Hazards 3


Earthquakes and volcanoes occur at constructive margins:

A constructive margin occurs where two plates are moving apart.

The mantle is under pressure from the plates above. 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 plate cracks, making a fault line and causing an earthquake. Further earthquakes may also occur along the fault line once it's been created.

Constructive margins create two different landforms, depending on where they are:

1. Ocean ridge:

  • Where diverging plates are underwater, an ocean ridge forms, e.g. 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, e.g. Iceland has been formed by the build-up of underwater volcanoes along the Mid-Atlantic Ridge.

2. Rift valley:

  • 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, e.g. the East African Rift System is a series of rift valleys that stretches from Mozambique to the Red Sea. It's formed because the Nubian and Somalian plates are diverging.
  • Volcanoes are found around rift valleys, e.g. Mount Kilimanjaro and Mount Kenya are volcanoes in th East African Rift Valley.

Earthquakes and volcanoes also occur at destructive margins:

destructive margin occurs where two plates are…


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