Constructive eg, Mid Atlantic Ridge (10,000km)
- Plates are pulled apart by convection currents
- Reduction in pressure leads to the mantle melting and forming magma
- Thinning crust may bow upwards due to pressure below & low density
- Magma produces submarine volcanoes
- Transform faults are produced at right angle to the ridge system
Low level explosivity eruptions and NO SUBDUCTION
Conservative eg, San Andreas fault
- Pressure comes from the friction that is made from the plates sliding past each other.
No volcanoes, no new landoforms as there is no melting of a subducted plate
San Andreas- Pacific plate moving NW/ American plate moving SE
Plate boundaries continued.
Hot Spot eg, Hawaii
Plumes of magma are rising from the asthenosphere, eventhough they are not necessarily near a plate boundary.
- If the crust is thin/weak the magma may escape as a volcanic eruption.
- Lava is basaltic and free-flowing.
- Earthquakes- smooth and regular.
Continental-Continental eg, Himalayas
- 2 plates collide with each other and are forced upwards creating fold mountains.
- No volcanoes as no magma is rising towards the surface.
Himalayas- Indo-australian plate is moving north(5cm per year)/Eurasian plate is moving south and they collide.
More plate boundaries
Oceanic-Continental eg, Peru-chile trench
- Oceanic plates slides beneath the continental plate and melts at the subduction zone.
- Volcanoes are explosive as there is more pressure due to there being more plate to get through.
Peru-chile trench- Nazca plate subducts the SA plate. Andes are formed (fold).
Oceanic-Oceanic eg, Japan Trench
- Older of the 2 oceanic plates subducts and erupt immediately with 50% silica content.
- Island arc is formed (magma breaks through).
Japan Trench(Marina islands)- Pacific plate subducts the Philippine plate.