Deep within the earth (at least 15 miles deep) it’s so hot that some rocks melt and become a thick flowing substance called magma. Normally, material in the mantle is kept in a semi-solid state due to the high pressure exerted upon it. However if pressure is released locally by folding, faulting or other movements at plate boundaries some of the semi-solid material becomes molten. The gas-filled magma is lighter than the solid rock around it therefore the magma rises towards the surface and collects in magma chambers. Eventually some of the magma pushes through weak vents and fissures in the earth’s surface until it reaches the peak of the volcano and explodes through the opening called the central vent. The erupted magma is then known as lava. Alternatively rock fragments called tephra can be fired out during an eruption. The explosivity of an eruption depends on the viscosity of the magma – for example gases can easily escape from thin, runny magma resulting in violent eruptions. The viscosity is typically dependent on the silica content of the magma.
Where do eruptions occur?
Volcanoes most frequently occur at plate boundaries – where plates collide, spread apart or slide past each other.
1. Conservative plate margins – where two plates diverge away from each other, magma rises from the mantle to fill any possible gaps and in doing so creates new oceanic crust. This initially forms sub-marine volcanoes that can eventually grow above sea level. An example of this type of landform is Surtsey on southern Iceland which was formed after an underwater eruption in 1963.
2. Destructive plate margins – where continental plates and oceanic plates converge, the latter is forced to dip downwards producing a subduction zone. The oceanic lithosphere descends to approximately 100 kilometres (60 miles) and is slowly converted into magma during a process known as partial melting which then rises to the earth’s surface. Where this buoyant magma breaks through, volcanoes occur normally forming a long chain of fold mountains (e.g. the Andes) or offshore on an island arc (e.g. Japan, West Indies). Allegedly, 80& of the world’s active volcanoes are situated above subduction zones. Since the rising magma at destructive margins is more acidic than at constructive margins, it is more viscous and flows less easily.
Both processes are shown diagrammatically below using the example of the boundary between the Juan de Fuca plate and the North American plate.
