Canadian Shield

• 1. Introduction

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  The Canadian Shield, also known as the Laurentian Shield or the Laurentian Plateau, spreads across more than half of Canada and most of Greenland. It is a geologic shield, meaning a vast region where Precambrian igneous and high-grade metamorphic rock lies exposed at the surface. Beneath the continent it forms the North American Craton, also called Laurentia, the ancient geologic core on which the rest of North America sits. The exposed part alone covers approximately 8,000,000 square kilometres. Yet most of it looks nothing like a mountain range. This documentary asks how a landscape once crowned with enormous peaks became rolling hills. It asks how the region was assembled from fragments of older crust, and what ice did to it. It also asks why the rock beneath the lakes and forests holds so much nickel, gold, silver, copper and even diamonds.

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• 2. Roots of Vanished Mountains

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  About 12,000 metres is the height the source gives for the mountains that once stood across the shield. They were tall, volcanic and far higher than any peak on Earth today. Over roughly 500 million years, erosion wore them down to the low relief seen now. Mountains have deep roots that float on the denser mantle much as an iceberg floats at sea. As the summits wear away, the roots rise and are eroded in turn. That is why the rocks at the surface of the shield were once far below the ground. The exposed roots take the form of greenstone belts. In these belts, volcanic rock altered by metamorphism is surrounded by granitic rock, and the belts range in age from 3.6 to 2.7 billion years. Much of that granite belongs to the tonalite-trondhjemite-granodiorite family, which is characteristic of Archean continental crust. Today most of the region sits between 300 and 610 metres above sea level. A few monadnocks and low ranges, including the Laurentian Mountains, rise above it. The deep heat and pressure that once acted on these rocks created ideal conditions for mineralization, and that detail matters later in this story.

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• 3. Assembling the Craton

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  Estimates put the age of the Canadian Shield at 4.28 billion years. The shield is a collage of Archean plates, together with young volcanic arc fragments and sedimentary basins from the Proterozoic Eon. These pieces were progressively joined between about 2.45 and 1.24 billion years ago. The most substantial growth came during the Trans-Hudson orogeny, between roughly 1.90 and 1.80 billion years ago. The shield was the first part of North America to be permanently lifted above sea level. Seas have encroached on the rest of the continent repeatedly, but they have left the shield almost wholly untouched. It is also the greatest area of exposed Archean rock on Earth. Some of its features are record-setting. The Sturgeon Lake Caldera in Kenora District, Ontario, dates to 2.7 billion years and is one of the best preserved mineralized Neoarchean caldera complexes in the world. The Mackenzie dike swarm is the largest known on Earth. The shield is also described as part of an ancient continent called Arctica, formed about 2.5 billion years ago. That ancient ground met a far younger force, the ice sheets of the Pleistocene.

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• 4. Scraped by Ice

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  The weight of continental ice sheets during the Pleistocene Epoch pressed the land surface down and created Hudson Bay. The ice also tilted up the northeastern rim, known as the Torngat. In northern Labrador and Baffin Island, the land now rises above 1,500 metres. Elsewhere the ice scooped out thousands of lake basins and carried away much of the region's soil. Rock in the north and east was scraped bare. The shield is described as a U-shaped part of the craton, marking where glaciation had its greatest effect. The result is a deranged drainage system, a classic example, in which rivers and lakes are scattered without an orderly pattern. Glaciation disturbed the watersheds, and post-glacial rebound added to the disorder. Soil is thin and bedrock often shows through. In the lowlands the soil is very dense, unsuitable for forests, and full of marshes and bogs called muskegs. The remaining ground is coarse, holds moisture poorly and is frozen with permafrost all year. Seen from above, the shield is a map of where this ice did its work, and the map is huge.

