The world of science has never witnessed, measured and documented an event such as the one that is now taking place beneath Bárðarbunga. The attention of most Icelanders is now focussed on the Holuhraun lava flow and its fiery eruption, for obvious reasons. The lava flow has now reached over 70 square kilometers in extent, and probalby about one cubic km in volume (the mean thickness is somewhat uncertain) and it is therefore approximately equal to the magma volume erupted during the Surtsey eruption from 1963 to 1965. This is a major eruption. But the eruption itself is actually somewhat of a diversion, because the main event is not in Holuhraun lava field, but rather in the volcanic center of Bárðarbunga. But there the spectacle is hidden from view beneath an ice cap of some 600 to 800 meters in thickness. I believe that no earth scientist can be in doubt that the subsidence that is now taking place in the Bárðarbunga ice cap is closely linked to the Holuhraun eruption. From 16. August to 29. September we were all witnesses of the formation of a magma dike, which linked Bárðarbunga to the fissure eruption in Holuhraun, about 50 km to the north. Since then the eruption has been stady and so has the subsidence of the Bárðarbunga caldera. It is likely that subsidence started as soon as the dike began to form in mid August, but accurate measurements of subsidence were not started until 14. September. Then the subsidence was already 22 meters in depth, and since then it has subsided an additional 23 meters, or a total subsidence of 45 meters to date. The subsidence is now at about 20 cm per day, but was initally about 50 cm per day and has decreased steadily from the start. I have earlier pointed out in my blog that the subsidence of Bárðarbunga follows remarkably well a curve or trend, as shown on the graph above (data from the web site of Veðurstofan). The curve is best described as a polynomial of this sort: y = -0.0012x2 + 0.4321x. The least squares fit of the data is R² = 0.99946. This is actually an incredibly close fit. Remember that if all the data points fall on the curve, then R² = 1.0000. It is very unusual in geology that a phenomenon obeys a mathematical formula so closely. This closeness of fit most probably only occurs in very large events, such as the current subsidence of the floor of the Bárðarbunga caldera, as it subsides into the magma chanber beneath the volcano. The subsiding block may be about 10 km in diamter and about 8 km in thickness, with a volume of rock in excess of 600 cubic kilometers! It is no wonder that its subsidene is continuously accompanied by earthquakes of size 4 and 5.
It is especially notable that the subsidence curve slows down, i.e. the subsidence has been systematically decreasing with time. This gives us a unique opportunity to estimate the time when subsidence ceases, which is most likely also the time when magma stops flowing out of the magma reservoir under the caldera and the time when the eruption in Holuhraun stops. I have therefore extended the mathematical curve to the point, where it becomes horizontal, i.e. when subsidence stops. This occurs in the model after about 170 days from the time that measurements began, on 14. September. The curve shown above thus predicts end of eruption at the end of February or early March 2015. But there are many factos, which could affect the magma flow when the flow rate slows down, especially the resistance in the magma dike beneath Holuhraun, and several other factors. All of these factors work towards an end of the eruption earlier than predicted above. As I pointed out in my last blog, it is clear that the erupting fissure is very close to the fissure that erupted in Holuhraun in 1797. The eruption today appears to be a virtual repeat of the eruption that took place at the end of the eighteenth century. This is encouraging, and supports the view that the current activity will most likely be restricted to holuhraun and unlikely to result in magma ascent in Bárðarbunga. The current holuhraun eruption is already world famous among earth scientists, but it is still not the largest ongoing volcanic eruption today. Kilauea volcano on Hawaii has erupted nonstop since 1983 and it has now erupted a volume of magma about 4 cubic kilometers, or about four time the volume in Holuhraun.