The Canadian Shield–also called the Laurentian Plateau, or Bouclier canadien (French)–is a massive geological shield covered by a thin layer of rocks that forms the nucleus of the North American or Laurentia craton. It is an area mainly covered by igneous rock which relates to its long volcanic history. It has a deep, common, joined bedrock region in eastern and central Canada and stretches North from the Great Lakes to the Arctic Ocean, covering over half of Canada; it also extends south into the northern reaches of the United States. Population is scarce, and industrial development is minimal, although the region has a large hydroelectric power potential.
The Canadian shield is a physiographic division, consisting of five smaller physiographic provinces, the Laurentian Upland, Kazan Region, Davis, Hudson, and James. The shield extends into the United States as the Adirondack Mountains (connected by the Frontenac Axis) and the Superior Upland. The Canadian Shield is U-shaped, but almost semi-circular, which yields an appearance of a warrior's shield or a giant doughnut, and is a subsection of the Laurentia craton signifying the area of greatest glacial impact (scraping down to bare rock) creating the thin soils.
The Canadian Shield is a collage of Archean plates and accreted juvenile arc terranes and sedimentary basins of Proterozoic age that were progressively amalgamated during the interval 2.45 to 1.24 Ga, with the most substantial growth period occurring during the Trans-Hudson orogeny, between ca. 1.90 to 1.80 Ga. The Canadian Shield was the first part of North America to be permanently elevated above sea level and has remained almost wholly untouched by successive encroachments of the sea upon the continent. It is the Earth's greatest area of exposed Archaean rock. The metamorphic base rocks are mostly from the Precambrian Era (between 4.5 billion and 540 million years ago), and have been repeatedly uplifted and eroded. Today it consists largely of an area of low relief 300 to 610 m (980 to 2,000 ft) above sea level with a few monadnocks and low mountain ranges (including the Torngat and Laurentian Mountains) probably eroded from the plateau during the Cenozoic era. During the Pleistocene epoch, continental ice sheets depressed the land surface (see Hudson Bay), scooped out thousands of lake basins, and carried away much of the region's soil.
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, the northernmost part of Lower Michigan and all of Upper Michigan, northern Wisconsin, northeastern Minnesota, the central/northern portions of Manitoba away from Hudson Bay, the Great Plains, northern Saskatchewan, a small portion of northeastern 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 8,000,000 km2 (3,088,817 sq mi). It covers even more area and stretches to 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.
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 (about 12,000 metres or 39,000 ft) with much volcanic activity, but over the millennia the area was eroded to its current topographic appearance of relatively low relief. It contains some of the most ancient volcanoes on Earth. It has over 150 volcanic belts (now deformed and eroded down to nearly flat plains) that range from 600 to 1200 million years old.
Each belt probably grew by the coalescence of accumulations erupted from numerous vents, making the tally of volcanoes in the hundreds. Many of Canada's major ore deposits are associated with Precambrian volcanoes.
The Sturgeon Lake Caldera in Kenora District, Ontario is one of the world's best preserved mineralized Neoarchean caldera complexes, which is some 2.7 billion years old. The Canadian Shield also contains the Mackenzie dike swarm, which is the largest dike swarm known on Earth.
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.
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 billion years ago, during the Neoarchean era. It was split into Greenland, Laurentia, Scotland, Siberia, East Antarctica and is now roughly situated in the Arctic around the current North Pole.
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- Using and analog GPS (ie a compass), hold it my the large rock and record its behavior. What do you think causes this?
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