Chapter 21 Continental Crust Continental Crust It is thick 30 60km It is old 250 4000 my It is light 2 75 g cm3 It is silicic dioritic to granitic in composition It has a stable interior called the craton It grows at active margins It does not subduct Craton and Shield The craton is the stable interior part of the continent The craton may be covered by a thin 2km veneer of sedimentary rocks The shield is that portion of the craton that is free of sedimentary cover Shields occur in Canada Southern Africa Western Australia and Scandinavia Mountain Belts An orogeny is an episode of mountain building An orogenic belt is a co genetic belt of mountain ranges e g Alps Himalayas Rocky Mountains Mountain belts tend to have thicker sedimentary cover 2 10 km Growth of Continents Continents grow at active margins They grow by addition of accreted or exotic terranes A exotic terrane is a small block of crust scraped off a subducting plate Much of SE Alaska and British Columbia is composed of accreted terranes Exotic Terranes Terranes A terrane is a co genetic block of crust Continents are composed of terranes or blocks of similar age and origin The terrane we are on gives a metamorphic age of 1800 my The CO WY border is also a terrane boundary The Wyoming terrane is Archean
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