The Canadian Shield, also called the Laurentian Plateau, or Bouclier canadien (French), is a large area of exposed Precambrianigneous and high-grade metamorphic rocks (geological shield) that forms the ancient geological core of the North American continent (the North American Craton or Laurentia). Composed of igneous rock resulting from its long volcanic history, the area is covered by a thin layer of soil. With a deep, common, joined bedrock region in eastern and central Canada, it 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. Human population is sparse, and industrial development is minimal, while mining is prevalent.
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 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 more than 3.96 billion years old. The Canadian Shield once had jagged peaks, higher than any of today's mountains, but millions of years of erosion have changed these mountains to rolling hills.
The Canadian Shield is a collage of Archean plates and accreted juvenile arc terranes and sedimentary basins of the Proterozoic Eon 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 Archean rock. The metamorphicbase rocks are mostly from the Precambrian Supereon (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 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, 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, the exposed area of the Shield covers approximately 8,000,000 km2 (3,088,817 sq mi). The true extent of the Shield is greater still and stretches from the Western Cordillera in the west to the Appalachians in the east and as far south as Texas, but these regions are overlaid with much younger rocks and sediment. The underlying rock structure also includes Hudson Bay.
The Canadian Shield is among the oldest on earth, with regions dating from 2.5 to 4.2 billion years. 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, very tall mountains (about 12,000 metres or 39,000 feet) with much volcanic activity, but over hundreds of millions of years, the area has been eroded to its current topographic appearance of relatively low relief. It has some of the oldest (extinct) volcanoes on the planet. It has over 150 volcanic belts (now deformed and eroded down to nearly flat plains) whose bedrock ranges 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 reach 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 mineralizedNeoarcheancaldera complexes, which is 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 and Siberia and is now roughly situated in the Arctic around the current North Pole.
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 ice age, which covered the Shield and scraped the rock clean.
The lowlands of the Canadian Shield have a very dense soil that is not suitable for forestation; it also contains many marshes and bogs (muskegs). The rest of the region has coarse soil that does not retain moisture well and is frozen with permafrost throughout the year. Forests are not as dense in the north.
The Shield is covered in parts by vast boreal forests in the south that support natural ecosystems as well as a major logging industry. This boreal forest area includes ecoregions such as the Eastern Canadian Shield taiga that covers northern Quebec and most of Labrador, and the Midwestern Canadian Shield forests that run westwards from Northwestern Ontario. Hydrographical drainage is generally poor, the soil compacting effects of glaciation being one of the many causes. Tundra typically prevails in the northern regions. Many mammals such as caribou, white-tailed deer, moose, wolves, wolverines, weasels, mink, otters, grizzly bear, polar bears and black bears are present. In the case of polar bears (Ursus maritimus) the Shield area contains many of the denning locations such as the Wapusk National Park.
Mining and economics
The Canadian Shield is one of the world's richest areas in terms of mineralores. 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 one of the best known, is Sudbury, Ontario. Sudbury is an exception to the normal process of forming minerals in the Shield since the Sudbury Basin is an ancient meteoriteimpact crater. Ejecta from the meteorite impact was found in the Rove Formation in May 2007. The nearby, but less known Temagami Magnetic Anomaly, has striking similarities to the Sudbury Basin. This suggests it could be a second metal-rich impact crater.
In northeastern Quebec, the giant Manicouagan Reservoir is the site of an extensive hydroelectric project (Manic-cinq, or Manic-5). This is one of the largest-known meteoriteimpact craters on Earth.
The Flin Flon greenstone belt in central Manitoba and east-central Saskatchewan is one of the largest Paleoproterozoic volcanic-hosted massive sulfide (VMS) districts in the world, containing 27 copper-zinc-(gold) deposits from which more than 183 million tons[clarification needed] of sulfide have been mined.
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 closely associated with cratons, which provide the deep lithosphericmantle required to stabilize diamond as a mineral. The kimberlite eruptions then bring the diamonds from over 150 kilometres (93 mi) depth to the surface. Currently the Ekati and Diavik mines are actively mining kimberlite diamonds.
