Iceland: volcano breaks records

B rdarbunga's outbreak provided the first live insights into caldera formation

The Bárdarbunga eruption in 2014 was the largest in Europe for 240 years in terms of lava emissions. Here lava fountains at the Holuhraun column. © Peter Hartree / CC-by-sa 2.0
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The eruption of the Icelandic volcano Bárdarbunga just over two years ago broke several records: it was the strongest in Europe for more than 240 years and the largest caldera formation ever directly observed. More importantly, for the first time, volcanologists have discovered that it is not this collapse of the magma chamber that triggers the eruption, but, conversely, that the eruption only causes the collapse, as the researchers report in the journal "Science".

Calderen are impressive witnesses of large volcanic eruptions: sinks of up to one hundred kilometers are created when the ceiling of an underground magma reservoir collapses during explosive eruptions, causing the surface of the earth to sag. However, whether this collapse first triggers the eruption or, conversely, that the eruption causes the Magmareservoir to collapse remains unclear.

The simple reason: volcanic eruptions with caldera formation are very rare. "Between 1900 and 2014, there were only six documented cases, " report Magnus Gudmundsson of the University of Iceland and his colleagues. And of these, few could be followed from the beginning and studied closely.

Biggest break since 1784

But that changed in August 2014 with the eruption of the glacier volcano Bárdarbunga in Iceland. The volcano and its more than 150 kilometers long system of columns and Nebenschloten lies largely under the Vatnajökull glacier. During the six-month eruption, 1.5 square kilometers of lava leaked. It was the largest European volcanic eruption since the 1784 Laki eruption - though fortunately not as explosive.

Lava fountains in the Holuhraun plain at the Bárdarbunga eruption in the fall of 2014 © University of Iceland / rmann H skuldsson

The B rdarbunga erupted at the end of an underground canal, 45 kilometers northeast of the main chimney in the Holuhraun field. "It is interesting that the eruption site and the magma chamber 45 kilometers away were hydraulically coupled, " explains Thomas Walter from the German GeoForschungszentrum Potsdam (GFZ). As a result, the lava escaped in an ice-free area and this prevented an explosive steam explosion, which had thrown a huge ash cloud into the atmosphere. display

Caldera formation in the center of the volcano

But the deciding factor was that while the lava fountains on the edge of the glacier rose, the volcano researchers recorded a subsidence of the subsurface above the central magma chamber of the B rdarbunga. Over the course of six months, this caldera expanded to eight miles by eleven kilometers and became 65 meters deep. Initially, the caldera sank in the center of the volcano at around one meter per day, then gradually slower.

Depression in the ice above the caldera of the glacier volcanic B r arbunga RUV / University of Iceland

"With an area of ​​about 110 square kilometers, this is the largest caldera burglary that has ever been observed instrumentally, " says co-author Eoghan Holohan of the GFZ. "The results provide the clearest picture yet of the origin and development of this mysterious geological process."

Earthquakes reveal underground events

What happened during the formation of caldera at B rdarbunga in the underground, the researchers were able to reconstruct, inter alia, using seismic measurements. Because the sagging of the Calderenkessel was accompanied by a whole series of light earthquakes. "The typical structure of earthquake waves in volcanic eruptions can be used to derive the processes in depth, directly above the magma chamber, " explains Sebastian Heimann from the GFZ.

The evaluation showed: Not the breaking of the magma chamber blanket triggered the eruption, but conversely the outflow of the magma caused the collapse. However, the nature of the rock over the magma chamber also played an important role. "The results show that the physical properties of the reservoir shaft and the magma channel have both the caldera collapse and the magma outflow regulated, " the researchers report.

The B rdarbunga eruption has given the volcanic explorers valuable initial insights into the processes that created the mighty caldera. "Whether and to what extent, however, our findings from mafic volcanoes like Bárdarbunga can also be transferred to silicate-rich systems remains open for the time being, " say Gudmundsson and his colleagues. (Science, 2016; doi: 10.1126 / science.aaf8988)

(Science / GFZ, 15.07.2016 - NPO)