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The Canadian Shield— also called the Precambrian Shield, Laurentian Shield, or Laurentian Plateau— is a large thin-soiled area over a part of the North American craton (a deep, common, joined bedrock region) in eastern and central Canada and adjacent portions of the United States, composed of base rock dating to the Precambrian Era (between 4.5 billion and 540 million years ago). The Canadian Shield is almost circular which gives it an appearance of a warrior's shield or a giant horseshoe, and is a subsection of the Laurentia craton signifying the area of greatest glacial impact (scraping down to bare rock) creating the thin soils.
Drainage is generally very poor on the shield, mostly due to the glaciation. The southern part of the shield has thick forests while the north is covered with tundra. The region is largely undeveloped but has great water-power potential and is a source of minerals, timber, and fur-bearing animals.
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.
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. 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 of relatively recent origin, and one theory of their origin suggests that the Shield was at some time in the past above a hotspot 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.