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122-521: Algoman orogeny added landmass to the Superior province by volcanic activity and continental collision along a boundary that stretches from present-day South Dakota, U.S., into the Lake Huron region near Sudbury , Ontario , Canada. It is 1,400 km (870 mi) long, and separates the older Archean gneissic terrane to the south from younger Late Archean greenstone -granite terrane to

244-447: 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 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

366-658: A hotspot near Sudbury and was completed by around 2,100 million years ago . This is when the Wyoming province is hypothesized to have drifted away from the Superior province. The pattern of sedimentation from this rifting environment continued into the Penokean orogeny, which is the next major tectonic event in the Great Lakes region. During the Penokean orogeny (1,850 to 1,900 million years ago), compression deformed

488-423: 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

610-425: A 2,100- to 1,800-million-year-old passive sedimentary margins. Much of the southeastern Superior province was bisected by the more than 300,000 km (120,000 sq mi) 2,172- to 2,167-million-year-old Biscotasing Diabase Swarm which trended northeast from Sudbury. In southcentral Wyoming province there is a 2,170 ± 8-million-year-old quartz diorite dike of Wind River Range . By 2,100 million years ago ,

732-493: 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

854-498: A close, about 2,500 million years ago ; it lasted less than 100 million years and marks a major change in the development of the Earth's crust. The Canadian shield contains belts of metavolcanic and metasedimentary rocks formed by the action of metamorphism on volcanic and sedimentary rock. The areas between individual belts consist of granites or granitic gneisses that form fault zones . These two types of belts can be seen in

976-442: 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

1098-427: 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

1220-430: 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 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

1342-431: A high grade of metamorphism, intrusion and basement remobilization typical of Archean terranes. Migmatites, batholithic intrusive and granulitic metamorphic rocks show foliation and compositional banding; the rocks are uniformly hard and so thoroughly deformed that little foliation exists. Most Yellowknife Supergroup metasediments are tightly folded ( isoclinal ) or occur in plunging anticlines . The Archean rocks forming

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1464-621: 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

1586-623: 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

1708-476: A mass of basic to ultrabasic inclusions of varying size and frequency of occurrence. Sudbury gabbro varies between a gabbro and a norite, depending upon the local silicates' ratios. The quartz biotite gabbro is medium- to coarse-grained, the Climax quartz monzonite is medium-grained. In the eastern Sudbury area the rock is highly crystalline hornblendic gneiss, which apparently dips at a rather low angle toward

1830-507: 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

1952-588: A result, plates and continents may have been smaller. No broad blocks as old as 3 Ga are found in Precambrian shields . Toward the end of the Archean, however, some of these blocks or terranes came together to form larger blocks welded together by greenstone belts . Two such terranes that now form part of the Canadian shield collided about 2,700 to 2,500 million years ago . These were

2074-565: A rifted belt. Recent paleomagnetic and geochronological data from the central Wyoming craton support the hypothesis that the Huronian (in southern Ontario) and Snowy Pass (in southeastern Wyoming) supergroups were adjacent to each other at 2,170 million years ago and may have evolved as a single sedimentary rift basin between 2,450 and 2,100 million years ago. These Huronian and Snowy Pass sedimentary rocks are similar, each having 2,450- to 2,100-million-year-old epicratonic rifts succeeded by

2196-610: A roughly northeasterly heading, while the southern border dips south to follow the northeast shore of Lake Superior. The three subprovinces are separated by steeply dipping shear zones caused by continued compression that occurred during the Algoman orogeny. These boundaries are major fault zones. The boundary between the Wabigoon and Quetico subprovinces seems to have been also controlled by colliding plates and subsequent transpressions . This Rainy Lake – Seine River fault zone

2318-488: A sequence of events, approximately 75 million years in duration, leading to the formation of a new crustal segment. The oldest rocks, at 2,650 million years old, are basic metavolcanics with largely calc-alkaline characteristics. Radiometric dating indicates ages of 2,640 to 2,620 million years are recorded for the syn-kinematic quartz diorite batholiths and 2,590 to 2,100 million years for the major late-kinematic bodies. Pegmatitic adamellites , at 2,575±25 million years, are

