Saturday 16 July 2016

Understanding how caldera-collapse drove the 2014-15 Bárdarbunga eruption in Iceland.

Between August 2014 and February 2015 the Bárdarbunga volcanic system in Iceland underwent the largest volcanic eruption in Iceland (or anywhere else in Europe) since the 1783–1784 eruption on Mount Laki, producing 1.4 cubic kilometers of basaltic lava, not from the main caldera, which is burried beneath the Vatnajökull Ice Cap, but rather from the Holuhruan Vent Field, which is 48 km from the caldera and is connected to it by a network of fissures. During the course of this eruption the ice sheet above the caldera subsided by 65 m, leading volcanologists to conclude that the eruption was driven by the collapse of the caldera into the underlying magma chamber, only the seventh such eruption observed sinc the advent of seismic monitoring in the early twentieth century; the earlier cadera collapse eruptions being Katmai 1912, Fernandina 1969, Tolbachik 1976, Pinatubo 1991, Miyakejima 2000, and La Reunion 2007, all of which were substantial volcanic episodes.

 Lava eruptiong from the Holuhruan Vent Field in Sepember 2014. Eggert Norddahl/Bergsveinn Norddahl/VolcanoCafe.

In a paper published in the journal Science on 15 July 2016, a team of scientists led by Magnús Gudmundsson of the Institute of Earth Sciences at the University of Iceland describe the results of a study of the 2014-15 Bárdarbunga eruption using data from seismic monitoring stations and aerial radar observations made of the caldera during the eruption.

The eruption began on 16 August 2014 with a series of small earthquakes beneath the southeastern part of the caldera, followed by the development of a new rift, which originally propagated to the southeast, reaching 7 km from the caldera within 15 hours. This rift then changed direction, migrating northeast and reachin the Holuhraun Vent Field in two weeks (the rift eventually spread to 41 km beyond the vent field). Material passing from the magma chamber through the rift reached the  Holuhraun Vent Field on 31 August 2016, leading to the onset of the visible eruptive episode.

The loss of material into the new rift system led the magma chamber to begin to deflate, placing stress on the rocks around the margin of the caldera, which were now forced to support the weight of rock and ice above the chamber. On 23 August 2016 the first of a series of small tremors on the northern part of the rim as recorded, followed by the spreading of such activity around the rim over the next few days. This in turn was followed by substantial subsidance in the central part of the caldera, as new faults developed around the rim, enablimg a plug of material above the magma chamber to subside into the chamber. This in turn forced further magma out of the chamber and into the rift system, driving further volcanism on the Holuhraun Vent Field.

 The Bárdarbunga caldera and the lateral magma flow path to the Holuhraun eruption site. (A) Aerial view of the ice-filled Bárdarbunga caldera on 24 October 2014, view from the north. (B) The effusive eruption in Holuhraun, about 40 km to the northeast of the caldera. (C) A schematic cross section through the caldera and along the lateral subterranean flow path between the magma reservoir and the surface. Gudmundsson et al. (2016).

See also... outburst flood from Grímsvötn's Ice Lake                                                              On 27 September 2015 volcanologists from the Icelandic Meteorological Office noticed a drop in the level of ice above a subglacial lake in the Grímsvötn caldera (a lake beneath the Vatnajökull ice-cap kept liquid by heat rising from the magma chamber... Dioxide emissions from the 2014-15 Holuhraun Lava Field Eruption.                      In mid-August 2014 seismic monitoring stations in Iceland began to record small Earth-tremors beneath the Bárðarbunga Volcano, which rises through the Vatnajökull Glacier... 5.4 Earthquake beneath the Vatnajökull Glacier in Iceland.                         The Icelandic Met Office, which also monitors seismic activity, recorded a Magnitude 5.4 Earthquake at a depth of 4.1 KM beneath the Vatnajökull Glacier...
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