- ^ abThe Atlas of Canada. "Physiographic Regions Map"(JP2). Natural Resources Canada. Retrieved 2017-08-01.
- ^ abEncyclopædia Britannica. "Canadian Shield". Retrieved 2009-02-10.
- ^Stephen Marshak. Essentials of Geology. 3rd ed.
- ^"Canadian Shield" in Columbia Encyclopedia. 6th ed., 2005.
- ^James-Abra, Erin. "Canadian Shield". Canadian Encyclopedia. Retrieved 24 January 2018.
- ^Corrigan, D. (2008). "Metallogeny and Tectonic Evolution of the Trans-Hudson Orogen"(PDF). Archived from the original(PDF) on April 9, 2008. Retrieved 2008-03-05.
- ^Peterson Field Guide to Geology of Eastern North America by Roberts, David & Roger Tory Peterson.
- ^Alberta Heritage - Alberta Online Encyclopedia - The Canadian Shield Region of Alberta
- ^Tsuyoshi Iizuka, at al., Geology and Zircon Geochronolgy of the Acasta Gneiss Complex, Precambrian Research, 153 (2007) pp. 179 - 208
- ^Clark, Bruce W. (1999). "Geologic History". Making Connections: Canada's geography. Scarborough, Ontario: Prentice Hall Ginn Canada. p. 95. ISBN 0-13-012635-7.
- ^Caldera Volcanoes Retrieved on 2007-07-20
- ^Mark Pilkington and Walter R. Roest, Removing varying directional trends in aeromagnetic data,Geophysics, vol. 63 no. 2 (1998), pp. 446–453.
- ^"Barbeau Peak". Bivouac.com.
- ^World Wildlife Fund (2001). "Northern Canadian Shield taiga". WildWorld Ecoregion Profile. National Geographic Society. Archived from the original on 2010-03-08.
- ^C. Michael Hogan (2008) Polar Bear: Ursus maritimus, Globaltwitcher.com, ed. Nicklas StrombergArchived December 24, 2008, at the Wayback Machine.
- ^3-D Magnetic Imaging using Conjugate Gradients: Temagami anomalyArchived 2009-07-11 at the Wayback Machine. Retrieved on 2008-03-12
- ^Norris, Jessica (2007). "Report on the 2007 Diamond Drilling Program McClarty Lake Project, Manitoba"(PDF). Aurora Geosciences Ltd. Archived from the original(PDF) on 2008-05-30. Retrieved 2008-02-22.
The Canadian Shield refers to the exposed portion of the continental crust underlying the majority of North America. The crust, also known as the North American Craton, extends from northern Mexico to Greenland and consists of hard rocks at least 1 billion years old. With the exception of the Canadian Shield, the rocks of the North American Craton are buried deep within the continent and covered by soil and other material. At 5 million km2, the Shield makes up roughly 50 per cent of Canada’s land mass. Shaped like a horseshoe — or the shields carried during hand-to-hand combat — the Canadian Shield extends from Labrador in the east to include nearly all of Québec, much of Ontario and Manitoba, the northern portion of Saskatchewan, the northeast corner of Alberta, much of the Northwest Territories and Nunavut and into the Arctic Archipelago. (It also reaches into parts of the United States, in New York, Wisconsin and Minnesota.) While at times a barrier to settlement, the Shield has also yielded great resources, including minerals, coniferous forests and the capacity for hydroelectric developments.
How Was the Canadian Shield Formed?
The Canadian Shield formed over 3 billion years through processes such as plate tectonics, erosion and glaciation. Plate tectonics refers to the movement and collision of the Earth’s outer crust. When these crustal plates collide they may weld together, forming larger landmasses. The Shield can be thought of as a jigsaw puzzle of different crustal blocks, sometimes known as “provinces,” welded together over time. Each block is bounded by a belt of younger rock created when the blocks collided.