2440-523: 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

2562-403: A single continental mass, and culminated in the early Proterozoic, where deformation took place under low to intermediate pressures. After suturing, the region was tectonically quiet for a few hundred million years. The Algoman Mountains had been built and then eroded into sediments that covered the area. Fragmentation of this Archean supercontinent began around 2,450 million years ago under

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2684-492: A single layer may be exposed at the surface many times by subsequent erosion. As the greenstone belts were forming, volcanoes ejected tephra into the air which settled as sediments to become compacted into the greywackes and mudstones of the Knife Lake and Lake Vermilion formations. Greywackes are poorly sorted mixtures of clay , mica and quartz that may be derived from the decomposition of pyroclastic debris;

2806-510: A succession of metamorphosed volcanic and sedimentary rocks called greenstone belts . Most Archean volcanic rocks are concentrated within greenstone belts; the green color comes from minerals, such as chlorite , epidote and actinolite that formed during metamorphism. After metamorphism occurred, these rocks were folded and faulted into a system of mountains by the Algoman orogeny. The volcanic beds are 8 to 9 km (26,000 to 30,000 ft) thick. About 2,700  million years ago

2928-659: A trench during the collision of several island arcs (greenstone belts). Boundaries between the gneiss belt and the flanking greenstone belts to the north and south are major fault zones, the Vermilion and Rainy Lake – Seine River fault zones. The Wawa subprovince is a formerly active volcanic island chain, consisting of metamorphosed greenstone belts which are surrounded by and cut by granitic plutons and batholiths. These greenstone belts consist of felsic volcanics, felsic batholiths, felsic plutons and sediments aged from 2,700 to 2,670 million years old. The predominant rock type

3050-454: A zone of partial melting which is possible only under high temperature and pressure conditions. It is visible as a 500 m (1,600 ft) wide belt. Most of the flattened large crystals in the fault indicate a simple compression rather than a wrenching, shearing or rotational event as the two subprovinces docked. This provides evidence that the Quetico and Wawa subprovinces were joined by

3172-491: Is 50 km (30 mi) wide. The collision of the gneissic Minnesota River Valley (MRV) subprovince onto the southern edge of the Superior province was another process in the slow change in tectonics which marks the end of the Archean Eon. This gneissic terrane originally extended several hundred kilometers east to west, making it more of a protocontinent than a future Superior province belt. The boundary that separates

3294-492: 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 the most voluminous of all volcanic formations. Basaltic magmas within Earth are thought to originate from

3416-466: Is a major northeast–southwest trending strike-slip fault zone; it trends N80°E to cut through the northwest part of Voyageurs National Park in Minnesota and extends westward to near International Falls , Minnesota and Fort Frances , Ontario. The fault has transported rocks in the greenstone belt a considerable distance from their origin. The greenstone belt is 2 to 3 km (0 to 0 mi) wide at

3538-506: Is a white, coarse-grained, foliated hornblende tonalite. Minerals in the tonalite are quartz, plagioclase, alkali feldspar and hornblende. In extensive regions of the Slave province of northern Canada, the magma that later became batholiths heated the surrounding rock to create metamorphic regions called aureoles about 2,575 million years ago. These regions are typically 10 to 15 km (6 to 9 mi) wide. The creation of aureoles

3660-401: 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

3782-414: 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 the hypabyssal and plutonic equivalents of basalt. During

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3904-512: 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 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

4026-545: 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 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

4148-761: 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

4270-425: Is essentially a large downfold or downfaulted block. The areas between individual belts are fault zones consisting of granite or granitic gneiss. Its western part contains a regional pattern of east–west-trending 100 to 200 km (60 to 120 mi) wide granitic greenstone and metasedimentary belts (subprovinces). Western Superior province's mantle has remained intact since the 2,700-million-year-ago accretion of

4392-636: Is known as the Great Lakes tectonic zone (GLTZ). This zone is 50 km (30 mi) wide and extends in a line roughly 1,200 kilometers long from the middle of South Dakota , east through the middle of the Upper Peninsula of Michigan , into the Sudbury, Ontario region. The region remains slightly active today. Rifting in the GLTZ began about 2,500  million years ago at the end of

