The Canadian ShieldGeography . Geology
The Canadian Shield refers to the exposed part of the continental crust underlying most of North America. This crust, the “North American craton”, extends from northern Mexico to Greenland and is composed of hard rocks that are at least a billion years old. The rocks of the North American Craton, with the exception of those that form the Canadian Shield, are deeply buried and covered by soil and other materials. Covering 5 million km2, the Shield represents about 50% of Canada’s land mass. Reminiscent of a horseshoe shape or the shields used in hand-to-hand combat, the Canadian Shield begins in the east in Labrador, includes most of Quebec, most of Ontario and Manitoba, the northern part of Saskatchewan, the northwestern corner of Alberta, and most of the Northwest Territories, Nunavut, and the Arctic Archipelago. (It is also found in parts of the United States, in New York, Wisconsin, and Minnesota.) The Shield once stood in the way of settlement, but it is also the source of valuable resources, such as minerals, coniferous forests and sites for hydroelectric dams.
Formation of the Canadian Shield
The Canadian Shield was formed over 3 billion years ago by plate tectonics, erosion and glaciation. Plate tectonics refers to the movement of different parts – or plates – of the Earth’s surface crust and the resulting collisions. When two crustal plates collide, they can merge and form larger land regions. The Shield can be seen as a jigsaw puzzle of several crustal blocks, sometimes called “provinces”, that have fused together over time. Each block is surrounded by a belt of younger rocks created by the collision with the neighboring block.
When the Earth deforms as a result of these collisions, geologists refer to this as “orogeny”. Orogenesis results in an upward thrust of the crust and thus the creation of mountain ranges. The Shield is the result of a series of orogenies and occupies an area that was once mountainous. One of the most important orogenies is the Grenville Orogeny, which occurred about a billion years ago and created the Grenville Mountain Range. This range stretches from Quebec to Ontario and occupies the eastern part of the continent as far south as Texas. Comparable in size to the Himalayas of today, the Grenville Mountains are perhaps the largest mountains ever to have appeared on Earth.
The relief of the Canadian Shield was subsequently eroded by weathering events such as wind and rain. Acting like sandpaper on wood, these forces slowly eroded the mountains into the low-lying surface typical of today’s Shield about 800 million years ago. In the case of the Grenvillian Mountains, it is estimated that the rock was probably eroded tens of kilometers thick in this way.
This weathering process was interrupted by a number of glacial periods over the past 2.5 million years. Like weathering events, glaciers round out the landforms and move sediments as they are created and advance across the land. When they retreat, they release the material they were carrying up to that point, and the sediment deposited is what forms the soil.
The last ice age to shape the region was the Wisconsin Ice Age, which began about 110,000 years ago. At that time, the Shield was covered by the Laurentide Ice Sheet, a huge expanse of ice up to 2 miles thick. As the ice retreated from the southern part of what is now Canada – a process that began only 11,000 years ago – it carved out the Great Lakes basins and the thousands of lakes that dot the Canadian Shield today.
The Shield can be divided into seven distinct geological regions, called “provinces”: the Nain, Grenville, Southern, Lake Superior, Churchill, Slave and Bear provinces, each of which contains rocks of different ages, origins and types, as well as different mineral deposits. The Southern Province, for example, includes the mining districts of Sudbury, Ontario, known for its copper and nickel production. To the north of this region, the Lake Superior Province is one of Canada’s most important sources of metals, with deposits of copper, gold, iron and silver. The Bear Province is home to copper and uranium deposits, while the Slave Province is explored and mined for diamonds. The Churchill and Grenville provinces contain deposits of uranium, lead and zinc. The Nain province is the smallest of the seven regions and is not a major source of minerals.
Physical Characteristics and Topography
The Canadian Shield is characterized by thousands of small lakes, shallow soil and rolling hills. The lakes are primarily the result of glacial erosion that shaped the landscape during the last ice age. Glacial striations (grooves visible on the rock surface) and drumlins (long hills composed of glacial sediments) are also evidence of past glacial presence.
In addition to the multitude of small lakes that dot the Shield, the region is bordered by several of Canada’s largest lakes and longest rivers: 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 northern shore of the St. Lawrence River.
The Canadian Shield is home to a large number of wildlife. Southern lakes and rivers are home to a variety of fish species including trout, burbot and northern pike. In addition to fish, the lakes are often visited by a variety of waterfowl such as wood ducks, Canada geese and black ducks. The area is also home to other birds such as boreal owls, great horned owls, blue jays and white-throated sparrows, as well as caribou, deer, wolves, lynx, moose, black bears and beavers on the mammal side.
