Download Convergent Plate Boundaries

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