4514-661: Is partially interrupted by the Southern province. Immediately to the south, the Quetico subprovince extends as far west in north-central Minnesota, and extends further to the northeast. It is completely interrupted by a narrow band of the 1,100- to 1,550-million-year-old Southern province to the northeast of Thunder Bay . The Wawa subprovince is the most southerly of the three; it begins in central Minnesota, continues northeast to Thunder Bay, Ontario, Canada, (where its southern border just skims north Thunder Bay) and then extends east beyond Lake Superior. The northern boundary continues in

4636-531: 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

4758-624: Is shown to the right. At this time the two provinces are in contact at only one point north of the Blue Draw Metagabbro; that point of contact was 875 km (540 mi) from Sudbury and 95 km (60 mi) southwest of Duluth, Minnesota. The Blue Draw Metagabbro is now 935 km (580 mi) west of Sudbury and remains about 150 km (90 mi) south of the Superior-Wyoming provinces' junction. The 2,125- to 2,090-million-year-old mafic magmatic events affecting

4880-402: 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

5002-608: Is the continental landmass that is hypothesized to have rifted away from the southern Superior province portion of Kenorland, before moving rapidly west and docking with the Laurentia supercontinent 1,850 to 1,715 million years ago. Sedimentation from the GLTZ-rifting environment continued into the Penokean orogeny, which is the next major tectonic event in the Great Lakes region . Several earthquakes have been documented in Minnesota, Michigan's Upper Peninsula and Sudbury in

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5124-440: 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

5246-455: 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

5368-539: 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

5490-458: 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"

5612-1228: 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

5734-585: The Superior province and the large Minnesota River Valley terrane, the former composed mainly of granite and the latter of gneiss . This led to the mountain-building episode known as the Algoman orogeny in the U. S. (named for Algoma , Kewaunee County, Wisconsin ), and the Kenoran orogeny in Canada. Its duration is estimated at 50 to 100 million years. The current boundary between these terranes

5856-676: The igneous and sedimentary rocks, heating and pressing the rocks to metamorphose them into hard greenish greenstones. They began with fissure eruptions of basalt , continued with intermediate and felsic rocks erupted from volcanic centers and ended with deposition of sediments from the erosion of the volcanic pile. The rising magma was extruded under a shallow ancient sea where it cooled to form pillowed greenstones. Some of Minnesota's pillows probably cooled at depths as great as 1,000 m (3,300 ft) and contain no gas cavities or vesicles . Most greenstone belts, with all of their components, have been folded into troughlike synclines ;

5978-524: 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

6100-483: The magnetite series, rather than the low-oxygen concentration of the magnetic titanium oxides. Penokean-age rocks in the northern Wisconsin and the Upper Peninsula of Michigan contain areas of low-pressure, low- to high-temperature metamorphism. The folding and metamorphism increased in intensity to the south and southeast, and produced the isolated gneissic 1,755-million-year-old Watersmeet Domes which straddle

6222-558: 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; 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

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6344-614: The Algoman Mountains. This was followed by intrusions of granite plutons and batholithic domes within the gneisses about 2,700  million years ago ; two examples are the Sacred Heart granite of southwestern Minnesota and the Watersmeet Domes metagabbros (metamorphosed gabbros ) that straddle the border of Wisconsin and Michigan's Upper Peninsula . After the intrusions solidified, new stresses on

6466-567: The Algoman orogeny. The orogeny affected adjacent regions of northern Minnesota and Ontario in the Superior province as well as the Slave and the eastern part of the Nain province , a far wider region of influence than in subsequent orogenies. It is the earliest datable orogeny in North America and brought the Archean Eon to a close. The end of the Archean Eon marks a major change in

6588-479: The Blue Draw Metagabbro, is a 1 km (0.62 mi) thick layered sill. The East Bull Lake intrusive suite, in the southern Superior province near Sudbury, Ontario, aligns spatially with the Blue Draw Metagabbro if the Superior and Wyoming cratons are restored to the Kenorland configuration proposed by Roscoe and Card (1993). These layered mafic intrusions are of similar thickness and identical age, and occur along

