Some of Earth’s oldest rock still floors Canada’s lake country
The Canadian Shield—also Laurentian Shield or Plateau—exposes Precambrian igneous and high-grade metamorphic rock that cores the North American Craton, Laurentia. Ages from about 2.5 to 4.2 billion years make it among the planet’s oldest crust, scraped bare by ice into classic deranged drainage of lakes and rivers.
The shield is the exposed heart of the North American craton, the continent's ancient core. Counting its Greenland portion it forms a rough circle centred on Hudson Bay. It spans much of Greenland, all of Labrador, most of Quebec north of the St. Lawrence, much of Ontario, northern Manitoba and Saskatchewan, a corner of Alberta, a large part of the mainland Northwest Territories and Nunavut, and Baffin Island, and it dips into the United States in New York's Adirondacks, Upper Michigan, northern Wisconsin and north-eastern Minnesota. Geographers divide it into the Davis, James, Kazan and Laurentian regions.
Its age is staggering. One estimate for the shield's age reaches 4.28 billion years. It once carried peaks around 12,000 metres high and plenty of volcanoes, but over some 500 million years erosion reduced them to rolling, low-relief country. Because mountains float on the mantle with deep roots, erosion made those roots rise in turn, so the rock now at the surface formed far underground, where heat and pressure favoured the growth of mineral deposits. The exposed roots appear as greenstone belts, metamorphosed volcanic rock 3.6 to 2.7 billion years old surrounded by granitic rock largely of the tonalite–trondhjemite–granodiorite family.
Standout features include the Sturgeon Lake Caldera in Ontario's Kenora District, one of the best-preserved mineralised calderas of the Neoarchean at 2.7 billion years, and the Mackenzie dike swarm, the largest known on the planet. The shield belonged to Arctica, an ancient continent assembled about 2.5 billion years ago.
Ice finished the job. Glaciers scraped the region down to bare rock, leaving thin soils through which volcanic bedrock often shows. They also scrambled the watersheds, and together with the land's post-glacial rebound created a textbook deranged drainage pattern of countless lakes and rivers. High mountains do survive in the far north as the Arctic Cordillera, running from Ellesmere Island to northern Labrador and peaking at Barbeau Peak, 2,616 metres.
Source: Canadian Shield
