Canadian Shield
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The Canadian Shield is a large craton in eastern and central Canada and adjacent portions of the United States, composed of bare rock dating to the Precambrian Era (between 4.5 billion and 540 million years ago). It is also called the Precambrian Shield, or Laurentian Shield, or Laurentian Plateau.
Regional extent
The Shield is approximately U-shaped, with the U's points in the arctic. It covers much of Greenland; Labrador; most of Quebec north of the St. Lawrence River; much of Ontario outside the southern peninsula between the Great Lakes; the Adirondack Mountains of northern New York; parts of Michigan, Wisconsin, and Minnesota; the central portion 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.
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 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.
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 north of Canada there are many mining towns extracting these minerals. The largest, and best known, is Sudbury, Ontario. Sudbury is an exception to the normal process of forming minerals in the Shield since the Sudbury Basin is believed to be 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 of relatively recent origin, and one theory of their origin suggests that the Shield was at some time in the past above a hot spot in Earth's mantle (much like the one that formed Hawaii, but under land rather than ocean). The spot lifted the surrounding landscape as the continent drifted over it, forming the pipes in various locations. The line of subsurface mountains that runs from the eastern seaboard of the United States nearly to Europe before culminating in the Challenger Seamount would, if run backwards in time, follow a path that matches what is suggested.
The Shield is also covered by vast boreal forests that support an important logging industry.de:Kanadischer Schild fr:Bouclier canadien ja:カナダ楯状地