The Earth's Structure
Inner Core -- hottest part of the earth.
Outer Core -- a liquid layer.
Mantle -- dense, mostly solid layer between the outer core and the crust.
Crust -- relatively thin outer layer of the earth, split into plates (52 altogether, 14 major, 38 minor).
Convection Currents -- occur in the mantle, cause the plates to move and determines the direction of movement.
Continental Crust -- older, less dense, cannot sink, cannot be renewed or destroyed.
Oceanic Crust -- newer, denser, can sink, can be renewed or destroyed.
Destructive Margin - convection currents in mantle cause the plates to move together, oceanic crust is denser than continental so it sinks below it (subduction) the oceanic crust is destroyed and melts to form magma, violent volcanic eruptions take place.
Constructive Margin - convection currents in the mantle cause the plates to move away from each other, cracks start to form and magma forces its way up these cracks to form volcanoes, this usually happens under the ocean and so when the magma reaches the surface, it solidifies and new land is formed.
Conservative Margin - the plates slide past each other in a similar direction at slightly different angles or speeds, causing them to get stuck, the build up of pressure causes them to be suddenly released and this sudden release of pressure causes an earthquake, crust is neither being destroyed nor made.
Collision Margin - two continental plates collide and because continental crust cannot sink or be destroyed, both plates are pushed upwards to form fold mountains, earthquakes are common along these margins but no volcanoes.
Fold Mountains and Ocean Trenches
Fold Mountains -- two continental plates push together and the layers of sedimentary rock that has built up on the sea floor folds, this folded rock then rises above sea level as fold mountains, e.g. the himalayas, the andes and the alps.
Ocean Trenches -- deep sections of the ocean, usually where an oceanic plate is sinking below a continental plate.
Types of Volcanoes
- found on destructive boundaries.
- pyroclastic flow - hot steam, ash, rock and dust.
- steep sides.
- infrequent but very violent.
- found on constructive boundaries.
- low with gentle sloping sides.
- formed by eruptions of runny lava.
- frequent but quite gentle.