Fig 1. From Holuhraun lava field 4 March 2015. The central part of the crater Baugur (Circle) on March 4th, view to the North along the crater. The encrusted surface of the lava lake has collapsed. Its remains are now a course, black rubble at the bottom of the crater. Small vents of blueish gas can be seen sporadically at the crater floor. The crater rim on the right hand side gave way and and allowed an outlet onto the lava field beyond; the channel is about 50 m wide and 40 m deep. (Ármann Höskuldsson, IMO)
It has now been a month since the IMO proclaimed that the eruption in the Holuhraun area was over following an overflight on March 3rd followed up by a field trip on March 4th. As early as October last year, Haraldur Sigurdsson, the doyen of Icelandic volcanologists, prognosticated that the eruption would come to an end at the beginning of March, an uncannily accurate prediction it may seem.
But is it so uncanny? Several of our readers made similar prognostications based on the observed rate of subsidence and how that rate it seemed to decline. Fast-winding their curves yielded similar results, the only uncertainty being whether or not a pocket of eruptible, evolved magma would be breached, re-heated and then erupt. As of date, this has not happened nor has the jökulhlaup feared should a major eruption begin under the Vatnajökull glacier. In the end, the 2014-5 eruption was a very pretty, albeit on occasion for the inhabitants of affected areas, a very unpleasant and sulphurous experience.
There is an inverse relationship between the cumulative seismic moment, the energy released in the form of earthquakes, the rate of observed subsidence and the volume of magma erupted in the Holuhraun area. No doubt, scientists at NordVulk will publish several papers in the coming years and months explaining this gravitationally-assisted or –powered eruption in great detail. On February 14th, the IMO reported the extent of the two lava field as having reached 85 km² (84,6 km² + 0,4 km²), a figure since not improved upon. The total amount of erupted magma is estimated to have been at leat 1.4 km3 (Feb 1st) but that is not the full story.
This is a rifting event. The rift is 45 km long and roughly 17.5 km high. Calculating the width is though a bit more interesting. If we take the east and west dilation between the Dyngjuháls (DYNC) and Kverkfjöll (Gengissig, GSIG) is by now 44.5cm. To get the correct values we have to take to recognize that the GPS-station are a bit far from the rift so the value is larger. Now, if we compare with the apparent dilation on further out stations we can roughly calculate that the true value is around 135cm. That would give a volume of intruded magma sitting in the rift of 1.05 km3.
Fig . Manually processed earthquakes since August 16th 2014 (IMO)
But this is not the end of the story. In addition to the intrusion towards Holuhraun, there was a failed intrusion along another fissure line to the NNE which leads to another volcano, Kistufell. This intrusion stopped once the eruption at Holuhraun began.
Further north and separated from Holuhraun by a graben, there was a lot of activity going on in the Herðubreið area. This is nothing new as the area has been geologically active for years, but from the earthquake signatures, it would seem that this is all tectonic readjustments and not an ongoing intrusion. That there will eventually be an eruption in this area is a given, but perhaps not now.
Fig . Tugnafellsjökull volcano from Sprengisandsdalur. (VC reader Jamie)
What from a volcanological point of view is a much more interesting development is that a radial fissure opened up from Bárðarbunga to the Tungnafellsjökull/Vonaskard volcano with its twin calderas. The activity began about six weeks into the eruption and continued right unto the end with several hundreds of manually checked, i.e. greater than M2.5 if the same principle has been followed, earthquakes ranging from a deep 13 km to a shallow 2 km. There can be little doubt that there has been a substantial magmatic emplacement in this volcano, and as there have been very deep earthquakes (up to 30 km) since 2013, this volcano should be closely watched, especially since almost nothing is known about her eruptive history. There are however no signs that an eruption is “imminent”.
Finally, there is Bardarbunga itself. Icelandic geologists and volcanologists have repeatedly warned against complacency. It is such a huge volcano, with such vast reservoirs of magma and capable of inducing very large eruptions as far away as Torfájökull and Veidivötn 1477 (VEI 6) and Askja 1875 (VEI 5). The 2014-5 Holuhraun eruption is actually very small in comparison to what she can do and indeed by many was expected to do. Right at the beginning, we cautioned our readers to expect a Long Wait and the waiting is not yet over.
I’d like to end on a more personal note. It will have escaped few that Carl predicted this eruption just before all the fun began and he has now received a very rare accolade from one of our favourite authors of PDFs – “Congratulations on Bardarbunga”.