mantle Similar to an iceberg or blocks of wood floating on water You can only

Mantle similar to an iceberg or blocks of wood

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mantle - Similar to an iceberg or blocks of wood floating on water - You can only see about 1/3 of the iceberg above the water
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- Effect of crustal thickness - Areas with thicker crust - Land surface is at higher elevation - Crust pushes down on mantle so crust/mantle boundary is deeper and get “crustal root” - For every 5-6 km crustal thickness increase - elevation of land surface increases by about one kilometer - Crust with higher density - Has lower surface elevations - Deeper crust/mantle boundary - “The crust of the earth must be a shell floating on a fluid interior. Thus the surface of the globe would be capable of being broken and disordered by the violent movements of the fluids in which it rested.” - Benjamin Franklin 1782 - Types of plate boundaries (all produce earthquakes) - Divergent: plates move apart; new lithosphere created; magmatic/volcanic activity - Convergent: plates move towards each other; lithosphere destroyed - Transform: plates slide past each other; lithosphere conserved - Rate of plate motion - Move a few centimeters per year - Divergent boundaries - Plates move apart - Decompression melting - new crust and lithosphere created - Boundary is elevated forming Mid-ocean ridges - due to heat (causes expansion) - As new crust moves away from the ridge, it cools and contracts and seafloor gets low (deeper) - Rift valleys centered on ridge axis (low spot where plates are pulling apart - Mid-ocean ridges wind around the earth and connect with one another (like seems on a baseball) - Mid-atlantic ridge emerging in Iceland - New crust formation - Melt from upwelling, asthenosphere rises, cools, and crystalizes to form new oceanic crust - New crust and lithosphere carried away from ridge - Age of crust increases away from the ridge - Evolution of divergent boundaries - Continental rifting to seafloor spreading - Stage 1: thinning and stretching of continental crust; thinned continental crust makes continental shelves - Stage 2: continental crust ruptures and separates; new ocean crust is made and fills in gap as two continents move apart - Stage 3: ocean basin widens; steady-state sea floor spreading and oceanic crust
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- Mid Atlantic ridge, on the margins we have Africa, South America, North America, and Eu- rope which are passive - Can tell the age of the crust through seafloor spreading - Earth’s Magnetic Field - Lines of magnetic force - Two important characteristics: - Direction of lines of force (arrows on the figure) - Angle that force lines make with the earth - Compass needle parallels lines of magnetic force - Magnetic poles NOT at same location as geographic poles, but close compass needle paral- lels lines of magnetic force - Polarity of earth’s magnetic field reverses occasionally - Reversals appear to be sudden; mechanism unclear - Polarity of magnetic field is recorded by iron-bearing minerals in igneous rocks when they crystallize - Magnetic sea-floor stripes - Magnetic polarity alternates in bands across the seafloor - Mirror symmetry across mid-ocean ridges - Inclination increases with increasing latitude - Vertical (90 degrees) at pole -
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