As one moves north into the tundra, the wildlife, like the vegetation, becomes increasingly rare. In the Arctic Shield, polar bears, Arctic foxes, Arctic hares, snowy owls and rock ptarmigan are found.
The Canadian Shield is dominated by the boreal forest ecosystem. The following conifers are common: white and black spruce; jack pine, red pine, white pine, Weymouth pine; balsam fir; tamarack; eastern hemlock; and white juniper. Deciduous trees include red and mountain maple, white and white birch, trembling aspen, black ash and balsam poplar.
North of the boreal forest is the Arctic tundra ecozone, characterized by low vegetation and little or no tree density. Because the tundra is underlain by permafrost, plants cannot develop a deep root system, which limits their growth.
The Canadian Shield is rich in natural resources, including minerals, timber and fresh water. The mining industry began to develop in the region in the mid-19th century and became a driving force behind the country’s economic development. A variety of minerals and gems have since been mined from the Shield, including gold, silver, copper, zinc, nickel, iron, uranium and diamonds.
The first hard rock mine in the Canadian Shield, near Madoc, Ontario, opened in 1866 after gold was discovered at the site. Significant gold deposits were also discovered in Ontario at Kirkland Lake in 1906 and Timmins in 1912, and in Quebec at Rouyn-Noranda in 1920. Silver was discovered at the same time near Cobalt, Ontario in 1903. The town prospered during the silver rush of the early 20th century before declining in the 1920s. Gold mines remained in operation near Kirkland Lake and Timmins, but those at Colbalt and Rouyn-Noranda were no longer active, as was the case in many early mining towns.
Today, the largest concentration of operating mines on the Shield, and in the world, is around Sudbury, Ontario. Copper, nickel, gold and palladium are mined here.
Several diamondiferous kimberlites are scattered throughout the Shield, with the largest deposit located near Lac de Gras in the Northwest Territories, about 300 km northeast of Yellowknife. Canada’s first diamond mine – the Ekati mine – opened at this location in 1998.
The Canadian Shield also contains iron ore, with several deposits near Wawa, Ontario. The largest iron deposit in Canada, however, is the Labrador Trough – the New Quebec Orogen – which forms a band in northeastern Quebec and western Labrador.
The Shield also contains significant uranium deposits around Great Bear Lake in the Northwest Territories in northern Saskatchewan and at Elliot Lake in Ontario. Today, Saskatchewan is the sole producer of uranium in Canada, with mining sites located primarily in the Cigar Lake area.
The predominance of the boreal forest throughout the Canadian Shield means that forestry is also a major industry. Many small towns and villages throughout northern Ontario and Quebec have developed around pulp and paper mills and other forestry-related activities (e.g., Atikokan, Ontario).
The many rivers that flow through the Canadian Shield region also provide a significant amount of hydroelectricity. Among the most productive dams are Churchill Falls, Labrador and James Bay, Quebec.
The Canadian Shield is the traditional territory of several Aboriginal peoples. The Innu occupy the Shield in what is now Quebec and Labrador, while the Cree, Ojibwe and Métis occupy large areas in Quebec, Ontario, Manitoba, Saskatchewan and Alberta. The traditional territory of the Dene and Inuit includes the parts of the Shield now included in the Northwest Territories and Nunavut. From the beginning of the country’s colonization and the settlement of Europeans, the beavers present throughout the Shield fuelled the fur trade (see also Exploration). Inspired by native techniques, coureurs de bois, voyageurs and explorers used birchbark canoes to travel and trade along the Shield’s many waterways. Later, beginning in the mid-19th century, railroad construction required the blasting of many rocky sections of the Shield, exposing valuable minerals in the process. The rocky landscape, shallow soil, muskeg and multitude of insects made the Shield inhospitable, but the development of resource-based industries such as mining and forestry encouraged newcomers to settle. Hydroelectric projects such as the dams at Churchill Falls, Labrador, James Bay, Quebec, and Kettle, Manitoba, continued to provide power to southern towns.
The first maps of this region were made by Alexander Murray who, between 1851 and 1852, studied the geology of the Gananoque, Ottawa and St. Lawrence valleys and the perimeter of the Shield between Kingston and Lake Superior.
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