The eruption at Bárðarbunga in Iceland, which created the Holuhraun lava field, is arguably the most significant volcanic event of 2014.
Closely monitored even before the first signs of unrest, it’s been intensively scrutinised by volcanologists, who are taking every opportunity to learn what they can about volcanic processes. Iceland is the product of a magmatic hotspot underlying a divergent plate margin and so exhibits two types of volcanism which interact and influence one another.
Geometry of the Rifts
As Dr Freysteinn Sigmundsson, lead author of one of the first published papers on the Bárðarbunga eruption, explained to Decoded Science:
“The geometry of the rifts is more complicated than on most Mid-Ocean ridges because there has been, on a geologic timescale, relative motion between the rift and the hotspot. This has resulted in rift jumps and some complication regarding the plate boundary structure.”
Despite this complication, he says: “Regarding individual rifting events in Iceland, then they are often taken as an analogy for rifting events on the ocean floor.”
The current eruption is the result of divergence, as the Eurasian and North American tectonic plates move apart. In most parts of the world this type of margin is under the oceans and difficult to see — and so the Icelandic eruption provides a valuable opportunity to study the rifting process.
The 2014 Bárðarbunga Eruption
In mid-August, the subglacial Bárðarbunga volcano in Iceland showed high levels of seismic activity which led to speculation about the possibility of an eruption. The volcano didn’t disappoint, and the onset of the eruption was confirmed on 23 August. Eruptive activity increased along splits, or fissures, several kilometres long in the crust and since then these fissures, erupting steadily, constantly and spectacularly, have created not just a new layer of Iceland covering (at the last official notification) 79.8 square kilometers; but also an apparently endless stream of other-worldly images.
New Research on Rifting and Dyke Propagation
Such fissure eruptions are common in Iceland and elsewhere but they are elusive from a scientific point of view. “Only a few such episodes have been monitored, as most divergent plate boundaries form mid-ocean ridges” notes the latest research from Dr Sigmundsson and his team.
In the light of this, although the physical expression (the injection of linear magmatic features known as dykes) is clearly established, the exact mechanism (or mechanisms) by which these dykes have been emplaced remain unclear.
In their paper, Sigmundsson and colleagues summarise the two alternative mechanisms — either a vertical injection from an underlying magma source, or a lateral emplacement from a source closer ti the surface.
The team’s study of Bárðarbunga indicated that the latter process was at work, as the dykes propagated northeastwards over distances of tens of kilometres. They observed that the nature of the topography influenced the process with obstacles slowing dyke growth, followed by a build up of pressure which overcame the obstacle and allowed the growth to continue.
“I think our observation of how barriers at the end of dyke segments were overcome by pressure buildup in the far end of the lateral dyke, and our model explaining the dyke trajectory are novel new findings that are relevant for general understanding of the rifting process,” said Dr Sigmundsson. “We have now a better understanding of how lateral dykes can form over long distances (how barriers on the way are overcome), and we also have an improved understanding of dyke trajectories.”
Looking Forward: Bárðarbunga in 2015
Although the main episode of dyke propagation seems to be over (the study notes that dykes grew to lengths of 45km, much of it over a period of just two weeks) the eruption is still continuing at the time of writing.
Seismic activity has decreased in the four months of the eruption but continues at significant levels with three earthquakes of larger than M4.5 in the past week (there were 36 in the preceding 30 days). What happens next remains unclear.
In the past, such fissure eruptions have continued for many months and produced enormous quantities of lava (the eruption of Laki, in 1783-5, added an estimated 15 cubic km to the island and the 565 square km it covered dwarf the current output).
In their most recent bulletin, issued on 19 December, Iceland’s civil authorities propose three ‘most likely scenarios’ — a continuation of the eruption ‘for many months’; lengthening of the fissure below the icecap; or an eruption within the volcano itself. Either of the latter two would be expected to produce extensive flooding and the third has potential to produce volcanic ash.
The authorities don’t, however, indicate a degree of likelihood for any of the options and note that “other scenarios cannot be excluded.” Whatever volcanologists have learned — and continue to learn — from the ongoing eruption, there are still things we don’t know about its future development and its longevity. But it may well be that the eruption at Bárðarbunga continues to feature in science news in 2015.