A large geographic area in eastern and central Canada, composed of bare rock dating to the Precambrian era. Also called the Precambrian Shield, or Laurentian Shield, or Laurentian Plateau.
The shield is approximately U-shaped, with the points in the arctic. It covers much of Greenland; Labrador; all of Quebec north of the St. Lawrence River; much of Ontario outside the southern peninsula between the Great Lakes; upstate New York; parts of Michigan, Wisconsin, and Minnesota; the central portion of Manitoba away from Hudson's 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. In total it covers approximately 8 million square kilometers.
Such a large area of exposed, old rock requires some explanation. The current layout of the Shield is one of very thin soil lying on top of the bedrock, with many bare outcrops. This arrangement was caused by the last Ice Age, which only recently retreated from the shield and scraped the rock clean in doing so. 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.
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 the Earth's mantle (much like the one that formed Hawaii, but under land rather than ocean). The spot lifted the surrounding landscape as continental drift moved 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.