A couple of weeks ago I started a series about which volcanoes I think are most likely to erupt in Iceland. The most likely of them is Hekla right now, but as time goes by that will sooner or later be Grimsvötn.
Grimsvötn is the most frequently erupting volcano in Iceland however you count it. A normal eruption frequency for this volcano is about 10 to 15 times per century. The current average is about once every seven years. During the twentieth century Grimsvötn erupted 12 times. During the same time there were 26 other volcanic eruptions in Iceland giving a staggering average of one eruption every 3 years.
It though becomes even more interesting if you consider that the 26 eruptions were done by no less than 10 different volcanoes. The excess eruptions were from Askja, Krafla and Hekla. So how about we take a brief look on Iceland’s most prolific volcano before we get down to what to look out for in Grimsvötn.
Quite often in older literature (and not so old) Grimsvötn is mentioned as a small volcano. This judgment is though not true in any aspect. The reasoning behind calling it small is that it most often has benign small eruptions, but as the 2011 VEI-4 eruption thought us he can throw bigger ones at any time.
If we instead look at historical records and magma reservoir capacity a completely different image grows. First of all we must recognize that Grimsvötn has the largest rate of magmatic infusion of any volcano in Iceland and that the rate is even. We must also recognize that even with the rate of eruptions he is suffering from, Grimsvötn all the time is getting a larger magmatic reservoir for every eruption, or in other words, he erupts less than he infuses.
Next thing to remember is that Grimsvötn is a serial caldera event volcano. Currently there have been no less than 3 caldera forming events at the volcano. The last one happened right after deglaciation with the VEI-6 majestic Saxunarvatn tephra. After that Grimsvötn had several thousand years of small scale eruptions around VEI-2. But as time went by VEI-3 and VEI-4 eruptions started to happen as the volcano had rebuilt itself.
Currently the inferred magma reservoir is a staggering 400 cubic kilometers and at the rate the volcano is increasing in eruption size (even though the eruption frequency is high) it is highly likely that Grimsvötn will go caldera again in the next few thousand years.
Lately there has been a conflagration of small earthquakes at the Vatnajökull glacier, but these are mostly not caused by Grimsvötn, instead these are either caused by the diminishing ice-cap, or by other volcanoes hidden under the ice. Currently there is not much pointing towards an imminent eruption of Grimsvötn, but in about 2 to 3 years time we will most likely sit and wait for an eruption. So what are the signs to look out for?
The Road to Eruption
Back in the autumn of 2010 I stated that Grimsvötn would erupt in late spring of 2011 and that May was the most likely month for an eruption. If I had been a magician I would not tell anyone how I did that, but I am not a magician so let me tell about my bag of tricks.
Some volcanoes are very hard to predict, but Grimsvötn is not one of those, actually he is probably Iceland’s most easily predicted volcano. Let us start with GPS and magma infusion.
For every eruption Grimsvötn has infused more magma than prior to the previous eruption. Or in other words, before the 2011 eruption the GPS was higher up than prior to the 2004 eruption. The point where more magma had infused came about sometime late 2008 and with an even rate of infusion the excess magma was quite big. The same goes for every recorded Grimsvötn eruption. Right now we are still waiting for Grimsvötns GPS to surpass the value prior to the 2011 eruption, but it will happen sometime late 2015. So, from a GPS standpoint we can say that there will most likely be no eruption before late 2015 and that if we take into account the surplus magma issue we are looking at an eruption sometime late 2017 to 2018 for the next eruption.
In a way the GPS works as a sanity check for Grimsvötn when we see a few small signs of activity there. Now for the big one in my bag of tricks, the Cumulative Seismic Moment chart.
Cumulative Seismic Moment is the amount of pent up seismic strain as it is released in the form of earthquakes, or in other words how much the internal pressure of the volcano has increased as evidenced by the sum of earthquake energy release. The more the graph goes up, the higher the pressure should be. And somewhere along the line the lid above the pressure filled magma reservoir will break and an eruption starts. For Grimsvötn this point is surprisingly stable from eruption to eruption.
On the graph above Gjálp is not visible, but the Cumulative Seismic Moment was the same prior to onset of the eruption. So, we have 3 data points where the breaking point has been the same, and no reason to really believe that it has not been the same back in time, or that it will be that much different in the future.
So, basically what I did was draw up to lines on a piece of paper, one was the GPS upwards motion and the other was the CSM-value and then I just extrapolated the values forward in time and came up with May 2011 as the most likely month. Isn’t it boring when a magician reveals his tricks? But the science behind it is very interesting!
As most of you now have noticed on the Cumulative Seismic Moment chart the value is pretty much not moving right now. That is because there have been very few earthquakes in Grimsvötn, and that is due to the large VEI-4 eruption lowering the pressure in the magma reservoir more than the previous eruptions did. So more magma is needed to come into the system before the pressure goes up compared to the previous recorded eruptions. If we just extend the line we will get the next eruption somewhere in 2030.
Time to sanity check with the GPS, we know that the GPS value will surpass the 2011 value in late 2015 so sometime prior to that earthquake activity inside of Grimsvötn will start to pick up and the graph will start to rise rapidly again. If we imagine that the rate of CSM will be roughly the same as at the second half of the two other graph-lines we see that it will be just slightly later than compared to the 2004 and the 2011 eruptions, so somewhere 2018 to 2019. But this is still a conjecture; we will know better as time goes by. I am only certain of one thing, I will have to revise the timeline as we get closer to the next eruption, but my reasoning will most likely be the same.
The second run of the Riddle competition started with a frenzy. But the competition is still wide open. For those who have missed it this time there is one volcanologist, one faultline and three volcanoes to find. As usual two points will be awarded until I append any clues, after that point one point will be awarded.
- Desolated and truncated + Image – Eve Cone in Canada (Shérine France, 2pt). The artist Eve is on the image, and the cone is truncated, and it is in Desolate Field.
- Blue drink of pumice raft – Curacoa in Tonga (Sissel, 2pt). Almost the same spelling as the famous blue drink Curacao, Curacoa ejected a large pumice raft during it’s last eruption.
- Imperial hot spring God of trembling sound – Naruko or Narugo in Japan (Shérine France, 2pt). Naruko was awarded a Shrine by the Imperial Court after the last eruption. It is also known as the Mountain of Trembling Sound.
- Electric will from the wanderer – Steve Sparks (Kelda, 2pt). He is the Chaning Wills professor of Volcanology and his teacher was George P.L. Walker.
- Sharp dressed man from bearded Greek – Point–Eureka Heights fault system in Texas (KarenZ, 2pt), the band ZZ Top wrote the song “Sharp dressed man” and they are from Texas, Archimedes was a bearded Greek (all members of ZZ Top are bearded except for Beard) who are famous for saying “Eureka!”. He also famously said “Do not disturb my circles” before being killed by a Roman soldier, and a fault system would really disturb your circles…
|4 Sissel||3 Kelda||2 KarenZ|
Volcano of the Month Voting
This month there are 3 volcanoes to choose from, these are Nishinoshima, Tungurahua and Ubinas.