The Earth’s crust is unstable, especially at plate margins.

Cross section showing structure of the Earth

The Earth is divided into four distinct layers: inner core, outer core, mantle and crust.

Inner core: this is the hottest part of the Earth with temperatures of approximately 5,500°C . It is solid and made up of iron and nickel.

Outer core: this is also made up of iron and nickel but it is liquid. It is also extremeley hot with similar temperatures to the inner core.

Mantle: this is the thickest layer (approximately 2,900km thick). It is made up of semi-molten rock (magma). In the upper parts it is more solid than the lower parts where it is more liquid.

Crust: this is the outermost layer of the Earth. It is a thin layer broken up into sections called plates. There are two different types of crust: continental and oceanic.

Oceanic Continental

  • Thin (up to 10km thick)
  • Heavy
  • Sinks into the mantle
  • Relatively new (180 million years)
  • Forms constantly at constructive plate margins
  • Destroyed at destructive plate margins
  • Thick
  • Light
  • Doesn’t sink
  • Very old (up to 4 billion years)
  • New crust isn’t formed
  • Cannot be destroyed

Convection currents make the Earth’s plates move ( http://www.youtube.com/watch?v=xEYLx_bhtoA ).

Where convection currents diverge (move apart) the plates move apart, where convection currents converge (move together) plates also move together. Plates meet at a plate boundary.

World tectonic plates map

There are three types of plate boundary.

At this type of plate boundary the convection currents diverge and the two plates move apart. They tend to be found under the sea e.g. Mid Atlantic Ridge. As the two plates move apart magma rises up from the mantle. When the magma reaches the surfaces it cools and solidifies to form oceanic crust. This process repeats itself and over time a volcano is created. Earthquakes are also created when friction builds up as the plates move over the mantle.

At destructive plate margins the convection currents converge and two plates move together. When a oceanic plate and a continental plate move together the oceanic plate is subducted (forced down) as it is denser. As it is subducted into the mantle the oceanic crust melts to form new magma. It also take sea water down with it which, when combined with the magma, makes it less dense than the mantle. This means that it will rise up through the continental plate to form a volcano. Eventually the sea water will turn into steam and this makes the volcano very explosive. Earthquakes are also created at this plate boundary as friction builds up when the oceanic plate moves underneath the continental plate.

At conservative plate boundaries two plates are moving past each other (usually at different speeds). As the plates move past each other friction builds up which is then released as an earthquake. No volanoes occur at this plate boundary.