A constructive plate boundary is where two plates are moving apart.
Mantle is under pressure from above. When they move apart, the pressure is released at the boundary.
Release of pressure causes the mantle to melt, producing magma.
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 faster than others. This causes pressure to build up. When the pressure becomes too much the plate cracks, making a fault line and causes an earthquake. Further earthquakes may also occur along the fault line once it's been created.
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