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A shield is that part of the continental crust in which these usually Precambrian basement rocks crop out extensively at the surface. Shields can be very complex: they consist of vast areas of granitic or granodioritic gneisses , usually of tonalitic composition, and they also contain belts of sedimentary rocks, often surrounded by low-grade ...
There is little evidence of continental crust prior to 3.5 Ga. [12] About 20% of the continental crust's current volume was formed by 3.0 Ga. [13] There was relatively rapid development on shield areas consisting of continental crust between 3.0 and 2.5 Ga. [12] During this time interval, about 60% of the continental crust's current volume was ...
A craton is an ancient part of the Earth's continental crust which has been more or less stable since Precambrian times. Cratons whose ancient rocks are widely exposed at the surface, often with relatively subdued relief, are known as shields.
The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4.28 billion years [18] [19] and have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on other cratonic regions such as those on the Fennoscandian Shield.
The Canadian Shield (French: Bouclier canadien [buklje kanadjɛ̃]), also called the Laurentian Shield or the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), the ancient geologic core of the North American continent.
The term craton is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable. [4] Cratons are composed of two layers: a continental shield, in which the basement rock crops out at the surface, [5] and a platform which overlays the shield in some areas with sedimentary rock. [6]
A craton typically consists of a continental shield surrounded by a continental platform. The shield is a region where ancient crystalline basement rock (typically 1.5 to 3.8 billion years old) is widely exposed at the surface. The platform surrounding the shield is also composed of ancient basement rock, but with a cover of younger sedimentary ...
Laurentia basement rocks. Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America.Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although originally it also included the cratonic areas of Greenland and the Hebridean Terrane in northwest Scotland.