Volcanoes are rare locations on the Earth’s crust where molten rock (magma) spews to the surface as lava, often accompanied by superheated gas and debris.
Geologists see volcanoes as outward evidence of the inner workings of plate tectonics, the theory that the crust is fragmented into 15 oceanic and continental plates that diverge, converge and slide beneath one another over time.
Approximately 400 of Earth’s 500 known active volcanoes lie atop subduction zones, places where an oceanic plate slips beneath another oceanic or continental plate. The ‘Ring of Fire’ traces a circle of highly active subduction zones around the Pacific Ocean.
In a subduction volcano, magma is formed 100 to 200km beneath the surface when water and carbon dioxide seep from the sinking oceanic shelf, lowering the melting point of the surrounding rock.
Rift volcanoes form along the great seams of two separating plates. The mid-Atlantic ridge, which separates the North American and African plates, is one of these seams. As the plates pull apart, magma bubbles up through hundreds, even thousands, of small volcanoes to fill the cracks, creating new ocean floor.
Five per cent of volcanoes are located far from the seams of tectonic plates. So-called hot spot volcanoes are fuelled by deep sources of magma pumped to the surface through powerful convection currents in the molten mantle. Since the deep fuel source remains fixed while the plate slides above, the result is often a string of volcanoes, much like the Hawaiian Islands.