CONVERGING/DESTRUCTIVE BOUNDARY
Where two plates move together/collide, the outcome depends on whether the plates contain mainly continental or oceanic crust. Violent earthquakes and violent volcanoes are found at these margins.
Continental crust and oceanic crust
Continental crust and oceanic crust
- When a continental and an ocean plate converge, the lighter less dense continental plate moves up over the heavier, denser oceanic plate which is forced down into the heat of the mantle.
- As the oceanic plate descends, the increase in pressure can trigger major earthquakes (Benioff zone), it cracks and the heat produced by the friction causes it to melt. This creates molten rock and gases which move upwards through weaknesses in the continental crust above, forming volcanoes.
- Where the oceanic plate sinks down into the subduction zone, a deep sea trench is formed.
Example- The Andes were formed as the Nazca (oceanic) Plate pushed into and descended under the South American (continental) Plate.
Oceanic crust and oceanic crust
A similar process takes place when two oceanic plates meet. This happens near Japan where the Pacific plate pushes into and moves under the edge of the Eurasian plate (oceanic crust at this location). The resulting volcanoes form an almost continuous string of islands termed an Island Arc.
Other examples- Aleutians in Alaska, New Zealand and the Caribbean arc of islands (Lesser Antilles) involving the Caribbean and N. American plate.
Continental crust and continental crust
Other examples- Aleutians in Alaska, New Zealand and the Caribbean arc of islands (Lesser Antilles) involving the Caribbean and N. American plate.
Continental crust and continental crust
- Where two plates formed of continental crust collide, fold mountains are formed as neither crust is dense enough to sink beneath the other.
- Sediments are squeezed into folds and slowly pushed up by the steady advance of the two plates, eventually forming thick layers. Two mountain ranges have formed in this way, the Alps and the Himalayas. This process of mountain building is known as orogeny.
- Violent earthquakes are found at these margins but NOT volcanoes because there is no subduction or melting of any crust to form new magma in order to create the volcano.