Continental CrustChapter 21Continental 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.Craton and Shield• The craton is the stable interior part of thecontinent.• The craton may be covered by a thin(<2km) veneer of sedimentary rocks.• The shield is that portion of the craton thatis free of sedimentary cover.• Shields occur in Canada, Southern Africa,Western Australia, and Scandinavia.Mountain Belts•An orogeny is an episode of mountainbuilding.•An orogenic belt is a co-genetic belt ofmountain ranges. (e.g. Alps, Himalayas,Rocky Mountains)• Mountain belts tend to have thickersedimentary cover (2-10 km).Growth of Continents• Continents grow at active margins• They grow by addition of accreted or exoticterranes.•A exotic terrane is a small block of crust“scraped off” a subducting plate.• Much of SE Alaska and British Columbia iscomposed of accreted terranes.ExoticTerranesTerranes•A terrane is a co-genetic block of crust.• Continents are composed of terranes or blocksof similar age and origin.• The terrane we are on gives a metamorphicage of 1800 my.• The CO-WY border is also a terrane boundary• The Wyoming terrane is
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