Yesterday, the Bardarbunga crisis celebrated its first month. As our readers have already remarked, the IMO has put out an update that summarises the earthquake data over the past month. In all, some 25,000 earthquakes have been registered by the automated system of which no less than 5,900 have been manually checked by a seismologist. That is approximately 200 earthquakes per day on average or eight quakes per hour, during “rush hour”, quite a few more. This is really quite a staggering achievement by the IMO!
Yesterday in an interview carried by Icelandic News Agency MBL , Volcanologist Ármann Höskuldsson said that the Holuhraun eruption is abating. This could indicate that the first scenario might be about to happen: “Subsidence of the Bárðarbunga caldera stops and the eruption on Holuhraun declines gradually.” However, at this stage no such conclusion can be drawn as the subsidence continues even if the eruption may seem to be in decline. However, the IMO today claims that “Measurements show that the lava field in Holuhraun continues to expand. There are no signs of decreasing lava production” and ” The subsidence of the Bardarbunga caldera continues with the rate of about 50 cm over the last 24 hours.”
If we look at what could possibly be one of the best clues available, the chemical composition of the lava, there are some interesting comparisons to be drawn with those for the 2010 Eyjafjallajökull. First, Holuhraun as released by the IMO on Friday, September 5th:
The four first samples in the table were taken from the lava erupted in Phase II of the current Holuhraun eruption. The last sample (HRW-04) is from pre-existing Holuhraun lava, i.e. from previous eruptions in the area. The key elements to look at are SiO2 and MgO. The percentage of SiO2 indicates how evolved the magma is. Juvenile, basaltic magma usually has an SiO2 content of around 45%. The first of the evolved magmas, Andesite, usually has an SiO2 content in the 55% – 60% range. The second, MgO, can give an indication as to how fresh from the astenosphere/mantle the magma is. However, this seems to vary slightly from location to location. Now on to the Eyjafjallajökull samples from 2010:
As is readily apparent, the SiO2 content of the first fissure eruption at Fimvörduhals was substantially lower and consistent with juvenile basalt whereas the samples taken from the main eruption were basaltic-andesitic to andesitic in composition. The reverse is true for the MgO content, indicating that the Fimvörduhals eruption consisted of juvenile magma more or less directly from the astenosphere/mantle.
Volcanologists have mentioned the likelihood that the magma erupted this far at Holuhraun may be more evolved magmas that have been pushed out by the initial intrusion and subsequent continuing collapse at Bardarbunga, something that the above chemical analysis may support. Also, the report by Guðmundur Heiðar Guðfinnsson and Sigurður Jakobsson published by the IMO on Monday, September 8th, suggest this may be the case as they identify the magma as partially being “olivine-normative tholeiite” (0-5 wt% ol). The SEM analysis of some samples indicates that the (Holuhraun) erupted magma contains minor amount of plagioclase phenocrysts (<1%) and even scarcer olivine phenocrysts and they conclude that the lack of Fe-Ti oxides suggests that the samples had quenched before Fe-Ti oxides could form. As this is evidence of evolved magmas, it supports the conclusion that the magmas erupted this far are older and evolved magmas from the Bardarbunga magma reservoir. This view has been confirmed by the foremost theoretical volcanologist, Professor Haraldur Sigurðsson, on http://vulkan.blog.is/blog/vulkan/
Furthermore, the HRW-04 sample from a previous, older eruption is very similar to the recent ones. This indicates that the current modus operandi is not something new, that at least one previous eruption has followed the same pattern of a new intrusion pushing out older magmas from underneath Bardarbunga. This may be heartening news as the present subsidence of the caldera then most likely also is not a new feature, that it has happened at least once before without the entire system going into a full-scale caldera eruption and thus, that there is an improved chance of this not happening now. To judge by the current depth of the caldera, this must have happened several times previously. The salient question is if this time, the caldera will collapse far enough to trigger the ultimately inevitable great eruption.
Thus the long wait begins. Just how long is anyone’s guess.
Henrik (a.k.a. Pyrite)
Only the IMO and the Allmannavarnir can issue volcanic warnings and only London VAAC can issue Flight Warnings.
We know a few wish to go there right now. But, remember to stay on the where the wind is not blowing, this is a very gassy eruption high in SO2. Even one breath of the exhaust could kill you, so please respect any and all warnings from the appropriate Icelandic authorities!