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Bárðarbunga and Holuhraun: Overview of Activity

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Bárðarbunga and Holuhraun: Overview of Activity

A close-up of a Holuhraun lava fountain and stream.

Photo: Geir Ólafsson.

The Scientific Advisory Board of the Icelandic Civil Protection has reviewed data about the development of events in Bárðarbunga volcano under Vatnajökull glacier and the volcanic eruption in Holuhraun north of the glacier, from the beginning of the unrest on August 16 until present day.

Most of the data collected by scientists show a decline in the subsidence of the Bárðarbunga caldera and the volcanic eruption in Holuhraun. Earthquake activity is though still strong and the flow of lave great in comparison to volcanic eruptions in Iceland over the last 100 years. The findings of the meetings, released yesterday, are as follows:

o Earthquakes in Bárðarbunga: Seismic activity has been very strong since mid-August. The activity peaked in first half of September, it has slowly decreased since then, but activity remains intense. The period of seismic unrest is one of the largest ever recorded in a volcano globally.

o Earthquakes in the dike from Bárðarbunga to Holuhraun: Strong seismic activity coincided with the progression of the dike in the second half of August, but it decreased after the volcanic eruption began in Holuhraun (first on August 29, then continuing on August 31). Earthquakes are still detected in the dike but they are small and relatively few.

o Subsidence of the Bárðarbunga caldera: In the days following the onset of the seismic unrest, the ice-covered base of the caldera began to subside up-to 80 cm a day, but the subsidence has since slowed and it is now around 25 cm per day. The subsidence is in the shape of a bowl and it is greatest in the center of the caldera, about 50 m (164 feet), but smaller to the edges.

o Crustal deformation: Extensive ground deformation major was recorded while the dike was forming, signaling the progression of the dike and subsidence towards Bárðarbunga. Interpretation of GPS data and analysis of satellite interferograms indicate that the volume of the magma in the dike is about 0.5 cubic km, and that it was fully formed by the beginning of the volcanic eruption. After the eruption started the subsidence has been steady, but slowly decreasing, towards Bárðarbunga.

o The volcanic eruption in Holuhraun: The volcanic eruption that began in Holuhraun on August 31 is characterized by a large and unusually steady flow of lava. The magma that comes up is a rather primitive basalt, with a chemical composition typical of the Bárðarbunga volcanic system. The petrology analysis of the magma suggests that it stabilized at 9-20 km depth, meaning that it could not have resided at a shallower depth in the crust.

The lava field is now 76 square km in area. It is more difficult to measure the volume of the lava than the corresponding area. Measurements by air on November 4 and 26 indicate that the volume of the lava is now about 1.0 cubic km but the uncertainty is 0.3 cubic km. The flow of lava was about 200 cubic meters per second in September but the average flow rate for November is estimated to have been under 100 cubic meters per second. The new lava field is the largest in Iceland since the Laki eruption (1783-1784) and probably the third largest lava field on the planet over that period.

o Gas: For the first time in 150 years, gas pollution from a volcanic eruption has had an impact in Iceland. The reason is the size and the long duration of the eruption. The flux of the sulfuric dioxide (SO2) in the gas cloud has been measured as high as 1,300 kg per second, but the average flux in the first month of the eruption was estimated to have been 400 kg per second. Measurements of the total flux of gas at the eruption site are difficult and the uncertainty of these values is very high. It is probable that the flux of gas will decrease in line with decreasing flow of magma, but this has not been confirmed by measurements. A few cases of high SO2 concentration have been recorded in populated areas over the last few weeks compared to September and October, but in those cases weather conditions may have had an equally large effect as the flux of gas coming from the eruption site.

o If the unrest continues in a similar fashion, the subsidence of Bárðarbunga and the volcanic eruption on Holuhraun will continue for at least some months. The situation could, of course, change so scenarios such a sub-glacial eruption remain a possibility.

Two earthquakes stronger then magnitude 5.0 have been detected in Bárðarbunga from noon on Monday. The former was magnitude 5.2, occurring on Monday, December 1 at 12:52 noon on the south-eastern rim of the caldera and the second was magnitude 5.4, occurring on Tuesday, December 2 at 02:18 am on the northern rim.

Since noon on Monday ten earthquakes between magnitude 4.0 and 5.0 were detected in Bárðarbunga and ten earthquakes between magnitude 3.0 and 4.0. In total about 120 earthquakes were detected in Bárðarbunga during that time.

Little seismic activity has been detected in the dike and around the eruption site in Holuhraun since Monday.

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