6710-463: 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

6832-554: The GLTZ show later dates, reinforcing the theorized closure from west to east. Late Archean lithologies in northwestern Wisconsin and the Upper Peninsula of Michigan are similar to the Sacred Heart granite and consist of gneisses and migmatites. The 1,850-million-year-old Penokean magmatism in Wisconsin represents margin-type igneous activity terminated by collision. Some of the Penokean granites show iron enrichment similar to

6954-581: The GLTZ – Negaunee, Newberry and two in Sault Ste. Marie – and the Sudbury area has had three earthquakes. Algoman orogeny The Algoman orogeny , known as the Kenoran orogeny in Canada, was an episode of mountain-building ( orogeny ) during the Late Archean Eon that involved repeated episodes of continental collisions , compressions and subductions . The Superior province and

7076-470: The GLTZ, the MRV protocontinent consumed the Superior province's oceanic crust as the subprovince came in from the south. Suturing of one continental block onto another usually occurs because a subduction zone exists beneath one of the blocks. The subduction zone consumes the oceanic crust connected to the other block. After the oceanic crust is consumed, the two blocks meet and the subducting oceanic crust pulls

7198-511: The Great Lakes tectonic zone with the Algoman mountain-building event and continued for tens of millions of years. During the formation of the GLTZ, the gneissic Minnesota River Valley subprovince was thrust up onto the Superior province's edge as it consumed the Superior province's oceanic crust. Fragmentation of the Kenorland supercontinent began 2,450 million years ago and was completed by 2,100 million years ago . The Wyoming province

7320-430: The Great Lakes tectonic zone, are consistent with rifting during this time period. Minnesota has been the most seismically active in the region of Minnesota, Wisconsin, Michigan's Upper Peninsula and southern Ontario. Several earthquakes have been documented in Minnesota in the last 120 years, with at least six in the GLTZ. The epicenters show a clear relationship to tectonic features of the state; four epicenters lie along

7442-404: The Great Lakes tectonic zone. Depths are estimated at 5 to 20 km (3 to 12 mi). The best-documented event occurred on July 9, 1975, near Morris, Minnesota , with a magnitude of 4.6, and a felt area of 82,000 km (32,000 sq mi) covering parts of four states. Wisconsin has had no earthquakes along the GLTZ, Michigan's Upper Peninsula has had four earthquakes in the vicinity of

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7564-680: The Lake Superior region. The best-known units are the Morton Gneiss and the Montevideo Gneiss complexes, along the Minnesota River Valley in southwest Minnesota. Rocks exposed in the Minnesota River Valley include a complex of migmatitic granitic gneisses, schistose to gneissic amphibolite , metagabbro and paragneisses. The complex of ancient gneisses is intruded by a younger, weakly deformed granite body,

7686-474: The Marquette area, the GLTZ is a northwest-striking zone of metamorphic rock about 2 km (1.2 mi) wide that was crushed by the dynamics of tectonic movements. Shear zone boundaries are subparallel and strike N60°W; the foliation in mylonite within the GLTZ strikes N70°W and dips S75°W. A stretching lineation (line of tectonic transport) in the mylonite foliation plunges 42° in a S43°E direction. In

7808-733: The Michigan-Wisconsin border. The Sudbury Basin structure is located in Greater Sudbury at the erosional boundary between the Archean Superior province and the overlying sequence of early Proterozoic continental margin deposits. The structure consists of the Sudbury Igneous Complex , a differentiated sequence of intrusive volcanic rocks – norite , gabbro and granophyre – overlain by breccias and metasedimenary rocks. The sublayer consists of

7930-463: The Minnesota River Valley terrane collided about 2,700 to 2,500 million years ago. The collision folded the Earth's crust and produced enough heat and pressure to metamorphose the rock. Blocks were added to the Superior province along a 1,200 km (750 mi) boundary that stretches from present-day eastern South Dakota into the Lake Huron area. The Algoman orogeny brought the Archean Eon to

