and no more than 5% alkali metal oxides. This places basalt in the B field of the TAS diagram. Such a composition is described as mafic . Basalt is usually dark grey to black in colour, due to a high content of augite or other dark-coloured pyroxene minerals, but can exhibit a wide range of shading. Some basalts are quite light-coloured due to

1344-564: Is on the lake, near the headwaters of the Bear River . There is an ice crossing from Délı̨nę to the winter road on the far side of the Great Bear River. On 5 March 2016, a tank truck fell partway through the ice road just a few days after the government had increased the allowed maximum weight limit to 40,000 kg (88,000 lb) on the road. The truck, which was 3 km (1.9 mi) outside of Délı̨nę and close to

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1408-453: Is paramount in the Délı̨nę peoples' identity, laws and culture. Hence, conserving it is critical for the Délı̨nę people. ɂehtsǝ́o Erǝ́ya, a Dene Elder, is widely regarded as a prophet , making over 30 prophecies which have been interpreted as having come true. His prediction for the end of times claims that, as the world dries up, the little remaining life will flock to, and end on the banks of

1472-583: Is rich in titanium compared to augite in tholeiitic basalt. Minerals such as alkali feldspar , leucite , nepheline , sodalite , phlogopite mica, and apatite may be present in the groundmass. Basalt has high liquidus and solidus temperatures—values at the Earth's surface are near or above 1200 °C (liquidus) and near or below 1000 °C (solidus); these values are higher than those of other common igneous rocks. The majority of tholeiitic basalts are formed at approximately 50–100 km depth within

1536-402: Is suppressed. Above this depth, submarine eruptions are often explosive, tending to produce pyroclastic rock rather than basalt flows. These eruptions, described as Surtseyan, are characterised by large quantities of steam and gas and the creation of large amounts of pumice . When basalt erupts underwater or flows into the sea, contact with the water quenches the surface and the lava forms

1600-440: Is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and buildup of volcanic gases . Hawaiʻi's Mauna Loa volcano erupted in this way in the 19th century, as did Mount Tarawera , New Zealand in its violent 1886 eruption. Maar volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through

1664-455: Is ultimately derived from Late Latin basaltes , a misspelling of Latin basanites "very hard stone", which was imported from Ancient Greek βασανίτης ( basanites ), from βάσανος ( basanos , " touchstone "). The modern petrological term basalt , describing a particular composition of lava -derived rock, became standard because of its use by Georgius Agricola in 1546, in his work De Natura Fossilium . Agricola applied

1728-491: Is used instead of Acasta gneisses . As the result of a detailed mapping and U–Pb dating, the oldest zircons of the Acasta Gneiss Complex were found to occur within a relatively homogeneous and mappable tonalitic gneiss by J. R. Reimink and others in 2014. They named it the Idiwhaa tonalitic gneiss . The Idiwhaa tonalitic gneiss has yielded abundant well-preserved igneous zircons with a U–Pb crystallization age of 4.02 Ga . It

1792-735: The Canada–US border ), the fourth-largest in North America, and the eighth-largest in the world . The lake is in the Northwest Territories , on the Arctic Circle between 65 and 67 degrees of northern latitude and between 118 and 123 degrees western longitude, 156 m (512 ft) above sea level . The name originated from the Chipewyan word satudene , meaning "grizzly bear-water people". The Sahtu ,

1856-539: The Clementine mission demonstrate that the lunar maria possess a continuum of titanium concentrations, and that the highest concentrations are the least abundant. Lunar basalts show exotic textures and mineralogy, particularly shock metamorphism , lack of the oxidation typical of terrestrial basalts, and a complete lack of hydration . Most of the Moon 's basalts erupted between about 3 and 3.5 billion years ago, but

1920-786: The Mackenzie River at Tulita . Rivers flowing into Great Bear Lake include the Whitefish River , Big Spruce River, Haldane River, Bloody River , Sloan River, Dease River and the Johnny Hoe River. Great Bear Lake lies between two major physiographic regions : the Kazan Uplands portion of the Canadian Shield and the Interior Plains . It was part of glacial Lake McConnell in

