Continental Crust Chapter 20 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 Continental Crust Because continental crust is thick and old it has experienced and recorded many orogenic events It is extremely heterogeneous Oldest rocks are about 4 billion years old 1 Cratons and Shields 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 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 Growth of Continents Continents grow at active margins They grow by addition of accreted or exotic terranes A exotic terrane accreted terrane is a small block of crust scraped off a subducting plate Much of SE Alaska and British Columbia is composed of accreted terranes 2 Mountain Belts orogeny is an episode of mountain building An 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 Plumes and Hot Spots A Plume is a small convective upwelling from the mantle Plumes may cause hot spot volcanism Yellowstone Hawaii Plume Hypothesis 3 Epeirogeny Glacial Rebound Orogeny and Epeirogeny Orogeny is an episode of mountain building Himalayas Rocky Mtns Alps Appalachians Epeirogeny is regional vertical movement of continental crust Glacial rebound in NE US Epeirogeny Continental Crust Western US has several terranes Coast Ranges Cascades Sierra Nevada Columbia Plateau Snake River Hot spot track Basin and Range Colorado Plateau Rocky Mountains Great Plains 4 Continental Crust Terms Orogeny Epeirogeny Terrane Accreted Terrane Craton Shield Mantle Plume Hot Spot Volcanism 5
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