8052-710: The Minnesota River Valley subprovince attached – and the current-day southeastern border of the Wyoming province abutted each other from the Sudbury area westerly about 625 km (390 mi) to the Wisconsin-Michigan state line on Lake Superior. The hotspot was 125 km (80 mi) south of the East Bull Lake suite, approximately under present-day Sudbury. The Blue Draw Metagabbros – in the Black Hills of South Dakota – were 625 km (390 mi) west of Sudbury and 150 km (90 mi) south of

8174-428: The Minnesota – South Dakota border. This crustal boundary is the Great Lakes tectonic zone (GLTZ). It is a 1,400 km (870 mi) long paleo suture that separates the more than 3,000-million-year-old Archean gneissic terrane to the south – Minnesota River Valley subprovince – from the 2,700-million-year-old Late Archean greenstone- granite terrane to the north – Wawa Subprovince of the Superior province. The GLTZ

8296-568: The Nain province of northeastern Canada and Greenland are separated from the Superior terrane by a narrow band of remobilized rocks. Greenland separated from North America less than 100 million years ago and its Precambrian terranes align with Canada's on the opposite side of Baffin Bay. The southern tip of Greenland is part of the Nain Province, this means it was connected to North America at

8418-458: The Sacred Heart granite. The Sacred Heart granitic bodies that occur along portions of the Minnesota River Valley are relatively unfractured and unfoliated, and may represent passive intrusions into folded metasedimentary rocks. It is a typical late-tectonic medium-grained pink granite that was intruded around 2,600 million years ago , after the suturing of the MRV gneissic terrane onto the Superior province. Similar intrusions farther east along

8540-517: The Sask and Wyoming cratons to form the first supercontinent , the Kenorland supercontinent. Through most of the Archean Eon, the Earth had a heat production at least twice that of the present. The timing of initiation of plate tectonics is still debated, but if modern-day tectonics were operative in the Archean, higher heat fluxes might have caused tectonic processes to be more active. As

8662-598: The Seven Sisters Islands; to the west the greenstone interfingers with pods of anorthositic gabbro . Radiometric dating from the Rainy Lake area in Ontario show an age of about 2,700 million years old, which favors a moving tectonic plate model for the formation of the boundary. The largest fault is the Vermilion fault separating the Quetico and Wawa subprovinces. It has a N40°E trend and

8784-675: The Sims-and-Day model, this last collision in the assembly of the Superior province resulted from northwest-directed tectonic transport of the Minnesota River Valley subprovince terrane against the terrane of the Superior province. The collision was oblique, resulting in dextral -thrust shear along the boundary. Early Archean rocks generally form elongate, domal or circular bodies that are several kilometers thick. Late-stage dikes and sills of diabase, quartz-feldspar fine-grained instrusive rocks ( aplite ) and quartz-feldspar-mica coarse-grained intrusive rocks ( pegmatite ) are common. Most of

8906-433: The Superior and Wyoming cratons show the hotspot having moved 500 km (310 mi) west from Sudbury, and the two provinces have rifted so that they are separated by 100 km (60 mi). That narrowest distance between the two cratons is 1,150 km (710 mi) from Sudbury, in east-central South Dakota. The Blue Draw Metagabbro is now 950 km (590 mi) west of Sudbury and 200 km (120 mi) south of

9028-400: The Superior and Wyoming provinces had completely separated. From about 2,100 to 1,865 million years ago the Wyoming craton drifted in a westward direction until it docked 1,865 million years ago with the Superior province, northwest of its original position. The final assembly of supercontinent Kenorland was finished by 2,600 to 2,550 million years ago; the southern Superior province – with

9150-460: The Superior province's southern border. Swarms of mafic dikes and sills are typical of continental rifting and can be used to time supercontinent breakup. Intrusion of the 2,475- to 2,445-million-year-old Matachewan-Hearst Mafic Dike Swarm and the 2,490- to 2,475-million-year-old East Bull Lake suite of layered mafic intrusive rocks are interpreted as indicating early Paleoproterozoic, mantle-hotspot driven rifting centered near Sudbury, Ontario, during

