Oceanic plate: A tectonic plate with oceanic crust at the boundary in question.
Japan Trench
Japanese island arc
Subduction zone
Solid upper mantle Oceanic crust
Asthenosphere
Melting of subducted crust
Lithosphere
Fig. 1.7 Convergent (destructive) plate boundary – oceanic plate colliding with an oceanic plate.
A-Z
Continental plate: A tectonic plate with continental crust at the boundary in question.
Two continental plates collide Example: The Eurasian Plate collides with the African Plate (Fig. 1.8)
There is very little subduction at this zone, as both plates tend to be similar in age, weight and density. Instead of moving down, the two plates buckle upwards slowly, creating fold mountain ranges such as the Alps. Magma can intrude into gaps created between layers of rock as they fold upwards. This magma cools slowly creating new rock under the surface of the earth.
Crust buckles upwards Alps
EURASIAN PLATE
Lithosphere Continental crust
Solid upper mantle
Asthenosphere Volcanic intrusions A-Z
Destructive plate boundary: Crust is destroyed at convergent plate boundaries when an oceanic plate is one of the plates in collision.
Constructive plate boundary: Crust is created at divergent plate boundaries.
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Fig. 1.8 Convergent plate boundary – continental plate colliding with a continental plate.
An oceanic plate collides with a continental plate Example: The Nazca Plate collides with the South American Plate.
The heavier, denser oceanic plate subducts under the lighter continental plate. As convection currents drag the plate downwards, a deep arc-shaped ocean trench forms at the subduction zone. Crust in the oceanic plate melts, creating a hot silica and gas-rich magma. This less dense magma burns a path to the surface of the continental plate, leading to a powerful volcanic eruption. Over time, a chain of volcanic mountains form parallel to the subduction zone (Andes volcanic arc).
