Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 443

404 EARTH/Crust Figure Diagrams showing the threefold layering of the oceanic (A) and continental (B) crust The continental crust is much thicker than the oceanic crust The two diagrams are not to scale Recycling of the Crust Plate tectonics requires that crustal slabs are recycled in subduction zones and taken down to the mantle (Figure 2) However, this is by no means an even process, with oceanic crust being subducted very rapidly, but continental crust resisting recycling The result of this is that the oceans are all products of the last plate dispersion cycle, which commenced about the beginning of the Mesozoic, and no oceanic crust is older than about 200 million years In contrast, continental crust has been preserved almost indefinitely, and there is some continental crust in the Slave Province of Canada which is more than 4000 million years old Minerals (zircons) in continental rocks in Western Australia are also up to 4300 million years old Physical Regions of the Crust Continental crust is subdivided into a number of physical regions Shields: deeply eroded expanses of low relief, which have been stable since Precambrian times Platforms: similar to the above, but mantled by thick sedimentary cover, which may be entirely or in part Phanerozoic in age Orogens: long, curved belts of folded rocks, usually forming mountain chains, mostly formed by continental collisions Rifts: linear, fault-bounded depressions, traversing continents; these are the structures which originate crustal splitting and dispersion, and lead to midocean ridge formation, but they may, as in the case of the East African Rifts, be aborted, i.e., never developed into oceans Oceanic crust is also subdivided into a number of physical regions Volcanic islands: scatterings or chains of islands which mark mantle hotspots Volcanic arcs: above subduction zones (e.g., Aleutian Islands), these may be represented by their