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Eruptions
There are many different styles of eruption, and many volcanoes exhibit different eruption styles. Even individual eruptions normally change their behaviour with time, so a whole range of activity and hazards can be seen at any volcano. Despite this diversity, by studying each volcano we can identify what sort of eruptions might be expected based on past activity, the shape of the volcanoes and the eruptive products.
Left: An explosive eruption of Calbuco volcano, Chile, in 2015. MAV Drone/Shutterstock. Right: An effusive eruption from Mount Etna, Italy. Wead/Shutterstock.
In general we can split eruptions into two categories: explosive and effusive. Explosive eruptions can send clouds of ash high into the atmosphere during violent explosions. Explosive eruptions can last minutes to hours and days. The molten rock beneath volcanoes, magma, contains dissolved gases under great pressure. When this pressure is released the gas rapidly expands and the explosion occurs (like when you open a bottle of fizzy drink). Effusive eruptions occur when the magma has already lost most of its gas. You get less ash, and instead more gentle flowing of lava, either in relatively fast streams of lava or slow squeezing out more-solid domes of lava. But, any eruption can be one or both explosive and effusive, or vary between the two.
Volcanologists have names for different eruption styles. The ones you’ll most often come across are:
Plinian eruptions are large explosive eruptions, which have sustained eruption columns – clouds of ash and gas which are thrust high into the atmosphere above the volcano, sometimes to greater heights than aircraft fly. Near the eruptive vent blocks of rock are thrown out or fall out of the eruption column. Plinian eruptions are normally, but not always, associated with viscous felsic magma, and normally occur at stratovolcanoes.
Right: Plinian eruption from Mount St. Helens, U.S.A., in 1980. The eruption column reached a height of ~24km. Photo: USGS
Vulcanian eruptions are larger than Strombolian explosions, but are also short-lived, noisy explosions that throw blocks and bombs to tens to hundreds of metres. Eruption columns up to about 10 km high are produced.
Right: Vulcanian eruption with ~2km eruption column at Sakurajima, Japan. Photo: S.Brown
Strombolian eruptions are small, short-lived explosive outbursts that throw red-hot glowing clots of lava tens to hundreds of metres, with very little ash produced. Each explosion is accompanied by a large bang. During the day the bursts may look grey, but at night the glowing rocks can look like a firework display.
Right: Strombolian eruption at Etna, Italy. Shutterstock/Wead.
Hawaiian eruptions are what we call effusive. They are characterised by the outflow of large volume of fairly fluid lava, which forms streams of lava that can travel for tens of kilometres. Lava fountains reaching kilometres into the sky at the vent are also quite common. Hawaiian eruptions are usually associated with low-viscosity basic magma, and are often found at shield volcanoes and in rift zones.
Right: A lava flow in Hawaii. Shutterstock/Yvonne Baur.
