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Environmental Sciences
ENVS 2250
Steve Sadura

Natural Disasters Unit 2 January 20, 2013 Plate Tectonics Plates  The crust of the earth is divided into 8 major plates and several smaller ones  In some cases, a plate carries a continent on its back Plate Movement  Plates move about 2-3 cm per year at the Mid-Atlantic ridge to a max of about 17 cm per year along the east pacific rise  Temperature gradients in the upper mantle cause convection cells and drags the overlying crust around with it  Slab Pull – older parts of oceanic crust sink down into the mantle when plates collide because they are colder then the warm mantle material underneath them  Ridge Push – as hot magma rises to the surface and forms new crust, it pushes the rest of the plate out of its way (far less significant than a slab pull in moving plates around) The Wilson Cycle  The life cycle of an ocean basin from its beginnings as a continental rift to its final death throes during a continent to continent collision  The idea is that oceans are born as rift, they grow for a while, begin to shrink when subduction takes place around their edges and then they finally collapse completely during continental collision to form a single larger continent  Ex. The Atlantic Ocean Hot Spot Traces  Hot Spot – a place in the mantle with a greater than average amount of radiogenic heat  This heat causes convection and the production of mantle plumes and as crustal plates pass over these stationary features, volcanic features develop Plate Boundaries Divergent Boundaries  Plates move apart  Just above the core, hot blobs of mantle material begin to move slowly upward, forming conveyer belt like convection currents within the semi- fluid part of the mantle – the asthenosphere  When convection currents approach Natural Disasters Unit 2 January 20, 2013 earth’s surface, it causes the overlying crust to bulge slightly, causing them to diverge  Material from the mantle rises to fill the gap, so all rift areas have extensive volcanism Ocean Rifts  Rifts on the ocean basins are the largest volcanic features on earth  Ex. Mid-Atlantic Ridge  The rift between the diverging plates fills with molten basaltic magma  Seawater quickly cools and solidifies the molten rock, forming new oceanic crust Continental Rifts  Continental crust splitting apart  Occurs through the same process as oceanic rifting  Most volcanoes of continental rifts are not particularly dangerous but they are highly varied o Strato-volcanoes – steep sided, vicious magma o Shield volcanoes – gradually sloping, fluid basaltic magma  Expected small earthquakes Convergent Boundaries  Plates collide  3 different types (oceanic-continental, oceanic-oceanic, and continental- continental) Oceanic-Continental  The dense oceanic plate actually pulls the rest of the plate into the weak lower mantle (slab pull)  Subduction zone  Remains solid for several hundred km’s beneath the earth’s surface  Earthquakes are a common occurrence at this boundary o Caused as the subducting plates grinds past the continental plate in a series of jerks  Volcanic mountain ranges along this type of boundary o Ex. The Andes Mountains o Ex. Cascades  Typically explosive and potentially dangerous Oceanic-Oceanic  One plate is forced beneath the other, melting as it descends  This recycled molten oceanic crust rises upward to form underwater volcanoes Natural Disasters Unit 2
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