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Regional extent When the Greenland section is included, the 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; Labrador; most of Quebec north of the St. Lawrence River; much of Ontario including northern sections of the southern peninsula between the Great Lakes; the Adirondack Mountains of northern New York; most northern part of lower and all of upper Michigan, northern Wisconsin, and north-east Minnesota; the central/northern portions of Manitoba away from Hudson Bay and the Great Plains; northern Saskatchewan; a small portion of north-eastern Alberta; and the mainland northern Canadian territories to the east of a line extended north from the Saskatchewan/Alberta border (Northwest Territories and Nunavut). In total it covers approximately 8 million square kilometers. It covers even more area and stretches till the Western Cordillera in the west and Appalachians 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. Geology Such a large area of exposed, old rock requires some explanation. 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 multitude of rivers and lakes in the entire region is caused by the watersheds 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 that were about 12,000 m in height and much volcanic activity, but over the millennia the area was eroded to its current topographic appearance of relatively low relief. 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. 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. 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. One of the largest, and best known, is Greater Sudbury, Ontario. Sudbury is an exception to the normal process of forming minerals in the Shield since there is significant evidence that the Sudbury Basin is an ancient meteorite impact crater. The Shield, particularly the portion in the Northwest Territories, 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 150 km depth to the surface. Currently the Ekati and Diavik mines are actively mining kimberlite diamonds. The Shield is also covered by vast boreal forests that support an important logging industry. See also | ||||||||||
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