Canadian Shield
Geology

The Canadian Shield is among the oldest geologic areas on Earth, with regions dating from 2.5 to 4.2 billion years. The multitude of rivers and lakes in the region is classical example of a deranged drainage system, caused by the watersheds of the area being disturbed by glaciation and the effect of post-glacial rebound. The shield was originally an area of very large, very tall mountains (about 12,000 m or 39,000 ft) with much volcanic activity, but the area was eroded to nearly its current topographic appearance of relatively low relief over 500 Ma. Erosion has exposed the roots of the mountains, which take the form of greenstone belts in which belts of volcanic rock that have been altered by metamorphism are surrounded by granitic rock. These belts range in age from 3.6 to 2.7 Ga. Much of the granitic rock belongs to the distinctive tonalite–trondhjemite–granodiorite family of rocks, which are characteristic of Archean continental crust. Many of Canada's major ore deposits are associated with greenstone belts.

The Sturgeon Lake Caldera in Kenora District, Ontario, is one of the world's best preserved mineralized Neoarchean caldera complexes, which is 2.7 Ga. The Canadian Shield also contains the Mackenzie dike swarm, which is the largest dike swarm known on Earth. 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 Ga during the Neoarchean era.

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 Island to the northernmost tip of Labrador. The range's highest peak is Nunavut's Barbeau Peak at 2,616 metres (8,583 ft) above sea level. Precambrian rock is the major component of the bedrock.
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