I thought that I should write a short recapitulation on what has been going on during the summer in Iceland for those who have been otherwise occupied with gardening, beach visits and other general summer activities. I also think that it would be good with a general recapitulation for everyone. As always I am not that good at doing short.
Iceland is run by two different cycles. The first one is related to the mantleplume. About every 130 to 150 years the mantleplumes activity increases, and that corresponds to increased activity in the volcanoes directly on top of the mantleplume. Icelandic authorities put out a heads up on this in 2010, and it is generally believed that the 2011 Grimsvötn eruption was the opening sequence of the heightened activity. The cycle’s peak normally lasts about 30 years, so the peak should come around the year 2026.
The second cycle is the Eastern Volcanic Zone (EVZ) rift cycle. These are the large rifting fissure eruptions like Eldgjá, Skaftár Fires (Lakí) and Veidivötn. There have been 4 of those in the last thousand years with an average mean time between eruptions of 267 years. There are by far the largest basalt floods on earth, and at least 2 of the last 4 flood basalt eruptions rated in as VEI-6.
Remember, I am not saying that there will be a large scale rifting eruption in Iceland, all I am saying that in a while the risk will start to go up. But remember that as far as we know about every third rifting episode never happens and Iceland skips until the next cycle. But if it happens the peak risk would be around 2040.
Now, let us leave cyclical speculations behind and turn our eyes to this summer’s activity. I will do my best to go from North to South in this exposé. I will for natural reasons be pretty brief in what I write about each place.
During the summer the volcano has been unusually active from a seismic standpoint. The seismicity was of two types, on was running from depth up to the magma reservoir and is probably related to small magmatic intrusions even though no magmatic signals was detected, nor is anything visible on the publicly available GPS-network.
The second activity was anomalous and was very shallow and got its explanation a little more than a week ago as 50 million cubic meters of the 1875 caldera wall decided to up and about down into the caldera lake. This raised the water level with two meters and caused a 30 meter high tsunami to go across the lake.
If the event had happened just a few hours earlier it would most likely have cost lives since there were tourists down by the lake in the vicinity of where the Berghlaup occurred.
There are currently no signs of an impending eruption at Askja. GPS motion is normal, the amount of seismic activity is normal, the only thing out of the ordinary is that it seems like the hydrothermal activity under the lake has increased sufficiently to allow the lake to remain ice free during the winter. This in and of itself is not a clear signal of increased activity in the volcano, instead it could just be the sign that a crack has formed at the bottom and water has come into contact with hotter material.
Northeast of Askja we find the central volcano of Herðubreið; it is one of the few remaining perfectly preserved Tuyas of the world. It formed during glaciation and the outpouring lava formed a cylinder instead of a cone. It is in my eyes one of the most beautiful volcanoes on the planet.
It is normally said that Herðubreið has not erupted since deglaciation, but I am not entirely convinced of that. No samples have been analyzed from the spatter cone situated on top. The spatter cone looks very pristine even though it has been suffering from the extreme weather conditions at the top of the mountain. It really shouldn’t look that perfect and un-weathered if it was 10 000 years old. That being said, no large eruption has occurred there since deglaciation.
In 2007 a seismic swarm started deep down under the volcano of Upptyppingar, in the end more than 5 300 deep tectonic earthquakes had taken place. Back then the general idea was that it was Upptyppingar that had a root filling intrusion, but as time has passed the subsequent swarms has moved from Upptyppingar at an angle until they reached Herðubreið. As it reached that point the magma seems to have started to go upwards.
The original intrusion has since it reached a spot under Herðubreið been accompanied with two smaller deep intrusions under Herðubreið that has moved straight up.
A few days ago French tourists reported a strange rumbling from Herðubreið that later turned out to be a small Berghlaup caused by melting ice and snow at the top.
Current assessment of Herðubreið is that of an old volcanic system waking up from a very long sleep. We can probably expect to see several new large earthquake swarms as the magma continues to move towards the surface. It is still not clear if the original intrusion has sufficient heat to be buoyant enough to reach the surface. My guess is that it will take another large intrusion or several small intrusions for an eruption to occur. There has been regional uplift in the area consistent with an intrusion, but it is not large.
Kistufell is situated straight on top of the Icelandic mantleplume core. The petrochemical analysis gives at hand that that a large part of the magma comes from the 670 kilometer discontinuity where it passes through subducted slab remnants and is consistent with a formative mantleplume in the lower mantle.
“The isotopic heterogeneity within the Iceland mantle plume may thus be viewed as a result of mixing between plume material rising from a layer of subducted slabs (which have partly maintained their geochemical integrity and heterogeneity) and lower-mantle material (FOZO) entrained in the initial stages of plume formation.” (Kresten Breddam, 2002, linked below)
Now, what on earth is the 670 kilometer discontinuity? Well, material above that has the spinel crystal structure and below you have perovskite structure. In short, if your basic magma has spinels in it you have magma from above the discontinuity. If you have a marked lack of spinels the magma formed deeper than the discontinuity.
And the Kistufell magma is poor in chromium spinels, and the few that are seems to have come from xenoliths from the magma conduits rather than from the basic basalt (ol-tholeiite). Also the high amount of Sr points towards a deep source.
Now over to garnets, they form at about 35 to 45 kilometers depth, and the Kistufell lava is very poor in garnets, so it is safe to assume that the magma has formed below that. This differentiate the Kistufell (and other mantleplume volcanoes) from other Icelandic volcanoes far away from the plume core.
There are also inclusions of material that points to the formative mantleplume punching through a tectonic slab graveyard situated above the 670km discontinuity.
Well, I think this might have been the largest digression of my meandering mind ever, but there has been a lot of discussion lately about mantleplumes, especially the Icelandic one. So actually debating the strongest proof for it quickly and linking to the original paper might set some people at ease (or straight). Now back to what I was intending to write all along.
Kistufell has been seismically active since 2007, up until a year ago the seismicity consisted of mainly shallow earthquakes above 10 kilometers depth, but that changed abruptly a year ago where the earthquakes started to occur from 35 kilometers depth up to around 12 kilometers with the main cumulative seismic release occurring around 20 kilometers. One could interpret it as the focal point for a deep magma reservoir that is getting a refill of new magma. It will be interesting to see if this magma will start to move upwards with time.
The only sign of volcanic activity are the deep earthquakes of volcanotectonic nature, otherwise nothing is showing on the GPS-network, and no other signs have been noticed of increased activity at this small highly eroded Tuya.
I feel that going further in one installment would be a bit too much, so I will continue with Bárdarbunga next time. And also, my digression became a bit technical so I will leave things here so people have time to mull it over.
I will just say that currently there is no sign of an eruption being close at any of these 3 volcanoes. And what we have seen so far is rather miniscule compared to what we would see if they were closing in on an eruption.