When the earth deforms as a result of these collisions, geologists call the deformation process an “orogeny.” Orogenies cause the earth to thrust upward, creating mountain ranges. Because the Shield was formed through a series of orogenies, it was once a mountainous region. One of the largest of these mountain-making events was the Grenville Orogeny. Occurring about 1 billion years ago, the Grenville Orogeny created the Grenville Mountain Range, stretching from Québec through Ontario and down the eastern side of the continent to Texas. Comparable to the present-day Himalayas in size, the Grenville Mountains may have been the largest mountain range ever created on Earth.
The mountains of the Canadian Shield were subsequently eroded by weather such as wind and rain. Comparable to sandpaper on wood, these forces slowly wore down the mountains, so that by about 800 million years ago, the low-relief surface of the Shield had been created. In the case of the Grenville Mountains, it’s estimated that tens of kilometres of rock may have been worn down.
These weathering processes were interrupted by a number of ice ages occurring over the last 2.5 million years. Like weather, as glaciers grow and move they also smooth the landscape, as well as move sediment. When they retreat, glaciers release the material they’ve been transporting and the deposited sediments become the origin of soils.
The most recent glaciation to affect the area was the Wisconsin glaciation that began around 110,000 years ago. During this glaciation, the Shield was covered by the Laurentide Ice Sheet, a giant expanse of ice as much as 3 km thick. As the ice retreated from the southern part of present-day Canada — a process that began as recently as 11,000 years ago — it cut the basins of the Great Lakes as well as the thousands of lakes throughout the Canadian Shield.
The Shield can be divided into seven geologically distinct regions sometimes referred to as provinces. They are the Nain, Grenville, Southern, Superior, Churchill, Slave and Bear provinces. Each is home to rock of different ages, types and formation characteristics, as well as different mineral deposits. The Southern Province, for example, is home to the mining district of Sudbury, Ontario, known for its production of copper and nickel. North of this region, the Superior Province is one of Canada’s most important sources of metals, including deposits of copper, gold, iron and silver. The Bear province includes deposits of copper and uranium, while the Slave province is being explored and mined for diamonds. The Churchill and Grenville provinces contain deposits of uranium, lead and zinc. Finally, the Nain province is the smallest of the seven regions, and is not a major mineral producer.
DID YOU KNOW?
The Canadian Shield contains some of the oldest rocks on Earth. In 2008, researchers estimated rock found on the northern shore of Hudson Bay, 40 km south of Inukjuak, to be 4.28 billion years old. Its age means the rock was created approximately 300 million years after the formation of Earth. Previously, the oldest rocks in the world were thought to be southeast of Great Bear Lake, in the Northwest Territories. These rocks were estimated to be 4 billion years old.
Physical Features and Landforms
The Canadian Shield’s most notable physical features are thousands of small lakes, thin layers of soil and rolling hills. Lakes are largely the result of glacial erosion during the last ice age. Other evidence of past glacial structures include striations (lines scraped into rocks) and drumlins (long hills of glacial sediment).
In addition to the thousands of small lakes throughout the Shield, the region also borders many of the larger lakes and waterways of Canada: the eastern shores of Great Bear Lake, Great Slave Lake, Lake Athabasca and Lake Winnipeg; the northern shores of Lake of the Woods, Lake Superior and Lake Huron; and the north shore of the St. Lawrence River.
The Canadian Shield is dominated by the boreal forest ecosystem. Common coniferous trees include white and black spruce; jack, red, white and eastern white pine; balsam fir; tamarack; eastern hemlock; and eastern red cedar. Deciduous trees include red and mountain maple; white and paper birch; trembling aspen; black ash; and balsam poplar.
North of the boreal forest is the Arctic tundra ecozone, characterized by low-growing vegetation and few to no trees. Because most of the tundra is underlain by permafrost, vegetation is not able to put down deep roots, stunting growth.
A wide range of wildlife calls the Canadian Shield home. Lakes and rivers in the south house a variety of fish species including trout, burbot and northern pike. In addition to fish, lakes are often spotted with a mix of waterfowl including wood ducks, Canada geese and American black ducks. Other birds include boreal owls, great horned owls, blue jays and white-throated sparrows, while mammals include caribou, deer, wolves, lynx, moose, black bears and beavers.
