Lately Iceland has suffered a long series of earthquake swarms that have ambled all over the island. Earthquake swarms are common in Iceland, that is not the issue, the problem is that they this time came in a swarm of swarms, all of them containing at least one earthquake above M3. Before we take a look at the latest earthquake swarm between Bárdarbunga and Kistufell we should take a look at the really broad picture of Iceland.
Magmapulse, riftcycle and deglaciation
A few years ago the authorities of Iceland issued a notification that Iceland would soon be entering a period of heightened volcanic activity, these periods of unrest comes at about a 130 year long cycle. One of the explanations given for this is pulses of uprising magma via the hotspot/mantleplume which is centered under Kistufell volcano.
At the same time we are closing in on the time where Iceland is most likely to have a rifting episode. These come with a roughly 270 year interval and are caused by buildup of tectonic strain as the North American plate and Eurasian plate are pulled apart at the Mid Atlantic Rift. The release of this strain is uneven in Iceland and a large portion of the spreading comes in fairly regular intervals. These heightened motion periods are well known to cause large scale volcanic eruptions like the Lakí, Veidivötn and Éldgja eruptions.
As our planet becomes warmer due to greenhouse gases and human energy release (rarely talked about component of global warming) into the atmosphere the glaciers around the planet have started to melt.
Iceland is home to Europe’s largest glacier, the Vatnajökull. Vatnajökull is situated on top of most of the Icelandic hotspot/mantleplume with Kistufell situated at the northwest corner of the glacier. As the ice melts the lost weight is tremendous, and this causes the entire crust to rise rapidly upwards. This in turn causes a diminished pressure at the mantle-crust boundary (MOHO) and depressurizing melt is more likely to occur.
Bárdarbunga is the largest active volcano in Iceland, it was fairly unknown by volcanologists until satellite imagery revealed the true size of the volcano and the size of its caldera. It is responsible for the largest effusive eruption in the last 10 000 years, the Thjorsahraun lava field consisting of 25 to 30 cubic kilometers of lava from its extensive fissure swarm.
The last cataclysmic eruption happened in 1477 when the Veidivötn fissure erupted between 8 and 10 cubic kilometers of lava. That eruption ended with a VEI-6 caldera forming event.
The last confirmed eruption in the Global Volcanism Program catalogue was in 1910. That is though not correct. The last confirmed eruption from the main caldera happened at 1300 hours local time on the 6th of November 1996, the eruption was witnessed by volcanologists that was monitoring the ongoing Gjálp eruption and the eruption caused a 4km high column from Bárdarbunga. The eruption is believed to have lasted for only 30 minutes.
In the volcanic catalogues the eruption of Gjálp is assigned to Grimsvötn volcano, but there are reasons to doubt that assignment and nobody has ever given a really good explanation for assigning the eruption to Grimsvötn.
Most evidence instead point towards it being Bárdarbunga causing the eruption, or that it was an independent fissure eruption caused mechanically by Bárdarbunga. I am just mentioning this and I let everyone be their own judges on this.
At 10.48 on the 29th of September an M5.6 earthquake struck inside the volcano of Bárdarbunga. This earthquake was the last of a decade long series of a very peculiar type of M5+ earthquakes that caused no volumetric change. It is interpreted in such a way that fluid is changing places between two different and very large portions of the magma reservoir. A while ago a paper was published that argued that a large blob of fresh hot magma was pushed upwards and that displaced old magma downwards causing a no net volume change.
After the M5.6 earthquake a very intense earthquake swarm started at Bárdarbunga that quickly migrated towards Grimsvötn and Grimsvötn was in the end also covered by the earthquake swarm. Shortly before midnight on the 30th of September the amount of earthquakes abated a bit and harmonic tremor was detected. This was interpreted as onset of an eruption somewhere in the region.
The next day a reconnaissance flight discovered a depression bowl in the ice sheet and subsequent flights found more bowls in a line. The bowls was roughly equidistant between Bárdarbunga and Iceland’s second largest volcano, Grimsvötn. The length of the fissure that had opened up was between 5 and 6 kilometers in length. At the same time they noticed that the ice sheet above the Grimsvötn caldera had started to rise as water flowed into the caldera lake. A signal that a Jökulhlaup would soon follow the eruption.
The next day flights showed that the eruption had broken through the glacier and ash was explosively erupted.
On the 5th of November a violent Jökulhlaup burst forth from Grimsvötn with a peak discharge of 45 000 cubic meters of water per second, this was far more than the scientists had expected. As such it is the most powerful Jökulhlaup scientifically observed and in the end the Grimsvötn sub glacial lake was completely emptied out. It is estimated that the temperature in the Grimsvötn was 10 degrees Celsius prior to the start of the Jökulhlaup and that the unusually high temperature caused a wider than normal channel to form.
After 13 days of erupting the Gjálp fissure closed. The eruption had formed an elongated low ridge shaped like an overturned boat hull. The estimated amount of lava deposited was between 0.8 and 1.5 cubic kilometers. This is the second known eruption of Gjálp with the prior known eruption happening at 1938. As such it is possible that Gjálp is a unique volcano in its formative stage to become a new Icelandic central volcano.
The main reason behind assigning the eruption to Grimsvötn is that the eruption opened up on the southern end closest to Grimsvötn. This is though a bit of odd reasoning since historical evidence points towards Icelandic fissure eruption most commonly start at the point most far away from the responsible central volcano and then progress inwards to the central volcano.
The latest earthquake swarm
The latest earthquake swarm in Iceland is situated between Bárdarbunga and the Kistufell volcano. Kistufell has had no known Holocene eruption but a slow and persistent series of earthquakes started at that volcano 6 months ago. The start of the series was at 25km (MOHO) and has progressed upwards over time forming an earthquake stack to a depth of about 4km. It is likely that this is a dyke formation, but nothing is currently pointing towards an upcoming eruption since there is just not enough activity for that to happen in a volcano after such a long repose time.
In 2010 Bárdarbunga suffered its last major earthquake swarm. Even though the ongoing earthquake swarm had a M3.7 earthquake as it started the overall picture is that there is not sufficient seismic activity to point to an onset of eruption, nor is there any clear magmatic signals detected. So it seems to be yet another tectonic swarm caused by the building tectonic strain.
I firmly believe that the swarm of swarms that Iceland has suffered from is a sign that the tectonic strain is closing in on a condition where a rift eruption can occur, but the problem as usual is time-frame. It is quite impossible to say if it will happen within months or within a couple of decades. It is also impossible to say if there will be a normal Icelandic fissure eruption like at Krafla or Gjálp, or if it will be a larger rifting fissure eruption. There is of course nothing saying that there will not be either types, or nothing at all happening.
I do though think that something will happen, after all Iceland is nearing both a high-point in activity and the maximum risk zone for a large fissure eruption caused by tectonic strain release at the same time as massive deglaciation occurs.
It is where, when and which volcanic system that is so far written in the stars since the swarms seem to be jumping all over the place. But one thing is clear, it will start as an earthquake swarm, but that swarm will be intensifying over time instead of diminishing. So, if you see a swarm lasting for days with M3 earthquakes and it progresses towards M4+ earthquakes, then you can expect to live in interesting times. So far we are not there.
Magnús T. Gudmundsson, Freysteinn Sigmundsson, Helgi Björnsson and Thórdis Hödnadóttir; The 1996 eruption at Gjálp, Vatnajökull ice cap, Iceland: efficiency of heat transfer, ice deformation and subglacial water pressure