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The Canadian Shield—also called the Laurentian Plateau, or Bouclier Canadien (French)—is a massive geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mainly covered by igneous rock which relates to its long volcanic history. It has a deep, common, joined bedrock region in eastern and central Canadamarker and stretches North from the Great Lakesmarker to the Arctic Oceanmarker, covering over half of Canada; it also extends south into the northern reaches of the United States. Population is scarce, and industrial development is minimal, although the region has a large hydroelectric power potential.

Geographical extent

The Canadian shield is a physiographic division, consisting of five smaller physiographic provinces, the Laurentian Upland, Kazan, Davis, Hudson, and James. The shield extends into the United States as the Adirondack Mountains and the Superior Upland. The Canadian Shield is U-shaped, but almost semi-circular, which yields an appearance of a warrior's shield or a giant doughnut, and is a subsection of the Laurentia craton signifying the area of greatest glacial impact (scraping down to bare rock) creating the thin soils.

The Canadian Shield is a collage of Archean plates and accreted juvenile arc terranes and sedimentary basins of Proterozoic age that were progressively amalgamated during the interval 2.45 to 1.24 Ga, with the most substantial growth period occurring during the Trans-Hudson orogeny, between ca. 1.90 to 1.80 Ga. 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 Archaean rock. The metamorphic base rocks are mostly from the Precambrian Era (between 4.5 billion and 540 million years ago), and have been repeatedly uplifted and eroded. Today it consists largely of an area of low relief (1,000–2,000 ft/300–600 m above sea level) with a few monadnocks and low mountain ranges (including the Torngatmarker and Laurentian Mountainsmarker) probably eroded from the plateau during the Cenozoic era. During the Pleistocene epoch, continental ice sheets depressed the land surface (see Hudson Baymarker), scooped out thousands of lake basins, and carried away much of the region's soil.

When the Greenlandmarker section is included, the Shield is approximately circular bounded on the northeast by the northeast edge of Greenland, with Hudson Baymarker in the middle. It covers much of Greenland, Labrador, most of Quebecmarker north of the St. Lawrence Rivermarker, much of Ontariomarker including northern sections of the southern peninsula between the Great Lakesmarker, the Adirondack Mountains of northern New Yorkmarker, the northernmost part of Lower Michiganmarker and all of Upper Michiganmarker, northern Wisconsinmarker, and northeastern Minnesotamarker, the central/northern portions of Manitobamarker away from Hudson Bay and the Great Plainsmarker, northern Saskatchewanmarker, a small portion of northeastern Albertamarker, and the mainland northern Canadian territories to the east of a line extended north from the Saskatchewan/Alberta border (Northwest Territoriesmarker and Nunavutmarker). In total, it covers approximately 8 million square kilometers. It covers even more area and stretches to the Western Cordillera in the west and Appalachiansmarker in the east, but the formations are still underground. The underlying rock structure does include Hudson Bay and the submerged area between North America and Greenland.

Climate

The climate varies significantly across the large extent of the Canadian Shield. In the southern parts the climate is seasonal; the average temperature in the winter is , and in the summer it is . The growing season of about 120 days coincides with summer daylight averaging about 15 hours, while winter daylight averages about 8.5 hours. In the northern parts, it is very cold. The average temperatures range from in the summer to in winter. The growing season is only 60 days. Winter daylight hours are about 5.5 hours, and in the summer the daylight hours are about 18.5 hours.

Geology

The multitude of rivers and lakes in the entire region is caused by the watershed of the area being so young and in a state of sorting themselves out with the added effect of post-glacial rebound. The Shield was originally an area of very large mountains (about ) with much volcanic activity, but over the millennia the area was eroded to its current topographic appearance of relatively low relief. It contains some of the most ancient volcanoes on Earth. It has over 150 volcanic belts (now deformed and eroded down to nearly flat plains) that range from 600 to 1200 million years old.

Each belt probably grew by the coalescence of accumulations erupted from numerous vents, making the tally of volcanoes in the hundreds. Many of Canada's major ore deposits are associated with Precambrian volcanoes.

The Sturgeon Lake Calderamarker in Kenora District, Ontariomarker is one of the world's best preserved mineralized Neoarchean caldera complexes, which is some 2.7 billion years old. The Canadian Shield also contains the Mackenzie dike swarm, which is the largest dike swarm known on Earth.

Typical Canadian Shield: pines, lakes, bogs, and rock.


Mountains have deep roots and float on the denser mantle much like an iceberg at sea. 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 the Arctic Cordillera. This is a vast deeply dissected mountain range, stretching from northernmost Ellesmere Islandmarker to the northernmost tip of Labrador. The range's highest peak is Nunavutmarker's Barbeau Peakmarker at above sea level. Precambrian rock is the major component of the bedrock.

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 called Arctica, which was formed about 2.5 billion years ago, during the Neoarchean era. It was split into Greenland, Laurentia, Scotlandmarker, Siberiamarker, East Antarcticamarker and is now roughly situated in the Arctic around the current North Polemarker.

Ecology

The current surface expression of the Shield is one of very thin soil lying on top of the bedrock, with many bare outcrops. This arrangement was caused by severe glaciation during the last ice age, which covered the Shield and scraped the rock clean.

The lowlands of the Canadian Shield have very dense soil that is not suitable for forestation, but it also contains many marshes and bogs. The rest of the region has coarse soil that does not retain moisture well and is frozen as permafrost year round. Forests are not as dense in the north.

The Shield is covered in parts by vast boreal forests in the south that support important natural ecosystems as well as a major logging industry. Hydrographical drainage is generally poor, the effects of glaciation being one of the many reasons. Tundra typically prevails in the northern regions. Many mammals such as caribou, wolverines, weasels, mink, otters, grizzlies, polar bears and black bear are present. In the case of polar bears (Ursus maritimus) the Shield area contains many of the denning locations such as the Wapusk National Parkmarker.

Mining and economics

The Shield is one of the world's richest areas in terms of mineral ores. It is filled with substantial deposits of nickel, gold, silver, and copper. Throughout the Shield there are many mining towns extracting these minerals. The largest, and one of the best known, is Sudburymarker, Ontariomarker. Sudbury is an exception to the normal process of forming minerals in the Shield since there is significant evidence that the Sudbury Basinmarker is an ancient meteorite impact crater. The nearby, but less known Temagami Magnetic Anomalymarker has striking similarities to the Sudbury Basin. This suggests it could be a second metal-rich impact crater.

In northeastern Quebec, the giant Manicouagan Reservoirmarker is the site of an extensive hydroelectric project (Manic-cinq, or Manic-5). This is one of the largest-known meteorite impact craters on Earth.

The Flin Flon greenstone belt in central Manitobamarker and east-central Saskatchewanmarker is one of the largest Paleoproterozoic volcanic-hosted massive sulfide (VMS) districts in the world, containing 27 copper-zinc-(gold) deposits from which more than 183 million tons of sulfide have been mined.

The Shield, particularly the portion in the Northwest Territoriesmarker, has recently been the site of several major diamond discoveries. The kimberlite pipes in which the diamonds are found are closely associated with cratons, which provide the deep lithospheric mantle required to stabilize diamond as a mineral. The kimberlite eruptions then bring the diamonds from over depth to the surface. Currently the Ekatimarker and Diavikmarker mines are actively mining kimberlite diamonds.

See also



References


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