1Continental CrustChapter 20Continental Crust•It is thick (30-60km), •It is old (250 - 4000 my), •It is light (ρ = 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 old2Cratons 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•Aterrane 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.3Mountain Belts•An orogeny is an episode of mountain building.•An orogenic beltis 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–HawaiiPlume Hypothesis4Orogeny and Epeirogeny•Orogeny is an episode of mountain building.–Himalayas Alps–Rocky Mtns Appalachians•Epeirogeny is regional vertical movement of continental crust–Glacial rebound in NE USEpeirogeny: Glacial ReboundEpeirogenyContinental 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 Plains5Continental Crust Terms•Orogeny•Epeirogeny•Terrane•Accreted Terrane•Craton•Shield•Mantle Plume•Hot Spot
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