Features

Question 1

How is an ocean ridge formed?

Answer 1

At a constructive boundary
2 plates mover apart due to convection currents and slab pull
Less pressure where plates moving - decompression melting and partial melt
Partial melt moves upwards, cools and solidifies
Crust sits higher as it is warmer, less dense and due to PM pushing it up
Example - Mid Ocean Ridge

Question 2

How are rift valleys formed?

Answer 2

At a constructive plate
Plates moving apart due to convection currents and distant slab pull
Plate thins and tensional faults form
Sections of the lithosphere drop due to gravity (grabens)
Rift valley is created
Volcanic activity at rift valleys as the partial melt will go between the cracks
Eg great rift valley Africa

Question 3

How is an ocean trench formed?

Answer 3

At a destructive boundary
2 plates moving towards each other
Convection currents
Denser plate subducted (oceanic)
Slab pull brings the rest of the plate
Seen on the surface as a depression
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