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

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

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

Continental - Continental

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