The Volcanic Explosivity Index: How Big is Iceland’s Bárðarbunga Eruption?


Home / The Volcanic Explosivity Index: How Big is Iceland’s Bárðarbunga Eruption?

Is the Holohraun eruption really as big as Mt St Helen’s? Image courtesy of USGS.

Things have gone a little quiet in Iceland, in terms of media reporting at least.

’Twas ever thus; something bigger, more important comes along.

The fissure eruption at Holuhraun, part of the Bárðarbunga volcanic system, made the headlines for a while but the much-speculated-on major eruption with all its associated disruption to aviation never materialised.

It’s a pity that it’s faded out of the headlines – especially because on the 24 September, Iceland’s national broadcaster reported that the eruption “is now considered to be VEI (Volcanic Activity Index) 5”.

This suggests that the eruption is larger than some of the planet’s most notorious events and can now be ranked alongside the likes of Mount St Helen’s in 1980 and Vesuvius in 79AD.

But is this really the case?

The Volcanic Explosivity Index (VEI)

The VEI is a logarithmic scale. Image courtesy of USGS.

Earthquakes are defined by their magnitude (a measure of the energy released) and volcanic eruptions are defined broadly by the volume of material — known as the dense rock equivalent, or DRE — which they produce.

A number of other factors influence the ranking: the USGS lists these as “height of eruption column, duration in hours, and qualitative descriptive terms.

The scale is known as the volcanic explosivity index (VEI) — perhaps a little misleading, in that the more spectacular explosive eruptions (such as that of Eyjafjallajökull in 2010) don’t necessarily produce enormous volumes of ejecta.

The scale was developed in 1982 by volcanologists Christopher Newhall and Stephen Self, who recognised that: “To be useful, studies of historic volcanism need: (1) up-to-date, readily manageable historical information about eruptions, including dates and the nature of activity, and (2) a basis for comparing the scale or magnitude of each type of activity.”

The VEI was the result. It’s a logarithmic scale (each category is ten times larger than the preceding one) and is a scale from 0-8. At the lower end of the scale are the smallest eruptions, which occur on a daily basis: these produce thousands of cubic metres of lava, have a plume height of less than 100m and are rated 0.

The drama of the volcanic eruption is captured in the terminology associated with it, as the scale moves on through eruptions termed by the University of North Dakota gentle (VEI1); explosive (VEI2); severe (VEI3); cataclysmic (VEI4); paroxysmal (VEI5); colossal (VEI6); super colossal (VEI7); and mega-colossal (VEI8).

Mega-colossal eruptions, which erupt over a thousand cubic kilometres of material and have a plume height of over 25km, are thankfully rare, occurring at intervals of tens of thousands of years. The last eruption of Yellowstone would have been in this category.

How Does Holuhraun Rate?

Holohraun just keeps going. Image credit: Ármann Höskuldsson

The Holuhraun eruption is, at the time of writing, around three weeks old. In that period it has erupted an enormous volume of lava and reached significant dimensions: Volcanologist John Stevenson tweeted the latest update from the University of Iceland on 25 September: “#Holuhraun thickness from @uni_iceland: edge=8m, flow=18-20m, max=30m. Avg=14m. 14m * 37km2 = 0.5km3. Over 3 weeks => 230-350 m3/s”.

These volumes don’t suggest a VEI5 (which requires output of 1.0 cubic km) but rather of VEI4 – especially given the absence of a significant plume of ash at altitude. Nevertheless, that’s a phenomenal amount of lava and there are no signs of it abating.

On 22 September Iceland’s Institute of Earth Sciences reported that: “The volcanic eruption in Holuhraun continues with similar rate as last few days. The eruption does not seem to be declining. The lava production continues with the same strength.”

Holuhraun in an Icelandic Context

Iceland is extraordinarily volcanically active. The Smithsonian Institution’s Global Volcanism Program (GVP) database lists 525 known eruptions excluding the current one and the figure is almost certainly a significant underestimate. On average, there’s an eruption in the country every five years or so.

Most are relatively small but the database includes 87 of at least VEI3.

The Holuhraun eruption continues to produce vast quantities of lava. Image courtesy of NASA Visible earth.

The VEI is, to a degree, subjective in terms of comparing the Holuhraun fissure eruption with earlier ones; while we can be reasonably certain of the volumes erupted in the present day, earlier eruptive volumes are estimates and may very well be significant under-estimates. And, arguably, the relatively low height of the current plume undermines the case for an assignation of VEI5 even if the volume is sufficient.

Historically, the most devastating eruption in Iceland is probably that of Laki in 1783. Like the current Holuhraun event, Laki was a fissure eruption which produced copious quantities of lava but also, owing to its vast emissions of toxic gases and aerosols, caused crop failure and starvation across Europe.

Laki is rated at just VEI4 on the GVP database.

According to the British Geological Survey “The eruption produced approximately 15 km3 of lava that flowed to cover an area of approximately 565 km2, and produced about 0.4 km3 of tephra (rock fragments and particles). This was the second largest basaltic lava flow in Iceland in historical times.”

These figures suggest that the VEI should be 6 rather than 4, illustrating the uncertainties surrounding the VEI concept.

Iceland Volcano: What Happens Next?

The eruptive episode in Iceland clearly isn’t finished yet and may well continue for a long period. It seems likely, if not probable, that the volumes erupted will increase to the point at which it does become, at least in terms of erupted volume, a major eruptive episode with potential consequences beyond the immediate area.

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