Constructive plate margins occure where two plates are moving apart.
The mantle is under pressure from the plates above and when they move apart, the pressure is released at the margin.
The release of pressure causes the mantle to melt, producing magma.
The magma is less dense than the plate above, so it rises and can erupt to form a volcano.
The plates don't move apart in a uniform way - some parts move faster than others. This causes pressure to build up. When the pressure becomes too much the plates cracks, making a fault line and causing an earthquake. Further Earthquakes may also occur along the fault line once it has been created.
Constructive margins create two different landforms, depending where they are:
Mid-Ocean Ridge formed in water.
Rift Valley formed on land.
Mid-Ocean Ridge and Rift Valley
Constructive plate margins underwater are where a mid-ocean ridge forms.
For example, the Mid-Atlantic Ridge is where the Eurasian plate and North American plate are moving apart.
Underwater volcanoes erupt along mid-ocean ridges and they can build up to be above sea level. For example Iceland has been formed by the build-up of underwater volcanoes along the Mid-Atlantic Ridge.
Where plates diverge beneath land, rising magma causes the continental crust to bulge and fracture, forming fault lines.
As the plates keep moving apart, the crust between parallel faults drops down to form a rift valley. For example, the East African Rift System is a series of rift valleys that stretch from Mozambique to the Red Sea (4000km). Its formed because the Nubian and Somalian plates are diverging. Some parts of the system are hundreds of metres deep and thousands of metres wide.
Volcanoes are found in the rift valleys. For example, Mount Kilimanjaro and Mount Kenya (the two highest mountains in Africa) are volcanoes in the East African Rift System.
Destructive Plate Margins
A destructive plate margin occurs where two plpates are moving towards each other.
Where continental crust and oceanic crust converge, the more dense oceanic crust is subducted under the less dense continental crust. This forms a deep sea trench e.g. the Peru-Chile trench in the Pacific Ocean.
Fold mountains also forms where plates meet. They're made up of sediments that have accumulated on the continental crust, which are folded upwards along with the edge of the continental crust.
The oceanic crust is heated by friction and contact with the upper mantle, which melts into magma.
The magma is less dense than the continental crust above and will rise back to the surface to from volcanoes.
As one plate moves under the other they can get stuck. This causes pressure to build up. When the pressure becomes too much the plates jerk past each other, causing an Earthquake.
Destructive Plate Margins
Most of the same processes occur where two plates of oceanic crust are moving towards each other - the denser of the two oceanic plates will be subducted, forming a deep sea trench and triggering earthquakes and volcanic eruptions.
Volcanic eruptions that take place underwarter at destructive plate margins create island arcs (clusters of islands that sit in a curved line, e.g. The Mariana Islands)
Where two plates of continental crust move towards each other neither is subducted so there arent any volcanoes but the pressure that builds up between them can cause earthquakes.
Fold mountains form when continental crusts converge e.g. The Himalayas.
Conservative Plate Margins
A conservative plate margin occurs where two plates are moving past each other.
The two plates get locked together in places and pressure builds up. As with destructive margins this causes the plates to jerk past each other (or crack forming fault lines), releasing the energy built up as an earthquake.
For example, the Pacific plate is moving past the North American plate. Many earthquakes occur along this margin and along its fault lines, e.g. Along the San Andreas fault in California.