Natural Hazards

PLATE TECTONICS

The crust of the Earth is composed of plates.  The plates are essentially floating on the mantle - a layer of partially melted rock (magma).  It reaches temperatures approaching 2000 degrees Celsius.  The hot inner core of the Earth heats the magma, so it gets lighter and rises.  As it approaches the surface it cools and gets heavier, so it sinks agiain.  These convection currects keep the magma moving and make the plates at the crust move.  The way the plates move is called continental drift.

CONSTRUCTIVE MARGINS

At constructive margins or divergent margins, the convection currents are diverging (moving away from each other).  This means the plates at the surface are being pulled apart.  Magma rises through the gap and becomes lava at the surface.  The material cools and solidifies to form new crust.  This forms ridges at the surface of the sea and basic volcanoes when the lava reaches the surface.  An example is the Mid-Atlantic Ridge, where the plates are moving apart at around 4cm per year.  The Atlantic is widening.  Volcanic islands form along the ridge, such as Sertsey, Iceland in 1963.

DESTRUCTIVE MARGINS

At destructive or convergent margins, the convection currents are converging, so the plates at the surface are moving towards each other.  There are two different plates involved - oceanic and continental.  The oceanic plate is denser, so it is subducted. Now it is heated by the mantle and the rocks become molten.  They rise to form explosive volcanoes at the surface.  Sometimes, the magma rises offshore to form island arcs such as the west Indies.  The Nazca plate is an example.  It is moving towards the American Plate and where they meet, the Nazca plate is subducted (because it is an oceanic plate) to form a subduction zome and an associated deep sea trench (the Peru-Chile Trench).  Volcanoes such as Cotopaxi have been formed where the magma has risen.  The Andes are an example of fold mountains and have been formed due to the movements of the Nazca Plate and the American Plate.

CONSERVATIVE MARGINS

Conservative margins are found where two plates slide past one another.  This is because the convection currents are moving in the same direction.  Crust is neither formed nor destroyed.  Earthquakes can occur if the two plates 'stick'.  When sufficient pressure builds up, one plate is jerked forward sending shockwaves to the surface.  An example of a conservative plate margin is the San Andreas Fault in California.  It marks the junction between the Pacific Plate and the American Plate.  The American Plate moves slower and slightly into the Pacific Plate.  In 1906, an earthquake in San Francisco moved the ground by 6m and 450 people were killed.  28,000 buildings were destroyed.

VOLCANOES

Features of a volcano:

• A magma chamber
• The vent
• A minor vent
• A secondary cone
• A volcanic crater
• Lava
• Layers of ash and lava

A volcano is active if it has erupted recently and is likely to erupt again.

A volcano is dormant if…