1984-1228: The Paraná Traps in Brazil, the Siberian Traps in Russia , the Karoo flood basalt province in South Africa, and the Columbia River Plateau of Washington and Oregon . Basalt is also prevalent across extensive regions of the Eastern Galilee , Golan , and Bashan in Israel and Syria . Basalt also is common around volcanic arcs, specially those on thin crust . Ancient Precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as greenstone belts , because low-grade metamorphism of basalt produces chlorite , actinolite , epidote and other green minerals. As well as forming large parts of

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2048-524: The hypabyssal and plutonic equivalents of basalt. During the Hadean , Archean , and early Proterozoic eons of Earth's history, the chemistry of erupted magmas was significantly different from what it is today, due to immature crustal and asthenosphere differentiation. The resulting ultramafic volcanic rocks, with silica (SiO 2 ) contents below 45% and high magnesium oxide (MgO) content, are usually classified as komatiites . The word "basalt"

2112-524: The lanthanide or rare-earth elements (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled. In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the europium anomaly . It arises because Eu can substitute for Ca in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form cations . Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are

2176-567: The 13,000 km (5,000 sq mi) of the Canadian Shield underlain by Acasta Gneiss, only a 50 km (19 sq mi) area has been mapped and studied great detail of its remoteness and patchy outcrops due to a thick cover of Pleistocene glacial deposits. The study of the age distribution of detrital zircons from the Pleistocene deposits inferred the unmapped Acasta Gneiss Complex covered by Pleistocene deposits consists of

2240-582: The Acasta Gneiss Complex began at the end of the Hadean and continued episodically over much of the Archean. The oldest felsic rocks that are currently exposed intruded pre-existing, even older, mafic crustal rock and crystallized well beneath the Earth's surface. Later, these rocks were thermally metamorphosed, intruded by additional felsic magma, and partially melted during Eoarchean thermal events occurred about 3.85 to 3.72 Ga and 3.66 to 3.59 Ga. The metamorphism and intrusion by felsic magmas continued throughout

2304-400: The Archean. This included major periods of intrusion by granitic magmas about 3.30 Ga, 2.88 Ga, and 2.70 Ga. Finally, these rocks were intruded by multiple Paleoproterozoic dike swarms and syenites about 1.80 Ga. The setting in which the tectonic activity that created the Acasta Gneiss Complex is proposed to have been a block of dominatly mafic protocrust, at least some of which crystallized in

2368-463: The Earth's crust, basalt also occurs in other parts of the Solar System. Basalt commonly erupts on Io (the third largest moon of Jupiter ), and has also formed on the Moon , Mars , Venus , and the asteroid Vesta . The dark areas visible on Earth's moon , the lunar maria , are plains of flood basaltic lava flows. These rocks were sampled both by the crewed American Apollo program and

2432-528: The Great Bear Lake, a lake seen as a physical beating heart to humanity. The Délı̨nę people have followed these prophecies closely, the cultural considerations being a driving force for self-governance and environmental sustainability. Basalt Basalt ( UK : / ˈ b æ s ɔː l t , - əl t / ; US : / b ə ˈ s ɔː l t , ˈ b eɪ s ɔː l t / ) is an aphanitic (fine-grained) extrusive igneous rock formed from

2496-594: The Hadean. This block of protocrust would have been very similar to modern Iceland in its geology. With zircon U-Pb crystallization ages as old as about 4.03 Ga, beds within the Acasta Gneiss Complex are, as of 2019, regarded to be the oldest known felsic rocks (crust) on Earth. Based on the analyses of Sm and Nd isotopes , mafic rocks of the Nuvvuagittuq Greenstone Belt , found on the shore of Hudson Bay , Inukjuak , Quebec , are argued to be

2560-604: The Nuvvuagittuq Greenstone Belt only demonstrate that these rocks are older than 3.768 Ga. 65°09′59.91″N 115°34′54″W  /  65.1666417°N 115.58167°W  / 65.1666417; -115.58167 Great Bear Lake Great Bear Lake ( Slave : Sahtú ; French : Grand lac de l'Ours ) is a lake in the boreal forest of Canada . It is the largest lake entirely in Canada ( Lake Superior and Lake Huron are larger but straddle

