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Convergent Plate Boundaries By Diana L. Duckworth Rustburg High School Campbell County, VA Features • Deep sea trenches and • Volcanic island arcs or • Volcanic mountain ranges or • Folded & faulted mountain ranges Process • Subduction • One plate is pushed down into the mantle where it melts producing magma for volcanoes Subduction • Oceanic lithospheric plate dives under another plate • Produces initially mafic lavas, but as islands grow, lavas become more felsic from subducted sediment • Metamorphic rocks produced in subduction complex Subduction Explains • Why there are no abyssal plains in the Pacific Ocean (sediment goes into trenches) • Why there are no continental rises in Pacific continental margins – (sediment goes into trenches) • Why there is no old sea floor (it subducts & melts) • Why there are explosive volcanoes in the ring of fire (subducted sediment contains water!) Evidence • Compressional earthquake foci get deeper under the continents (or island arcs) away from the trench. • Angle is 45° ocean Mantle Convection http://wc.pima.edu/~bfiero/tucsonecology/setting/geology_platetec.htm Three Types of Convergent Plate Boundaries • Oceanic – Oceanic lithospheric plates – Features include trench and volcanic islands • Oceanic – Continental lithospheric plates – Features include trench and volcanic mountains • Continental – Continental lithospheric plates – Features include folded and faulted mountains Oceanic - Oceanic http://wc.pima.edu/~bfiero/tucsonecology/setting/geology_platetec.htm Oceanic - Oceanic • As volcanic islands increase in size • More sediment contributed to trench • Magma becomes more felsic due to melting of sediments subducted on plate • Subduction of water & sea shells in sediments adds carbon dioxide and water vapor to magma – result explosive eruptions • Island arc can be welded on to nearby continent as subduction continues. Oceanic - Continental http://wc.pima.edu/~bfiero/tucsonecology/setting/geology_platetec.htm Continental - Continental http://wc.pima.edu/~bfiero/tucsonecology/setting/geology_platetec.htm Continental Collision • Continental crust cannot be subducted • Density is too low • Remnants of subducted ocean crust provide uplift (low density) until it melts • Erosion carves out mountain ranges & valleys • Intense folding & faulting & metamorphism • No volcanic activity remains; remnants of magma chambers become batholiths Growth of Continents • These processes cause continents to become larger through additions of terranes • Subduction can cease & new trench forms seaward (Asia) • Continental collision creates a thicker continental crust • Rifting will occur somewhere else