Natural Hazards (Tectonic)

WHY IS THERE A PATTERN OF EARTHQUAKES?

An Earthquake is a sudden and violent period of ground shaking. It is most commonly caused by a sudden movement of rocks within the earth's crust. These mainly occur at the margins of tectonic plates. Some earthquakes do not occur at plate margins, sometimes these may be caused by human activity such as underground mining, oil extraction or reservior constructon

TECTONIC PLATES

The Earht's crust is split into a number of plates about 100KM thick

There are two types of crust - dense thin oceanic crust and less dense, thick continental crust

Plates move in relation to each other due to convection (heat) currents from deep within the Earth. Gravitational pull may play a part.

At a constructive plate margin plates move apart. New crust is formed as magma rises towards the surface

At a destructive margim, where plates are moving towards each other, the denser oceanic plate may subduct (sink) beneath a less dense continental plate. Gravity pulls the oceanic plate into the mantle, dragging the plate away from the constructive margin

Tectonic acitivty at plate margins causes earthquakes and volcanoes.

WHY IS THERE A PATTERN OF VOLCANOES?

Volcanoes are fed by magma (hot molten rock) from deep within the Earth. This risses to the surface at constructive and destructive plate margins. Volcanoes also form at hot spots, where the crust is thin and magma is able to break throigh to the surface. 

CONSTRUCTIVE MARGIN

At a constructive margin two plates are moving apart. Magma is forcing its way to the surface. The diagram below shows what is happening at the constructive margin in the Mid-Atlantic Magma is forcing its way to the surface along the Mid-Atlantic Ridge. As it breaks throigh the overlying crust it causes earthquakes. On reaching the surface it forms volcanoes.
The magma at constructive margins is very hot and fluod. Lava erupting from a volcano in these areas will flow a long way before cooling. This results in typically broad and flat shield volcanoes

DESTRUCTIVE MARGIN

At a desctructive margin two plates are moving towards one another. The diagram below shows what is happening on the west coast of South America. 
Where the two plates meet a deep ocean trench has formed. The oceanic Nazca Plate which is relativley dense is subducted beneath the less dense South American Plate. Friction between the two plates causes strong earthquakes. As the oceanic plate moves downards it melts. This creates magma which is less fluid than at a constructive margin. It breaks through to the…