Lately the internet has been filled with the usual doom and gloom about Katla. And since there has actually been a bit of unusual behavior I find it merited to actually write about the volcano.
Background on Katla
The volcano resides under the Myrdalsjökull glacier and it is one of the larger volcanoes on Iceland. It has suffered at least two large caldera forming events, but the last one was sufficiently far back in geologic history to have given the volcano sufficient time to rebuild its magmatic system fully.
This last thing is evidenced in among by the for Iceland unusually large scale of the explosive eruptions coming from the volcano. In short, after a caldera forming event the magmatic system is normally damaged and needs considerable time to rebuild before the volcano can have large eruptions again. As time goes by the volcano incrementally has larger eruptions on average until the magmatic system has grown too large and forceful to be contained and a new caldera formation occurs.
If we look at the last two large caldera formations in Iceland after deglaciation we find that Grimsvötn had one around 10 000 years ago and that it follows pretty close to the theory of gradual eruption size increase. The 2011 eruption is probably the largest in the last 10 000 years from that volcano, but it still has thousands of years to go until the next VEI-6 caldera formation. Bárdarbunga on the other hand had its VEI-6 caldera formation in 1477 so the eruptions there are now pretty meek VEI-1 or VEI-2 eruptions.
So, even though both Grimsvötn and Bárdarbunga are much larger volcanoes, Katla is suffering from larger eruptions. The current average is that Katla has 10 times larger eruptions compared to Grimsvötn on average, and a whopping 100 to 500 times larger eruptions than Iceland’s largest volcano Bárdarbunga.
So yes, Katla is definitely a force to reckon with. My beef is rather that Katla is not showing any great signs of an impending eruption. So what then is all the fuss about? Let us take a look.
Current signs and portents
Lately there has been a low level Jökulhlaup coming out from the glacier. It has so far just given a slight increase in water levels; instead the warnings are because of the greatly increased amount of volcanic gasses that are released where the water comes out. And that gas can be deadly in high doses.
This is a sign that the hydrothermal activity has increased under the glacier and that somehow water has come into contact with a hot area. It is more likely that water has found a way down than that magma has found a way up.
Now over to the earthquakes that have been seen lately, first of all, there is not an unusually high level of earthquake activity. On the contrary the current levels are about the normal level of earthquake activity for the volcano. Instead it is the last two years that have been unusually calm. If we then go on and look at the noisiest year recorded since 1992 we better understand that it takes quite a lot for Katla to erupt.
Neither has the earthquakes pointed to magmatic components so this is most likely ordinary tectonic to magmatectonic earthquakes. Some have also pointed to the fact that some earthquakes have been rather deep, but this is also normal for this particular volcano.
Now over to “overdue”. Yes, Katla tend to be fairly regular in the time between eruptions, but there have been longer repose times than this. So talking about overdue is not giving anything to the discussion. Saying that the volcano has had a statistically slightly longer repose time than the average is more correct.
Now, some might think that I said above that Katla is likely to go caldera, but that was not what I was saying at all. I am just saying that Katla has gone further down the road to a potential future caldera formation than the other two large Icelandic volcanoes I compared with. In reality the chance for a caldera formation is probably somewhere in the region of 1 in 50 right now, and that is not really worth worrying about.
In the seismic strain release charts I found a small peculiarity that I had not pondered upon before. It seems like the earthquake swarms tend to migrate outwards from Katla. First Katla has a swarm, then a much larger swarm occur a few months later at the Gódabunga cryptodome, and yet another few months later a smaller swarm takes place under Eyjafjallajökull.
This leads me to hypothesize that the volcanic area is prone to magmatic pulses that first impacts at Katla and then spreads outwards to the west.
There is also the possibility that a larger than normal amount of magma is not going into Katla and instead moves into the proto-volcano of Gódabunga and secondarily into Eyjafjallajökull, and that might explain the longer than usual repose time of Katla. This is though just a bit of speculation on my part. It might though explain why Katla and Eyjafjallajökull on two occasions have erupted closely together in time.
Bárdarbunga is the largest Icelandic volcano currently active, and it is also the largest producer of lava in the world during the last 10 000 years. The reason for this is probably that slightly north of Bárdarbunga you find the center point for the Icelandic Hotspot.
As I mentioned above the volcano went caldera in 1477 after a prolonged rifting fissure eruption in the southern part of the fissure swarm (Veidivötn). It seems to greatly have affected the size of the eruptions from the volcano; the question is more if it also will have affected the large rifting fissure eruptions that have burst forth from the fissure swarm on average every 500 years.
The current increased activity has been taking place at a spot halfway between Bárdarbunga and a volcano further north on the fissure swarm named Kistufell. This swarm has over time slowly moved closer Bárdarbunga himself. Depth and signatures have ranged from the MOHO up to close to the surface and the signatures have ranged all the way from purely tectonic to volcanic/magmatic.
My personal belief is that Bárdarbunga is gearing up to another VEI-1 to VEI-2 eruption like the one it had in 1996 when it produced an ash column for a few hours that reached 3.5 kilometers height at the end of the more famous Gjálp eruption.
At the same time a low level highly persistant swarm has been running over at the Kistufell volcano. The swarm started with deep magmatic earthquakes that gradually have risen upwards. The vicinity between the volcanoes and the fact that Kistufell is point zero seems to suggest some sort of magmatic pulse moving upwards from the Hotspot/Mantleplume.
If Kistufell erupts all bets are really off for how it would behave. It has probably not erupted during the last 10 000 years and how such a long repose time volcano would behave is written in the stars, especially for a volcano that is sitting straight on top of a large powerful mantleplumes core center.
Personally I find it much likelier that the Bárdarbunga volcanic swarm erupts at the current time that that we have an eruption at Katla, but that is my opinion based on the current evidence. Others might disagree with me, but in the end the only really important opinions are those of the involved volcanoes.