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Impossible to predict how long an erutpion will last
A lava eruption started early Sunday morning on a fissure at the lava field Holuhraun, north of the subglacial volcano Bardarbunga, Iceland‘s largest. This is the third eruption since an intense episode of seismic activity started at and around Bardarbunga around the middle of August. The first eruption was located at Bardarbunga, but faded out before it managed to break through the thick glacier. A tiny second eruption started at Holuhraun in the early hours of Friday, but lasted only 3-4 hours. The third eruption is still ongoing.
The scientist at ISOR say it is impossible to predict how long the eruption at Holuhraun will last. That depends mainly on how clean a path the magma will have, from the source at Bardarbunga, to the eruption site. The Krafla Fires (an eruption in the Krafla volcanic system in the north of Iceland in the 1970s and 80s) came in short bursts, with the magma gathering steadily in a shallow magma chamber, and flowing from there intermittently into the fissure system.
The scientists at ISOR believe it is more likely that the magma in the current eruption does not stop in a shallow magma chamber like in the Krafla Fires, Rather it will flow directly in a channel under the magma chamber and out to the volcanic craters. In their estimation the upflow of magma is much larger under Bardarbunga than it was in the Krafla Fires. Therefore they expect one protracted eruption or multiple eruptions, each lasting for a short while.
Pressure was formed when the magma moved
In an article on the ISOR webpage (here, in Icelandic) the geologists present the timeline, for the last few weeks, of the Bardabunga earthquakes and eruptions. They say a fissure system, and magma corridor (dike intrusion) that formed at Bardarbunga, flowed east from the caldera of the volcano, first for 5 km east-southeast and then 20 km (12 mi) northeast.
There the flow of magma halted for a few days, until it broke through, migrating another 20 km north-northeast. The scientists at ISOR state that the first subglacial eruption on August 23rd, took place when there was a pause in the northward flow of magma. That caused pressure to build up in the magma underneath Bardarbunga.
Holuhraun lava field possibly formed in a similar scenario
The northward migration of magma then stopped suddenly and decisively, just north of Holuhraun lava field, about a week ago. The scientists say that the eruption in Holuhraun supports a theory they presented on the ISOR webpage on August 20th, that Holuhraun lava field was possibly formed originally in a similar scenario to the one being played out now, in the late 18th century.
11 earthquakes, magnitude 5 or above, have been registered at Bardabunga‘s main caldera since Aug. 16. Earthquakes of that size had previously not been detected in Vatnajokull glacier since 1996. The biggest one in the current episode of earthquakes, a magnitude 5.7, struck on August 26th. The ISOR article states that the earthquakes in the volcano itself are close to a ring which is thought to mark the faults which were created when the Bardarbunga caldera was formed, in prehistoric times. The quakes seem to be at a depth of around 10 km (6 mi). The cause is considered to be that the Bardarbunga caldera is sinking, because the pressure under it is lowering as the magma underneath flows away.
Indications of a large amount of magma at a shallow depth under the mountain
The scientists point out that inside the ring in question, very few earthquakes are detected. According to their article, this would indicate that the matter underneath is very hot, above or near melting point. This is is considered evidence of a very large magma chamber existing underneath Badarbunga caldera, at a very shallow depth.
A large explosive eruption possible
The scientists say that an eruption straight up from such a magma chamber, where acidic magma has been gathering for a long time, could be a large one and could produce large amounts of ash and pumice, similar to the eruption in Askja, a nearby Icelandic volcano, in 1875. There is currently no indication of an eruption of that size being imminent at Bardarbunga, even though there is considerable earthquake activity in the rim of the caldera. Still, the scientists say, that scenario cannot be ruled out.
This story, by the Icelandic National Broadcasting Service (RUV), was updated on 3 September 2014, at 12:02 GMT.