9272-529: The Wabigoon, Quetico and Wawa subprovinces ; the Wabigoon and Wawa are of volcanic origin and the Quetico is of sedimentary origin. These three subprovinces lie linearly in southwestern- to northeastern-oriented belts about 140 km (90 mi) wide on the southern portion of the Superior Province. The Slave province and portions of the Nain province were also affected. Between about 2,000 and 1,700  million years ago these combined with

9394-400: The Wyoming craton is thought to have completely separated from the southern Superior province, this is consistent with the occurrence of a 2,076- to 2,067-million-year-old hotspot centered just south of the Superior province and east of the MRV. The 2,125- to 2,101-million-year-old Marathon and 2,077- to 2,076-million-year-old Fort Frances dikes, both on the present-day Superior province north of

9516-410: The attached continental block under the other. During the collision with the Superior province, the MRV gneissic block was thrust up onto the Superior province's edge; this resulted in a thrusted crumpled fault tens of kilometers wide producing a mountain range, and a shear zone which defines the boundary between the two terranes. Tectonism along the zone began during the docking of the two terranes into

9638-405: The belt are schists and gneisses produced by intense metamorphism of greywackes and minor amounts of other sedimentary rocks. The sediments, alkalic plutons and felsic plutons are aged from 2,690 to 2,680 million years. The metamorphism is relatively low-grade on the margins and high-grade in the center. The low-grade components of the greywackes were derived primarily from volcanic rocks;

9760-589: The border of Michigan and northeastern Wisconsin. Compressive deformation during the Penokean orogeny reactivated the GLTZ, which followed deposition of the Marquette Range Supergroup sediments and resulted in a north-side up motion along steep brittle-ductile faults in the eastern, low-grade portion of the Marquette Trough In the western portion of the Marquette syncline , a second episode of GLTZ reactivation took place during

9882-478: 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

10004-852: The collision of two continental plates, about 2,690 million years ago . Structures in the migmatite include folds and foliations ; the foliations cut across both limbs of earlier-phase folds. These cross-cutting foliations indicate that the migmatite has undergone at least two periods of ductile deformation. The Wabigoon subprovince is a formerly active volcanic island chain, made up of metavolcanic-metasedimentary intrusions. These metamorphosed rocks are volcanically derived greenstone belts, and are surrounded and cut by granitic plutons and batholiths. The subprovince's greenstone belts consist of felsic volcanics, felsic batholiths and felsic plutons aged from 3,000 to 2,670 million years old. The Quetico gneiss belt extends some 970 km (600 mi) across Ontario and parts of Minnesota. The dominant rocks within

10126-447: 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

10248-405: The development of the Earth's crust : the crust was essentially formed and achieved thicknesses of about 40 km (25 mi) under the continents. The collision between terranes folded the Earth's crust, and produced enough heat and pressure to metamorphose then-existing rock. Repeated continental collisions, compression along a north–south axis, and subduction resulted in the uprising of

10370-438: The emplacement of granite at the core of the thermal dome. This phase occurred at lower pressure because of erosional unloading, but the temperatures were more extreme, ranging up to about 700 °C (1,300 °F). With deformation complete, the thermal dome decayed; minor mineralogical changes occurred during this decay phase. The region has since been effectively stable. Geochronology of several Archean rock units establishes

10492-515: The end of the Kenoran orogen. 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 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

10614-494: 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

10736-532: The gneissic rocks south of the Great Lakes tectonic zone in Minnesota, south of the Midcontinent Rift System in Wisconsin and in the Upper Peninsula of Michigan. Crystalline rocks are more prominent in Minnesota, where they underlie 8,882 km (3,429 sq mi), than they are in either Wisconsin or Michigan's Upper Peninsula. Recent radiometric age data indicates that there are four crystalline rock complexes 3,400 million years old in

10858-418: The greenstone belt caused movement horizontally along several faults and moved huge blocks of the crust vertically relative to adjacent blocks. This combination of folding, intrusion and faulting built mountain ranges throughout northern Minnesota, northern Wisconsin, Michigan's Upper Peninsula and southernmost Ontario. Igneous and high-grade metamorphic rocks are associated with the orogeny. By extrapolating

