Download Plate Boundaries

Plate Boundaries
Convergent Boundaries
 2 plates move toward each other
 Destructive plate margins
 Old plate material is being recycled
 Oceanic crust is being pushed back into
the mantle at the subduction zone
3 types of Convergent Boundaries
1. Oceanic – Continental
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When oceanic crust is subducted under
continental crust
Occurs because oceanic crust is more dense
As oceanic crust subducts into the mantle it
melts
Melted oceanic crust rise and forms
mountains and volcanoes (ex. Andes Mts.) –
called continental volcanic arc
1. Oceanic – Oceanic
When 2 oceanic plates collide
 One oceanic plate subducts the other
 Causes volcanic activity under water
(similar to oceanic – continental)
 Forms a volcanic island arc (ex. The
Aleutian Island)
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1. Continental – Continental
When 2 continental plates collide
 Only part of one continent subducts
 Causes folded mountains to form (ex.
Himalayas)
 Mts are folded along the margins
 Composed of metamorphosed
sedimentary rock
 Other examples Alps, Urals, Appalachians
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1. Transform Boundary (Fault)
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2 plates grind (slide) past each other
No destruction of lithosphere
Ex. San Andreas fault
Divergent Boundaries
 The separation of 2 plates
 Constructive plate margins –
formation of new lithosphere
 Plates move away from a ridge
margin
 New molten material wells up from
the mantle
 Molten material cools-new crust is
formed
 Continuous process
Features Formed:
 Continental Rifts –
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Splitting of continental crust (ex: East
African Rift Valley an d Rhine Valley)
 Continental crust is being stretched in
opposite directions due to upwarping
 Formation of mountains and volcanoes (ex:
Mt. Kenya and Mt. Kilimanjaro)
 Eventually a rift valley will form followed by
a sea
 Oceanic Ridges
 An underwater mountain chain 70,000
miles long
 Formed by the upwarping of the crust –
over time rift valley fills with water and
forms an ocean
 Sea-floor spreading – spreading of the
ocean floor
 Process where new lithosphere is
formed
 Average rate of 5 cm/yr