Geog251a - 353 MINERALS AND ROCKS Sea level Oceanic crust(basaltic Lithosphere Continental crust(granitic Lithosphere Mohorovicic discontinuity

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353 Earth’s Crust Earth’s solid and rocky exterior is the crust, which is composed of a great variety of rocks that respond in diverse ways and at varying rates to Earth-shaping processes. The crust is the only portion of the lithosphere of which Earth scientists have direct knowledge, yet its related surface materials form only about 1% of Earth’s planetary mass. Earth’s crust forms the exterior of the lith- osphere and is of primary importance in understanding surface processes and landforms. Earth’s deep interior components, the core and mantle, are of concern to physical geographers primar- ily because they are responsible for and can help explain changes in the lithosphere, particularly the crust, which forms the ocean floors and continents. The density of Earth’s crust is significantly lower than that of the core and mantle, and ranges from 2.7 to 3.0 grams per cubic centimeter. The crust is also extremely thin in compari- son to the size of the planet. The two kinds of Earth crust, oce- anic and continental, are distinguished by their location, com- position, and thickness ( Fig. 13.5). Crustal thickness varies from 3 to 5 kilometers (1.9–3 mi) in the ocean basins to as much as 70 kilometers (43 mi) under some continental moun- tain systems. The average thickness of continental crust is about 32 to 40 kilometers (20–25 mi). The crust is relatively cold, rigid, and brittle compared to the mantle. It responds to stress by fracturing, wrinkling, and raising or lowering rocks into up- warps and downwarps. Oceanic crust is composed of heavy, dark-colored, iron- rich rocks that are also high in silicon (Si) and magnesium (Mg). Its basaltic composition is described more fully in the next sec- tion. Compared to continental crust, oceanic crust is quite thin because its density (3.0 g/cm 3 ) is greater than that of continen- tal crust (2.7 g/cm 3 ). Forming the vast, deep ocean floors as well as lava flows on all of the continents, basaltic rocks are the most common rocks on Earth. Continental crust
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This note was uploaded on 09/20/2011 for the course GEOG 251 taught by Professor No during the Spring '11 term at Cambridge.

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Geog251a - 353 MINERALS AND ROCKS Sea level Oceanic crust(basaltic Lithosphere Continental crust(granitic Lithosphere Mohorovicic discontinuity

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