2624-480: The basalt/andesite field of the QAPF diagram. Basalt is further distinguished from andesite by its silica content of under 52%. It is often not practical to determine the mineral composition of volcanic rocks, due to their very small grain size, in which case geologists instead classify the rocks chemically, with particular emphasis on the total content of alkali metal oxides and silica ( TAS ); in that context, basalt

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2688-478: The characteristic igneous rocks formed at mid-ocean ridges. They are tholeiitic basalts particularly low in total alkalis and in incompatible trace elements, and they have relatively flat REE patterns normalized to mantle or chondrite values. In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatible elements. Because MORB basalt

2752-420: The community's fresh water intake, as well as a major fishing area, contained approximately 30,000 L (6,600 imp gal; 7,900 US gal) of heating fuel and was one of 70 truck loads intended to resupply the community. The fuel was removed from the truck by 2 am, 8 March. Three lodges around the lake are destinations for fishing and hunting. In 1995, a 32.65 kg (72.0 lb) lake trout

2816-447: The crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano. Amygdaloidal structure is common in relict vesicles and beautifully crystallized species of zeolites , quartz or calcite are frequently found. During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While

2880-580: The earliest sedimentary and volcanic strata that accumulated on Mesoarchean crystalline basement of the Slav craton. From decades of petrologic, geochronologic, and geochemical studies, it has become apparent that the Acasta Gneiss Complex, as it is now exposed in the Canadian Shield , is the result of the complex interplay between repeated periods of felsic magma generation from the partial melting of older, mafic crust, its crystallization as felsic plutonic rocks, and subsequent thermal metamorphism. The formation of

2944-545: The evolution of the Earth's mantle . Isotopic ratios of noble gases , such as He / He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiitic basalt (normalized to atmospheric values), but to 15–24 and more for ocean-island basalts thought to be derived from mantle plumes . Source rocks for the partial melts that produce basaltic magma probably include both peridotite and pyroxenite . The shape, structure and texture of

3008-740: The islands of Hawaiʻi , the Faroe Islands , and Réunion . The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is the rock most typical of large igneous provinces . These include continental flood basalts , the most voluminous basalts found on land. Examples of continental flood basalts included the Deccan Traps in India , the Chilcotin Group in British Columbia , Canada ,

3072-604: The magma as it decompresses during its approach to the surface; the erupted lava then solidifies before the gases can escape. When vesicles make up a substantial fraction of the volume of the rock, the rock is described as scoria . The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are more properly referred to either as diabase (also called dolerite) or—when they are more coarse-grained (having crystals over 2 mm across)—as gabbro . Diabase and gabbro are thus

3136-416: The magma to the surface, embedded in a finer-grained matrix . These phenocrysts are usually made of augite, olivine , or a calcium-rich plagioclase, which have the highest melting temperatures of any of the minerals that can typically crystallize from the melt, and which are therefore the first to form solid crystals. Basalt often contains vesicles ; they are formed when dissolved gases bubble out of

3200-710: The mantle. Many alkali basalts may be formed at greater depths, perhaps as deep as 150–200 km. The origin of high-alumina basalt continues to be controversial, with disagreement over whether it is a primary melt or derived from other basalt types by fractionation. Relative to most common igneous rocks, basalt compositions are rich in MgO and CaO and low in SiO 2 and the alkali oxides, i.e., Na 2 O + K 2 O , consistent with their TAS classification . Basalt contains more silica than picrobasalt and most basanites and tephrites but less than basaltic andesite . Basalt has

3264-406: The most voluminous of all volcanic formations. Basaltic magmas within Earth are thought to originate from the upper mantle . The chemistry of basalts thus provides clues to processes deep in Earth's interior . Basalt is composed mostly of oxides of silicon, iron, magnesium, potassium, aluminum, titanium, and calcium. Geologists classify igneous rock by its mineral content whenever possible;

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3328-476: The oldest rocks (crust) on Earth with estimated crystallization age of 4.313 Ga. Unfortunately, because of the basaltic composition and low- Zr content of the rocks comprising Nuvvuagittuq Greenstone Belt, they essentially lack the igneous zircons needed to reliably date them. As a result, their age and status as oldest known rock on Earth remains controversial. U-Pb crystallization ages of igneous zircons from felsic, trondhjemitic bands of intrusive origin within

