Canadian shield

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Canadian shield

The Canadian Shield (also: Laurentian Shield ) is the geological core of the North American continent. It covers the northern half of the subcontinent and consists of rocks, some of which are billions of years old. During the last glacial period , the Canadian Shield was almost completely covered by inland ice, and its surface shapes are still shaped today by the inland ice of that time.

location

The Canadian Shield includes the Canadian regions of Labrador , Northern Québec , Northern Ontario , the Hudson Bay Lowland, the Northwest Territories , the Yukon Territory and the Arctic Archipelago ; in the broader sense in the southeast also the regions up to the St. Lawrence Lowlands and in the southwest to Minnesota and Wisconsin . In total, the Canadian Shield covers approximately 4.8 million square kilometers .

geology

Geology of Canada: the Canadian Shield in red and brown hues is made of Precambrian rocks

The Canadian Shield geological structure is made up of Archaic and Proterozoic crystalline rocks. It is divided into several archaic cores ( cratons ), which are separated from one another by greenstone belts , according to the age and structure of the exposed rocks . The largest and best preserved of these are the North American craton in the east, the superior craton around Hudson Bay, and the slave craton in the northwest. The Wyoming craton in the west is not always included in the Canadian Shield , it is in the United States .

The world's oldest known rocks have been found in the Canadian Shield: in the western part in the slave craton the Acasta gneiss (4030 mya ), in the eastern central part in the superior craton the Nuvvuagittuq greenstone belt (up to 4300 mya). Both rock units come from the Hadean .

In the Hudson Bay area, the old rocks are partially covered by younger layers of the Mesozoic Era.

Glacial overmolding

The advance and subsequent melting of the inland ice has shaped the huge area glacial and abraded it into a gently undulating almost plain with a few protruding hills and ridges, between which thousands of lakes and ice reservoirs have formed. The loose topsoil was carried away to the south by ice and meltwater and deposited in the Great Lakes area.

After the huge ice sheet melted away, the area as a whole rose and bulged at the edge ( isostasis ). This post-glacial uplift created new forms of erosion and can be seen as the cause of the numerous rapids and waterfalls . Large areas are swampy.

climate

Climatically , the Canadian Shield is characterized by its opening to the cold north and its location in the area of ​​the Arctic Circle . The climate is strictly continental , polar in the north . The winters are very cold and long and the summers are cool. North of -2 degrees year isothermal starts the permafrost , the ground is impermeable to water through, so that in the summer months the surface water dams and marshes provide an ideal breeding ground for huge swarms of mosquitoes .

vegetation

The dominant vegetation zone in the north is the tundra , which extends to the south shore of Great Bear Lake , Great Slave Lake , Athabasca Lake and Hudson Bay. It is bordered in the south by the strip of northern coniferous forests (taiga), which here has a width of 1,000 to 1,400 km. At the southern edge of the Canadian Shield there are mixed forests that have been cleared for agricultural use on the fertile soils between the Rocky Mountains and the Great Lakes .

The Canadian Shield encompasses several eco-zones ; the largest is the Boreal Shield .

See also

literature

  • Gerhard H. Eisbacher: North America . In: Geology of the Earth . 1st edition. tape 2 . Ferdinand Enke Verlag, Stuttgart 1986, ISBN 3-432-96901-5 , p. 13 ff .

Web links

Individual evidence

  1. ^ Canadian Shield . In: Britannica online . Retrieved April 25, 2009.
  2. ↑ The Atlas of Canada - Boreal Forest ( Memento from November 1, 2012 in the Internet Archive ) (English)