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4.11: Ancient Parts of Continents- Cratons and Shields

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    Ancient Parts of Continents: Cratons and Shields

    A craton is a part of a continent that is stable and forms the central mass of the continent. The craton region of North America includes the region between the Rocky Mountains (to the west) and the Appalachian Mountains (to the east) and include the Canadian Shield.

    A shield is a large area of exposed Precambrian-age crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to over 4 billion years. For instance: the Canadian Shield is part of the North American craton region. Shallow inland seas have flooded over and retreated from North America's craton/shield region in the past billion years.

    World Physiographic Provinces Avalon Formation and Breakup of Pangaea

    Through geologic time new continental crust forms and accumulates along the margins of continents. The "floating" continental crust eventually crashes into other land masses, and these terranes may assemble into larger continental crustal plates. For instance, the formation of the ancient supercontinent Pangaea assembled through continental accretion. Pangaea later gradually split apart by continental rifting forming the world's continental landmasses that exist today. The geologic story of the formation and breakup of Pangaea are preserved in the rock record all along the Atlantic Margin of North America, Europe, and Africa.
    Breakup of Pangaea in the New Jersey region along the Atlantic Coastal Margin
    Figure 4.33. Continental shields and cratons
    Continental shields contain the oldest rocks preserved in the cores of continental landmasses. These regions formed by processes associated with continental accretion billions of years ago, long before the continents of the modern world existed. These ancient shield are part of the stable parts of continents (cratons) and in many places are partially covered by younger sedimentary rocks (such as in the Great Plains and Midwestern Low Plateau regions of North America).
    Figure 4.34. Formation of Pangaea by closing of the "proto-Atlantic" Iapetus Ocean about 300 million years ago. Figure 4.35. Breakup of Pangaea and formation of the Atlantic Ocean about 200 million years ago.