| Canadian Shield | |
|---|---|
| Stratigraphic range:Precambrian | |
| Type | Shield |
| Unit of | North American craton |
| Sub-units | Laurentian Upland Kazan[1] |
| Area | 8,000,000 km2[2] |
| Location | |
| Coordinates | 52°00′N71°00′W / 52.000°N 71.000°W /52.000; -71.000 |
| Region | North America |
| Country | Canada United States Greenland |
 The Canadian Shield is a broad region of Precambrian rock (pictured in shades of red) that encirclesHudson Bay. It spans eastern, northeastern, and east-centralCanada and theupper midwesternUnited States. | |
TheCanadian Shield (French:Bouclier canadien[bukljekanadjɛ̃]), also called theLaurentian Shield or theLaurentian Plateau, is ageologic shield, a large area of exposedPrecambrianigneous and high-grademetamorphic rocks. It forms theNorth American Craton (or Laurentia), the ancient geologic core of theNorth American continent.Glaciation has left the area with only a thin layer ofsoil, through which exposures of igneousbedrock resulting from its long volcanic history are frequently visible.[3] As a deep, common, joined bedrock region in eastern and centralCanada, the shield stretches north from theGreat Lakes to theArctic Ocean, covering over half of Canada and most ofGreenland; it also extends south into the northern reaches of the continentalUnited States.
The Canadian Shield is aphysiographic division comprising four smaller physiographic provinces: theLaurentian Upland,Kazan Region, Davis and James.[1] The shield extends into the United States as theAdirondack Mountains (connected by theFrontenac Axis) and theSuperior Upland. The Canadian Shield is a U-shaped subsection of the Laurentiacraton signifying the area of greatestglacial impact (scraping down to bare rock) creating the thin soils. The age of the Canadian Shield is estimated to be 4.28Ga (4.28 billion years). The Canadian Shield once had jagged peaks, higher than any of today's mountains, but millions of years of erosion have transformed these mountains to rolling hills.[4]
The Canadian Shield is a collage ofArchean plates and accreted juvenile arcterranes andsedimentary basins of theProterozoic Eon that were progressively amalgamated during the interval 2.45–1.24 Ga, with the most substantial growth period occurring during theTrans-Hudson orogeny, between c. 1.90–1.80 Ga.[5] The Canadian Shield was the first part of North America to be permanently elevated above sea level and has remained almost wholly untouched by successive encroachments of the sea upon the continent. It is the Earth's greatest area of exposed Archean rock. The metamorphic base rocks are mostly from the Precambrian (between 4.5 Ga and 540 Ma) and have been repeatedly uplifted and eroded. Today it consists largely of an area of low relief 300–610 m (980–2,000 ft) above sea level with a fewmonadnocks and low mountain ranges (including theLaurentian Mountains) probably eroded from the plateau during theCenozoic Era. During thePleistocene Epoch, continental ice sheets depressed the land surface (creatingHudson Bay) but also tilted up its northeastern "rim" (theTorngat), scooped out thousands of lake basins, and carried away much of the region's soil. The northeastern portion, however, became tilted up so that, in northernLabrador andBaffin Island, the land rises to more than 1,500 metres (5,000 feet) above sea level.
When theGreenland section is included, the Canadian Shield is approximately circular, bounded on the northeast by the northeast edge of Greenland, with Hudson Bay in the middle. It covers much of Greenland, all of Labrador and theGreat Northern Peninsula ofNewfoundland, most ofQuebec north of theSt. Lawrence River, much ofOntario including northern sections of theOntario Peninsula, the Adirondack Mountains[6] ofNew York, the northernmost part ofLower Michigan and all ofUpper Michigan, northernWisconsin, northeasternMinnesota, the central and northern portions ofManitoba, northernSaskatchewan, a small portion of northeasternAlberta,[7] mainlandNorthwest Territories to the east of a line extended north from the Saskatchewan-Alberta border, most ofNunavut's mainland and, of itsArctic Archipelago, Baffin Island and significant bands throughSomerset,Southampton,Devon andEllesmere islands.[2] In total, the exposed area of the shield covers approximately 8,000,000 km2 (3,100,000 sq mi). The true extent of the shield is greater still and stretches from theWestern Cordillera in the west to theAppalachians in the east and as far south asTexas, but these regions are overlaid with much younger rocks and sediment.
