_04_PlateTectTwo

_04_PlateTectTwo - GEL 1: Lecture 4: Plate Tectonics:...

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1 GEL 1: Lecture 4 : Plate Tectonics: (Ch. 4) 2) Convergent Plate Boundaries Plate boundary where two plates converge to produce chains of volcanoes, uplifted mountain belts, and deep-sea trenches. - the primary force at convergent boundaries is compression – squeezing and ‘shortening’ caused by the motion of the two plates toward each other - lithosphere is commonly destroyed along convergent margins by burial back into the mantle A - Continental-oceanic convergence Along convergent boundaries where an oceanic part of a plate dives beneath a continental part of a plate, lithospheric material is returned to the mantle along deep-ocean trenches. The process where oceanic lithosphere descends down into the mantle is called subduction . (e.g., Andes Mtns ., Cascades, Japan). - oceanic lithosphere is denser than continental lithosphere and thus subducts into the asthenosphere beneath the continental plate at rates of 10-15 cm/yr - as the subducting slab reaches depths of about 100 to 150 km, temperatures are reached that are capable of melting the crustal rock of the downgoing plate - water-saturated sediment is dragged down into the subduction zone where it contributes to lowering the melting point of rock - this buoyant molten rock (magma) rises in plumes toward the surface where most of it solidifies beneath the surface and some of it violently erupts to build volcanoes - the volcanoes align roughly parallel to the convergent margin, forming a mountain chain called a volcanic arc - along convergent boundaries, the process of subduction creates a deep oceanic trench that forms along the linear contact between the two plates (e.g., Peru-Chile trench, Japan trench, Indonesian trench) - deep-ocean trenches are the topographic expression of a subduction zone - the denser of the two plates will be the one to subduct (oceanic rocks are denser than continental rocks, so they typically subduct beneath the continental margin). Central America exhibits a deep ocean trench and associated volcanic arc along its length
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_04_PlateTectTwo - GEL 1: Lecture 4: Plate Tectonics:...

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