Plate margins

What is a destructive plate margin (convergent)

Where two tectonic plates are moving towards each other and one subducts underneath the other. Subducting plate is destroyed by melting.

Oceanic-Oceanic destructive plate margins

The denser oceanic plate subducts underneath the other and a deep ocean trench forms. Melting plate generates viscous, silica-rich magma resulting in highly explosive parallel to plate margin. Volcanic island arcs form for example Japan.

Continental-Continental destructive plate margins

• Neither plate subducts as they are similar density

• Plates collide crust crumples, compresses and forced upwards ,

• Fold mountains formed no volcanoes

• Strong earthquakes due to constant friction between two plates and pressure builds along fault line

• Example: Himalayas

Oceanic-Continental destructive plate margin

• Oceanic crust subducts as it is more dense

• Creates an oceanic trench

• Plate melts as it descends due to pressure, friction and heat

• Molten material will come back up to surface through the crust as an explosive composite volcano

• Fold mountains also form

• Friction along subduction zone leads to earthquakes

What is a constructive plate margin (divergent)

Plates are moving a part magma fills the gap and new lithosphere forms. In oceans mid ocean ridge system produced. In continents rift valley formed.

Oceanic-Oceanic constructive plate margin

Two oceanic plates diverge due to convection currents in asthenosphere. Gap is filled with magma, which has low viscosity and is runny so travels before solidifying. Happens over hundreds of thousands of year forming chain of volcanic mountains. Gentle volcanic eruptions. Low magnitude earthquakes. Iceland is an example which has gone above sea level.

Continental-Continental constructive plate margins

Two continental plates diverge, cracks and faults form. Magma fills cracks and faults so there is volcanic activity. Stretched crust sinks creating deep rift valleys. Shallow earthquakes. The East African Rift is evidence for this.

What is conservative plate margins

Plates sliding in the same or different direction side by side. No volcanoes as no material being destroyed. As they move paste each other friction builds up and can be released as an earthquake. Usually shallow earthquakes. San Andreas fault is an example of this plate margin.

How do ocean trenches form at destructive plate margins

• Found at subduction zones

• Oceanic plates are converging denser oceanic plate subducts

• As it subducts into mantle it bends at plate boundary creating long narrow depression in ocean floor

How do island arcs form at destructive plate margins

• Oceanic plates converging, denser oceanic plate subducts and partial melting occurs to produce magma

• Magma rises through overriding plates and erupts to form volcano

• Repeated eruptions create a chain of volcanic islands parallel to the ocean trench