The activity of a volcano can be described by one of three words.

• Active volcanoes still erupt and can often be seen smoking and flaring.
• Dormant volcanoes are unlikely to erupt, but sometimes take us by surprise.
• Extinct volcanoes have not erupted for a very long time and never will again.

How do volcanoes differ from each other?

There are many different types of volcanoes, each with particular characteristics and occurring against different geological backdrops.

As the ocean covers more of the planet than the land, and as volcanic hotspots tend to follow plate margins which are generally underwater, submarine volcanoes are the commonest. They often occur at mid-ocean ridges and vent their fumes into the water. When they erupt, they can form pillow lavas if they are deep enough under the ocean. Shallow eruptions, by contrast are similar to land volcanoes and produce billowing clouds of smoke and eject lava. They also produce a large amount of steam which can make the eruption quite explosive.

Some under-sea volcanoes produce “black smoke” containing metal sulphides which are precipitated at the surface. The metals are removed from the oceanic crust where water percolates through it.

Volcanoes come in three main shapes.

• Shield volcanoes have gently sloping sides and runny basaltic lava which can spread over a massive area, building up a huge shallow cone.

• Stratovolcanoes, produced by more viscous magma, are made up of alternating layers of lava and ash and form steeper sides, with higher gradients towards the top.

• A third category, called a scoria cone volcano, has very straight sides and a large crater.

At constructive plate margins, the crust is being pulled apart, creating tension which fractures the rock so that magma can erupt through it.

The runny basic magma can travel great distances, as evidenced by some Icelandic volcanoes.

What about eruptions?

Types of eruption can be classified by the Explosivity Index or by identification with a similar example (e.g. Hawaiian, Strombolian, Vulcanian, Plinian, Ultra-Plinian and Krakatoan. )

The Explosivity Index is a logarithmic scale (ie, explosivity increases ten times between each number on the scale). The most explosive volcanoes can eject hundreds of cubic kilometres of material onto the earth’s surface. Such a massive volume of magma being removed from a volcano often causes it so subside or collapse to form a huge depression called a caldera.  These can be more than 25 kilometres across and very deep. 

In explosive eruptions, the frothing over of magma at the vent, landslides as the volcano expands or the fallback of fragments from an erupting column, can produce pyroclastic flows: avalanches of hot ash, rock fragments and gas that move down the sides of the volcano as fast as 450 miles an hour. These are very destructive and the biggest cause of death in eruptions – for example in 1902 a pyroclastic flow in St. Pierre (West Indies) killed nearly 30,000 people. However, some are more destructive than others, and the speed of the flow is changed by the temperature, gradient and ratio of gas to solid in the flow.

As well as pyroclastic material, volcanoes also emit gases that can affect climate change. These gases include water vapour, carbon dioxide (which causes global warming) and sulphur dioxide. Smaller amounts of hydrogen, carbon monoxide, hydrogen chloride, hydrogen fluoride and helium are also released. The sulphur dioxide can combine with water to produce sulphuric acid, which contributes to global cooling and could lead to a volcanic winter. This can also deplete the ozone layer.

The type of eruption which a volcano produces depends largely on the composition of the magma, which is in turn dependent on the nature of the plate margin. Many variables can change the nature of an eruption and some are very gentle while others are explosive and create pyroclastic flows.

Geysers occur where the groundwater is heated up by magma sufficiently for it to evaporate. The steam expands and is driven up through lines of weakness to eject at the surface. This provides a geothermal source of energy which can be used to turn turbines and generate electricity.