3392-410: The oldest samples are 4.2 billion years old, and the youngest flows, based on the age dating method of crater counting , are estimated to have erupted only 1.2 billion years ago. From 1972 to 1985, five Venera and two VEGA landers successfully reached the surface of Venus and carried out geochemical measurements using X-ray fluorescence and gamma-ray analysis. These returned results consistent with

3456-540: The pre-glacial valleys reshaped by erosional ice during the Pleistocene . Since, the lake has changed from post-glacial rebound following the ice melting. Precambrian rocks of the Canadian Shield form the eastern margin of the McTavish Arm. These rocks of the Precambrian are sedimentary and metamorphic deposits supplemented by igneous intrusions forming dikes and sills . The community of Délı̨nę

3520-410: The rapid cooling of low- viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon . More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro . The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt

3584-475: The rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns. The character of submarine basalt eruptions is largely determined by depth of water, since increased pressure restricts the release of volatile gases and results in effusive eruptions. It has been estimated that at depths greater than 500 metres (1,600 ft), explosive activity associated with basaltic magma

3648-411: The relative volume percentages of quartz (crystalline silica (SiO 2 )), alkali feldspar , plagioclase , and feldspathoid ( QAPF ) are particularly important. An aphanitic (fine-grained) igneous rock is classified as basalt when its QAPF fraction is composed of less than 10% feldspathoid and less than 20% quartz, and plagioclase makes up at least 65% of its feldspar content. This places basalt in

3712-595: The robotic Russian Luna program , and are represented among the lunar meteorites . Lunar basalts differ from their Earth counterparts principally in their high iron contents, which typically range from about 17 to 22 wt% FeO. They also possess a wide range of titanium concentrations (present in the mineral ilmenite ), ranging from less than 1 wt% TiO 2 , to about 13 wt.%. Traditionally, lunar basalts have been classified according to their titanium content, with classes being named high-Ti, low-Ti, and very-low-Ti. Nevertheless, global geochemical maps of titanium obtained from

3776-403: The rock at the landing sites being basalts, including both tholeiitic and highly alkaline basalts. The landers are thought to have landed on plains whose radar signature is that of basaltic lava flows. These constitute about 80% of the surface of Venus. Some locations show high reflectivity consistent with unweathered basalt, indicating basaltic volcanism within the last 2.5 million years. Basalt

3840-462: The sea it usually forms pillow basalts. However, when ʻaʻā enters the ocean it forms a littoral cone , a small cone-shaped accumulation of tuffaceous debris formed when the blocky ʻaʻā lava enters the water and explodes from built-up steam. The island of Surtsey in the Atlantic Ocean is a basalt volcano which breached the ocean surface in 1963. The initial phase of Surtsey's eruption

3904-670: The term "basalt" to the volcanic black rock beneath the Bishop of Meissen's Stolpen castle , believing it to be the same as the "basaniten" described by Pliny the Elder in AD ;77 in Naturalis Historiae . On Earth, most basalt is formed by decompression melting of the mantle . The high pressure in the upper mantle (due to the weight of the overlying rock ) raises the melting point of mantle rock, so that almost all of

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3968-580: The upper mantle is solid. However, mantle rock is ductile (the solid rock slowly deforms under high stress). When tectonic forces cause hot mantle rock to creep upwards, pressure on the ascending rock decreases, and this can lower its melting point enough for the rock to partially melt , producing basaltic magma. Decompression melting can occur in a variety of tectonic settings, including in continental rift zones, at mid-ocean ridges , above geological hotspots , and in back-arc basins . Basalt also forms in subduction zones , where mantle rock rises into

4032-490: Was caught, the largest ever caught anywhere by angling . In 1930, Gilbert LaBine discovered uranium deposits in the Great Bear Lake region. The former mining area Port Radium , site of the Eldorado Mine , where pitchblende was discovered, was located on the eastern shore. Echo Bay Mines Limited leased the old camp and mill at Port Radium to recover silver and copper values from 1965 to 1981. Great Bear Lake

4096-400: Was highly explosive, as the magma was quite fluid, causing the rock to be blown apart by the boiling steam to form a tuff and cinder cone. This has subsequently moved to a typical pāhoehoe-type behaviour. Volcanic glass may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions. Pillow basalt

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