10980-617: The greenstone belt was subjected to new stresses that caused movement along several faults. Faulting on both small and large scales is typical of greenstone belt deformation. These faults show both vertical and horizontal movement relative to adjacent blocks. Large-scale faults typically occur along the margins of the greenstone belts where they are in contact with enclosed granitic rocks. Vertical movement may be thousands of meters and horizontal movements of many kilometers occur along some fault zones. Some time before 2,600  million years ago , masses of magma intruded under and within

11102-458: The high-grade rocks are coarser-grained and contain minerals that reflect higher temperatures. The granitic intrusions within the high-grade metasediments were produced by subduction of the ocean crust and partial melting of metasedimentary rocks. Immediately south of Voyageurs National Park and extending to the Vermilion fault is a broad transition zone that contains migmatite. The Quetico gneiss belt represents an accretionary wedge that formed in

11224-740: 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 ,

11346-472: The last 120 years along the GLTZ. During the Late Archean Eon the Algoman orogeny – which occurred about 2,750 million years ago – added landmass through volcanic activity and continental collision along a boundary that stretches from present-day South Dakota, U.S., into the Sudbury, Ontario, Canada, region. The farthest west into South Dakota is 99°W, which is about 55 km (34 mi) from

11468-710: 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

11590-411: 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 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

11712-468: The north. The zone is characterized by active compression during the Algoman orogeny (about 2,700 million years ago ), a pulling-apart ( extensional ) tectonics (2,450 to 2,100 million years ago), a second compression during the Penokean orogeny (1,900 to 1,850 million years ago), a second extension during Middle Proterozoic time (1,600 million years ago) and minor reactivation during Phanerozoic time (the past 500 million years). Collision began along

11834-434: The now-eroded and tilted beds upward, geologists have determined that these mountains were several kilometers high. Similar projections of the tilted beds downward, coupled with geophysical measurements on the greenstone belts in Canada, suggest the metavolcanic and metasedimentary rocks of the belts project downward at least a few kilometers. The action of metamorphism on the border between granite and gneiss bodies produces

11956-410: 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

12078-407: The onset of Kenorland breakup. Radiometric dating shows that the Wyoming province's Blue Draw Metagabbro was undergoing rifting at 2,480 million years ago , the same time the emplacement of the 250 km (160 mi) long belt of mafic layered intrusions in the Sudbury region. In the northern Black Hills of southwest South Dakota the 2,600- to 2,560-million-year-old Precambrian crystalline core,

12200-465: The original basaltic rock, which was on the bottom, occurs on the outer margins of the trough. The overlying, younger rock units – rhyolites and greywackes – occur closer to the center of the syncline. The rocks are so intensely folded that most have been tilted nearly 90°, with the tops of layers on one side of the synclinal belt facing those on the other side; the rock sequences are in effect lying on their sides. The folding can be so complex that

12322-468: The other 2,600 million years ago . The first was probably during formation of the terrane, the second was during suturing. The growth of the Superior province greenstone-granitic terranes ended with the suturing of the Minnesota River Valley gneiss terrane to the basaltic Wawa subprovince. Suturing, the last stage of closure, started in South Dakota and continued eastward. During the formation of

12444-621: The presence of this debris suggests that some explosive volcanic activity had occurred in the area earlier. The volcanism took place on the surface and the other deformations took place at various depths. Numerous earthquakes accompanied the volcanism and faulting. The Superior province forms the core of both the North American continent and the Canadian shield, and has a thickness of at least 250 km (160 mi). Its granites date from 2,700 to 2,500 million years ago. It

12566-475: 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

12688-442: The region's crystalline rock bodies of Late Archean age are part of the greenstone-granite terrane of northern Minnesota, northwestern Wisconsin and the western part of the Upper Peninsula of Michigan. Lithologies of the rocks are usually gneissose and migmatitic . Repeated metamorphism and deformation caused extensive recrystallization, intense foliation, shear zones and folding. There are east-northeast- to east-trending faults in