Moving north into the tundra, wildlife, like vegetation, becomes increasingly sparse. Animals in the Arctic portion of the Shield include polar bears, Arctic fox, Arctic hares, snowy owls and rock ptarmigan.
The Canadian Shield is rich in natural resources, including minerals, forests and freshwater. Mining began in the region in the mid-19th century and was key to Canada’s economic development. Various minerals and precious stones have been mined or continue to be mined on the Shield, including gold, silver, copper, zinc, nickel, iron, uranium and diamonds.
The first modern hard-rock mine in the Canadian Shield, near Madoc, Ontario, opened in 1866 after gold was found there. Substantial gold discoveries were also made in Ontario at Kirkland Lake in 1906 and Timmins in 1912, and in Québec at Rouyn-Noranda in 1920. Around the same time, silver was discovered near Cobalt, Ontario, in 1903. The town prospered during the silver rush of the early 20th century, before going into decline in the 1920s. While gold mines still operate near Kirkland Lake and Timmins, there are no longer any active mines in Colbalt or Rouyn-Noranda — a reality not uncommon for early mining towns.
Today, the largest concentration of active mines on the Shield — and in the world — is located around Sudbury, Ontario. Metals mined here include copper, nickel, gold and palladium.
Kimberlites — formations where diamonds can be located — are scattered throughout the Shield, with the largest deposit located near Lac de Gras, Northwest Territories, about 300 km northeast of Yellowknife. Canada’s first diamond mine — called Ekati — opened there in 1998.
The Canadian Shield also contains iron ore, including deposits near Wawa, Ontario. Canada’s largest iron ore deposit, however, is known as the Labrador Trough or New Québec Orogen, and runs in a strip through northeastern Québec and western Labrador.
Similarly, the Shield also contains major uranium deposits, found around Great Bear Lake in the Northwest Territories, in northern Saskatchewan, and at Elliot Lake, Ontario. Today, Saskatchewan is the sole producer of Canadian uranium, primarily from the Cigar Lake area.
DID YOU KNOW?
Sudbury, Ontario, was hit by a meteorite about 1.85 billion years ago, creating a crater known as the Sudbury Structure. The basin is 60 km long and 30 km wide and home to the rich metal resources found in the area. Metals such as copper and nickel are the result of the Earth’s crust melting after the meteorite hit.
Given the prominence of the boreal forest throughout the Canadian Shield, forestry is also a prominent industry. Many of the small towns and villages scattered throughout northern Ontario and Québec were founded around pulp and paper mills and other forestry-related activities (e.g., Atikokan, Ontario).
Due to its numerous rivers, the Canadian Shield region produces a significant amount of hydroelectricity. Some of the highest producing hydroelectric dams include Churchill Falls, Labrador, and James Bay, Québec.
The Canadian Shield is the traditional territory of several Indigenous peoples. The Innu made their home on the Shield in what is now Québec and Labrador, while the Cree, Anishinaabeg and Métis occupied large swaths of the region through Québec, Ontario, Manitoba, Saskatchewan and Alberta. The traditional territory of the Dene and Inuit includes the sections of the Shield now covered by the Northwest Territories and Nunavut. As Europeans began to settle and colonize the country, beavers found in the Shield region became a source for the fur trade (see alsoExploration). Learning from Indigenous people, coureurs de bois, voyageurs and explorers used the birchbark canoe to travel and trade along the Shield’s many waterways. Later, beginning in the mid-1800s, railway construction meant blasting through Canadian Shield rock, exposing valuable minerals in the process. While the bare rock, thin soils, muskeg and insects of the Shield made living there difficult, the development of resource-based industries, such as mining and forestry, promoted increased settlement. Hydroelectric developments such as those at Churchill Falls, Labrador, James Bay, Québec, and Kettle Rapids, Manitoba, continue to feed electricity to urban centres in the south.
The historic mapping of this area was done by Alexander Murray, who, in 1851–52, examined the geology around the Gananoque, Ottawa and St. Lawrence rivers, as well as the perimeter of the Shield from Kingston to Lake Superior. (See alsoGeological Survey of Canada.)
Nick Eyles and Andrew Miall, Canada Rocks: The Geologic Journey (2007).