During a scientific over flight a marked and unexpected drop was noticed in the caldera floor of the Bárdarbunga Central Volcano. The drop was 15 meters, and is as such the largest deformation of a caldera in Iceland. It is interpreted as the result of magma leaving the magmatic reservoir under the caldera floor.
If this number is valid for all of the 11 by 7km caldera it equals to a volume of drained magma of 808 million cubic meters, or just shy of a cubic kilometer. This does not take into account magma that has come into the system during this episode
.Now, where has all of this magma gone? Well between one quarter and one third has erupted out through the fissures. The current estimate is that between 250 and 300 million cubic meters have come out of all of the fissures so far. And that would leave between 500 and 750 million cubic meters inside the rifting fissure extending from Bárdarbunga Central Volcano.
A short note on the diminished amount of earthquakes. This is fully to be expected since the earthquakes are mainly a sign of increased pressure in a fissure system as magma pushes apart rock. Now that the fissures have opened the pressure will be more constant and no new rock would need to be ripped apart by the magma.
So far at Volcanocafé we have been able to keep ahead of the eruption at Bárdarbunga. We were the first in the world to publish information about the upcoming unrest of Bárdarbunga before the earthquake swarm started; we accurately predicted the most likely scenario with the following eruption. As did we do with what now has been confirmed to be happening at the caldera.
So, what will most likely happen at the caldera? For starters, it is normal for rapidly deflating large magma chambers to cause deflation caldera formations. Normally this does not lead to an eruption, or lead to just small eruptions since a deflating caldera floor is a sign of loss of pressure.
In this case we need to take into account that there are two large pools of water below the ice over the caldera floor, and that the ice in and of itself can rapidly transform into water. If that water finds a way down into the extremely hot magma reservoir the water will instantly transform into supercritical steam and a steam explosion will occur. In that case pure physics take over; if a small amount of water hits a small area of hot material a fairly benign explosive event happens. If a large amount of water hits a small area of hot material a prolonged event follows. If a small amount of water hits a large area of hot material a short rapid explosive event happens. And if a large amount of water finds a large area of warm material I would prefer to be more than 50 kilometers away.
Now that we have covered the options of water dumping into the magma reservoir we should briefly discus Jökulhlaups. For Bárdarbunga it would take quite a lot for it to cause a Jökulhlaup directly from an explosive event at the Caldera since there is not natural way for the water to leave. So, a lot of new water would need to be melted for that to happen. So either a prolonged event or a very large scale event would be needed.
As usual, follow the warnings from the Icelandic Authorities; they are the best on the planet at handling situations like this.
Correction: In a recently published article on volcanic gasses, I stated that volcanic ash could cause silicosis. Upon further review of the information out there, I found that this is unlikely. While volcanic ash is mostly silica, the most toxic form, (fine, crystalline silica,) is rarely abundant in volcanic ash. It would take years of exposure to crystalline silica rich ash to develop silicosis. Those with preexisting lung diseases, such as asthma, may be more prone to problems, up to and including death. Additionally, there is some evidence that children exposed to ash may be more prone to asthma. However, healthy individuals are unlikely to experience anything more than bronchitis-like symptoms.
The pertinent information on volcanic ash toxicity is presented here: http://www.geo.mtu.edu/~raman/papers2/HorwellBaxterBV.pdf