12810-595: 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

12932-403: 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

13054-456: The rocks chemically, with particular emphasis on the total content of alkali metal oxides and silica ( TAS ); in that context, basalt 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

13176-462: 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

13298-469: The sequences in the Lake Superior region. Recent geologic mapping in the Marquette, Michigan, U.S., area provides information of the structure for the zone along a 10 km (6.2 mi) strike. The GLTZ was an active dextral strike-slip zone south of Marquette, passing under the large Marquette anticline . P.K. Sims and W.C. Day suggest that the kinematics determined in the exposed GLTZ – which are consistent – are applicable to its entire length. In

13420-428: The southeast. A paleostress analysis of the eastern exposures near Sudbury shows continuing dextral offset during the Penokean orogeny. An episode of hotspot gabbro magmatism occurred 2,480 million years ago at the eastern edge of the Wyoming craton , south of current-day Sudbury. Continental rifting is exhibited by emplacement of mafic igneous rocks on each side of the rift margins. By 2,100 million years ago

13542-399: The subprovinces. Both folding and faulting can be seen in the Wabigoon, Quetico and Wawa subprovinces. These three subprovinces lie linearly in southwestern- to northeastern-oriented belts of about 140 km (90 mi) wide (see figure on right). The northernmost and widest province is the Wabigoon. It begins in north-central Minnesota and continues northeasterly into central Ontario; it

13664-670: 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

13786-424: The two colliding bodies is the Great Lakes tectonic zone; it is a fault zone of highly deformed rocks. Collision began along the GLTZ around 2,700 million years ago and continued for tens of millions of years. The collision is interpreted to have happened obliquely at an angle, beginning in the west. The MRV subprovince experienced two distinct high-grade metamorphic events, one 3,050 million years ago and

13908-509: The uplift of the post-Huronian 2,400- to 2,100-million-year-old granitic Southern Complex. The Northern and Southern complexes of the Upper Peninsula are highly migmatized and intensely foliated, with the intensity of foliation increasing toward margins. The western part of the Southern Complex shows intricate phases of folding and foliation. These Late Archean rocks form a roughly north–south belt lying south of Marquette extending to

14030-580: 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

14152-434: 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 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

14274-451: The westernmost contact of the two provinces on the Wyoming province. The 2,170-million-year-old intrusive events that affected the Superior and the Wyoming cratons indicate that the plume had moved 330 km (210 mi) west, centered in the opening between the Superior province and the rifting Wyoming province. The Wyoming province was rotating away, with the Blue Draw Metagabbro being the pivot point. Harlan's reconstruction of this pivot

14396-411: The youngest plutonic units. Metagreywackes and meta pelites from two areas traversing one of these aureoles near Yellowknife have been studied. Most of the Slave province rocks are granitic with metamorphosed Yellowknife metasedimentary and volcanic rocks. Isotopic ages of these rocks is around 2,500 million years ago , the time of the Kenoran orogeny. Rocks comprising the Slave province represent

14518-441: Was a continuous process, but three recognizable metamorphic phases can be correlated with established deformational phases. The cycle began with a deformation phase unaccompanied by metamorphism. This evolved into the second phase accompanied by broad regional metamorphism as thermal doming began. With continued updoming of the isotherms, the third phase produced minor folding but caused major metamorphic recrystallization, resulting in

14640-446: Was caused by the introduction of masses of magma. The Vermilion fault can be traced westward to North Dakota. It has had a 19 km (12 mi) horizontal movement with the northern block moving eastward and upward relative to the southern block. The junction between the Quetico and Wawa subprovinces has a zone of biotite -rich migmatite , a rock that has characteristics of both igneous and metamorphic processes; this indicates

14762-598: Was formed by the welding together of many small terranes , the ages of which decrease away from the nucleus. This progression is illustrated by the age of the Wabigoon, Quetico and Wawa subprovinces, discussed in their individual sections. Later terranes docked on the periphery of continental masses with geosynclines developing between the fused nuclei and oceanic crust. In general the Superior province consists of east–west-trending belts of predominately volcanic rocks alternating with belts of sedimentary and gneissic rocks. Due to down warping along elongate zones, each belt

14884-400: 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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