Update 9 September: Icelandic authorities have said that the 850m-thick ice sheet covering the Bardabunga caldera is sinking at an increasing rate from around a metre a day with continuing seismic activity underneath. This indicates, they say, a sub-glacial eruption in addition to the fissures which continue their surface eruption. With the sub-glacial eruption, however, there is clearly significant melt and although vulcanologists don’t know what’s going on under the ice, they are forecasting that at some stage the sheet will melt and cause a major flood of the town that was evacuated the other day for similar concerns.
At that point, they say, there will be a sudden decompression which will release not just surface water, but atmospheric ash: the longer this takes to build up, they caution, the larger the explosion will be. The current subsidence can only be described as a slow caldera collapse, says the University of Iceland´s Institute of Earth Sciences, and is the largest one yet measured in an Icelandic volcano.
Update 6 September: The alert remains at orange, but there’s no ash right now, and no effect on air traffic … but this video of the Bardarbunga eruption, copyright of ReykjavikHelicopters.com, is just stunning!
Update 29 August: Icelandic meteorologists have issued a red aviation warning near Bardarbunga after a surface fissure eruption began in the middle of the night. Iceland’s ATC has closed airspace above the eruption up to 1,500m: no volcanic ash has been seen so far but the situation will continue to be monitored, since vulcanologists say this eruption has clear similarities with Eyjafjallajökull in its early stages.
Update 24 August: European flight control is reporting that the Icelandic authorities have informed them that the Bárðarbunga alert status is being changed to orange and the danger area is being cancelled.
Original post 23 August: I really do not want to talk this up, but it is as well to be aware. The Icelandic authorities have confirmed that the anticipated eruption of Bardarbunga is now underway. At present it is “sub-glacial”, but a red alert has been issued because “significant ash emissions” are said to be likely. European flight control is saying that there is nothing visible on the surface yet, and they are not expecting this eruption to cause anything like as much disruption as Eyjafjallajökull did in 2010 – that, they say, was the biggest European air shutdown since WW2. How this latest eruption will develop is impossible to predict, but at least it is not likely to be as bad as that.