The Canadian Shield is among the oldest geologic areas on Earth, with regions dating from 2.5 to 4.2 billion years.[8] The multitude of rivers and lakes in the region is classical example of aderanged drainage system, caused by thewatersheds of the area being disturbed by glaciation and the effect ofpost-glacial rebound.[9] The shield was originally an area of very large, very tall mountains (about 12,000 m or 39,000 ft)[10] with much volcanic activity, but the area was eroded to nearly its current topographic appearance of relatively low relief over 500 Ma.[11][12]Erosion has exposed the roots of the mountains, which take the form ofgreenstone belts in which belts of volcanic rock that have been altered by metamorphism are surrounded by granitic rock. These belts range in age from 3.6 to 2.7 Ga.[13] Much of the granitic rock belongs to the distinctivetonalite–trondhjemite–granodiorite family of rocks, which are characteristic ofArcheancontinental crust.[14] Many of Canada's major ore deposits are associated with greenstone belts.[15]
TheSturgeon Lake Caldera inKenora District, Ontario, is one of the world's best preservedmineralizedNeoarcheancaldera complexes, which is 2.7 Ga.[16] The Canadian Shield also contains theMackenzie dike swarm, which is the largest dike swarm known on Earth.[17] The North American craton is the bedrock forming the heart of the North American continent, and the Canadian Shield is the largest exposed part of the craton's bedrock. The Canadian Shield is part of an ancient continent calledArctica, which was formed about 2.5 Ga during theNeoarchean era.
Mountains have deep roots and float on the densermantle much like aniceberg atsea. As mountains erode, their roots rise and are eroded in turn. The rocks that now form the surface of the shield were once far below the Earth's surface. The high pressures and temperatures at those depths provided ideal conditions for mineralization. Although these mountains are now heavily eroded, many large mountains still exist in Canada's far north called theArctic Cordillera. This is a vast, deeply dissected mountain range, stretching from northernmost Ellesmere Island to the northernmost tip of Labrador. The range's highest peak is Nunavut'sBarbeau Peak at 2,616 metres (8,583 ft) above sea level.[18] Precambrian rock is the major component of the bedrock.
The currentsurface expression of the shield is one of very thin soil lying on top of the bedrock, with many bareoutcrops. This arrangement was caused by severe glaciation during the ice ages that covered the shield and scraped the rock clean. The lowlands of the Canadian Shield have a very dense soil that is not suitable for forestation; it also contains many marshes and bogs (muskegs). The rest of the region has coarse soil that does not retain moisture well and is frozen withpermafrost throughout the year. Forests are not as dense in the north.
The shield is covered in parts by vastboreal forests in the south that support naturalecosystems as well as a majorlogging industry. The boreal forest area gives way to theEastern Canadian Shield taiga that covers northern Quebec and most of Labrador. TheMidwestern Canadian Shield forests that run westwards fromNorthwestern Ontario have boreal forests that give way to taiga in the most northerly parts of Manitoba and Saskatchewan.Hydrologic drainage is generally poor, the soil compacting effects of glaciation being one of the many causes.Tundra typically prevails in the northern regions.
Many mammals such asbeaver,caribou,white-tailed deer,moose,wolves,wolverines,weasels,mink,otters,grizzly bear,polar bears andblack bears are present.[19] In the case of polar bears (Ursus maritimus), the shield area contains many of theirdenning locations, such as theWapusk National Park.[20]The many lakes and rivers on the shield contain a plentiful quantity of different sports fish species, includingwalleye,northern pike,lake trout,yellow perch,whitefish,brook trout,arctic grayling, and many types of baitfish.[21] The water surfaces are also home to manywaterfowl, most notablyCanada geese,loons andgulls.[22] The vast forests contain a myriad population of other birds, includingravens andcrows,predatory birds and manysongbirds.
The Canadian Shield is one of the world's richest areas formineralores. It is filled with substantial deposits ofnickel,gold,silver, andcopper. There are many mining towns extracting these minerals. The largest, and one of the best known, isSudbury, Ontario. Sudbury is an exception to the normal process of forming minerals in the shield since theSudbury Basin is an ancientmeteoriteimpact crater.Ejecta from the meteorite impact was found in theRove Formation in May 2007. The nearby but less-knownTemagami Magnetic Anomaly has striking similarities to the Sudbury Basin. This suggests it could be a second metal-rich impact crater.[23] In northeastern Quebec, the giantManicouagan Reservoir is the site of an extensivehydroelectric project (Manic-cinq, or Manic-5). This is one ofthe largest-known meteorite impact craters on Earth, though not as large as the Sudbury crater.
TheFlin Flon greenstone belt in central Manitoba and east-central Saskatchewan "is one of the largestPaleoproterozoic volcanic-hosted massivesulfide (VMS) districts in the world, containing 27copper-zinc-(gold) deposits from which more than 183 million tonnes of sulfide have been mined."[24] The portion in the Northwest Territories has recently been the site of several majordiamond discoveries. Thekimberlite pipes in which the diamonds are found are closely associated with cratons, which provide the deeplithosphericmantle required to stabilize diamond as a mineral. The kimberlite eruptions then bring the diamonds from over 150 kilometres (93 mi) depth to the surface. TheDiavik mine is actively mining kimberlite diamonds.
Formed over 3 billion years through processes such as plate tectonics, erosion and glaciation
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