Sometimes I get a bit too science-y so this page defines some volcanic terms. Will be updated alongside new posts that introduce terms not previously mentioned.

Volcanic ash
Image by Wieprecht D. (1996).

In volcanology terms volcanic ash is called ‘tephra.’ It is classified as all material that is ejected from an eruption. This includes all sizes of material below and above 2 mm.


The process of gas emissions escaping magma before and during an eruption. The less gas that can escape before magma is erupted, the more explosive the eruption will be.

Image by Griggs J.D. (1983).

Lava fountaining is the ejection of lava caused by the rapid formation and expansion of vesicles (aka bubbles). Kilauea and its Pu’u ‘O’o vent (image above) provides a classic example.

Gas emissions

Volcanoes emit various gases, some of the most common being sulphur, sulphur dioxide, carbon dioxide and water vapour. Others include nitrogen, hydrogen, methane and argon.

Lava dome
Image by Robertson E. (2009).

It is a build up of viscous lava that has extruded from a volcano’s vent. Only volcanoes with lava of high viscosities produce lava domes. They are a dangerous hazard as they can act as a plug, effectively increasing pressure within the conduit.

Lava flow
Image by Photovolcanica (2014).

Molten rock that has reached the surface and flows under the influence of gravity. Due to various factors, flows can become highly complex and produce different surface features. They are categorised as either: pāhoehoe, a’ā or blocky.

Phreatic eruption

Image by Sanabria J. (1994).


Also known as steam or hydromagmatic eruptions. These eruptions involve the interaction of water.

Pyroclastic density current (PDCs)
Image by Photovolcanica (2014).

People will know them by them by their other name: pyroclastic flows. Or my favourite: nuée ardente (French for ‘burning cloud’).  PDCs are the flow of hot volcanic material, moving under the flow of gravity and fuelled by convecting heat. They can travel up to 700 mph with temperatures up to 1,000 °C.

Volcanic Explosivity Index (VEI)

A classfication system of 0-8 in a log scale. For example, a VEI 4 eruption is ten times greater than a VEI 3 eruption. Eruptions are placed on the VEI scale according to the amount of tephra ejected and the height of the eruptive column.

Vulcanian eruption

This type of explosive eruption is mainly associated with the pressure of trapped gases in a viscous magma puncturing through solidified lava, usually a lava dome.


Volcanic earthquakes are usually the key sign of volcanic activity – they show that magma is moving through weaknesses of the surrounding rock beneath the surface.

Strombolian eruption
Image by Photovolcanica (2014).

A type of eruption categorised by the ejection of incandescent tephra hundreds of metres high. Classic example is its namesake: Stromboli.

Sub-plinian eruption

Explosive eruptions with sustained ash columns with extensive tephra fallout. The difference between this type and Plinian eruptions (a magnitude bigger) is the amount of material that is erupted.


Image by G. Mackley (2007).
Image by Mackley G. (2007).

A lahar is a slurry of volcanic material, debris and water, usually down river channels connected to a volcano. The above image is from an eruption at Mt. Ruapehu, New Zealand in 2007.


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