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• 5. A Shield Without Borders

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  Greenland is the key to the shape of the shield. When its Greenland section is included, the whole is approximately circular, with Hudson Bay at the centre and the northeast edge of Greenland as its boundary. Geographers divide it into four smaller physiographic provinces: the Laurentian Upland, Kazan Region, Davis and James. In Canada it covers all of Labrador and the Great Northern Peninsula of Newfoundland. It takes in most of Quebec north of the St. Lawrence River and much of Ontario, including northern parts of the Ontario Peninsula. Farther west it covers central and northern Manitoba, northern Saskatchewan and a small corner of northeastern Alberta. It also reaches mainland Northwest Territories and most of Nunavut's mainland. In the Arctic Archipelago it includes Baffin Island and bands across Somerset, Southampton, Devon and Ellesmere islands. South of the border it continues as the Adirondack Mountains of New York, linked by the Frontenac Axis. It also includes the Superior Upland, the northernmost part of Lower Michigan, all of Upper Michigan, northern Wisconsin and northeastern Minnesota. Younger rock and sediment hide the rest, which runs from the Western Cordillera to the Appalachians and as far south as Texas. Not every mountain here has been worn down. The Arctic Cordillera, a deeply dissected range, runs from northernmost Ellesmere Island to the northernmost tip of Labrador, and its highest peak is Nunavut's Barbeau Peak at 2,616 metres.

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• 6. Forests, Tundra and Wildlife

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  In the south of the shield, vast boreal forests support natural ecosystems and a major logging industry. Moving north, they give way to the Eastern Canadian Shield taiga, which covers northern Quebec and most of Labrador. West of Northwestern Ontario, the Midwestern Canadian Shield forests turn to taiga in the most northerly parts of Manitoba and Saskatchewan. Forests thin out toward the north, and tundra typically prevails in the northern regions. Drainage is generally poor, partly because glaciation compacted the soil. The animals are varied. Beaver, caribou, white-tailed deer, moose, wolves, wolverines, weasels, mink, otters, grizzly bears, polar bears and black bears all live here. Many denning sites of polar bears, Ursus maritimus, lie within the shield, such as those in Wapusk National Park. The lakes and rivers hold walleye, northern pike, lake trout, yellow perch, whitefish, brook trout and arctic grayling, along with many kinds of baitfish. Canada geese, loons and gulls use the water surfaces. The forests shelter ravens, crows, predatory birds and many songbirds. Beneath all this life, the old rock holds another kind of wealth.

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• 7. Nickel, Gold and Craters

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  Sudbury, Ontario, is the largest and one of the best known mining towns on the shield. The shield is one of the richest mineral regions in the world, with substantial deposits of nickel, gold, silver and copper. Sudbury is an exception to the usual way minerals form here, because the Sudbury Basin is an ancient meteorite impact crater. Ejecta from the impact was found in the Rove Formation in May 2007. Nearby, the less-known Temagami Magnetic Anomaly shows striking similarities to the Sudbury Basin, which suggests it could be a second metal-rich impact crater. In northeastern Quebec, the giant Manicouagan Reservoir hosts an extensive hydroelectric project known as Manic-cinq, or Manic-5. It occupies one of the largest known meteorite impact craters on Earth, although it is smaller than the Sudbury crater. Meanwhile the greenstone belts mentioned earlier hold most of the ore. The Flin Flon greenstone belt in central Manitoba and east-central Saskatchewan is one of the largest Paleoproterozoic volcanic-hosted massive sulfide districts in the world. It contains 27 copper-zinc-gold deposits, from which more than 183 million tonnes of sulfide have been mined. The northwest of the shield produced a different prize.

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• 8. Diamonds From the Deep

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  More than 150 kilometres below the surface is where diamonds in the Northwest Territories begin their journey. Kimberlite eruptions carry them up from that depth through pipes. These pipes are closely associated with cratons, because a craton provides the deep lithospheric mantle needed to keep diamond stable as a mineral. The Northwest Territories portion of the shield has recently been the site of several major diamond discoveries. The Diavik mine was the longest running diamond mine in Canada. It stopped producing diamonds in 2026 after 23 years. Its closing marks a turning point for the region, though the ancient craton that delivered those stones remains.

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