Subsidence by hundreds of meters possible
In a short article published yesterday, geophysicists Magnus Tumi Gudmundsson and Pall Einarsson at the University of Iceland´s Institute of Earth Sciences state that the subsidence in Bardarbunga makes it even harder to predict future development of events in north-western Vatnajokull glacier. A magma intrusion has migrated some 40 kilometers from beneath Bardarbunga, to the north, leading to a lava eruption north of the glacier's margin, in the lava field of Holuhraun.
The current subsidence in Bardarbunga it to date the largest such event observed in Iceland. The last known example of a caldera collapse happened in the Askja volcano system in 1875, when a large explosive eruption occurred there, causing widespread ashfall in the eastern part of Iceland and reached Scandinavia. That caldera collapse created a lake in Askja, the deepest lake in Iceland.
However, Gudmundsson and Einarsson note that such events are well know internationally. For instance the caldera in Piton de la Fournaise volcano on Reunion Island in the Indian Ocean subsided by 340 meters in 10 days in 2007, when a large eruption occured just outside the caldera itself. Other recent examples are the Fernandina caldera on the Galapagos Islands and Tolbachik on the Kamchatka peninsula.
Gudmundsson and Einarsson point to three possible scenarios regarding Bardarbunga. (see pictures below). Firstly, that the subsidence will stop and the Holuhraun eruption slowly fades out.
Secondly, the subsidence could continue, along with the eruption in Holuhraun. Another eruption could start, possibly under the Dyngjujokull glacier. In this scenario, the eruption(s) could last for a longer time, with lava volume possibly measured in cubic kilometres. Should a subglacial eruption occur, explosive action and glacial floods should be expected. "The possibilty of the fissure propagating towards the southwest (of Bardarbunga), opening either subglacially or outside the margin of the glacier, can not be excluded."
The third scenario is an eruption within the Bardarbunga caldera. In this case, say Gudmundsson and Einarsson, the magma would find a way up through the rock column beneath the caldera, leading to an eruption inside Bardarbunga. "Such an eruption could melt large volumes of ice, causing a powerful glacial flood. Another possibility is that the water would not immediately escape the caldera, because of the previous subsidence. That meltwater would later produce a large flood. When the eruption would reach through the glacial ice above, explosive eruption with ashfall would be expected."