In August of 2017 Albert and I stuck our necks out on a limb and made a forecast each of what the future pattern of Grimsvötn was most likely to be. Or in other words, when would it erupt next.
As I reread our two separate forecasts, I am struck by how different methodologies we employed. Both versions are deeply entrenched in historic data and science. The data employed was obviously the same, and still our different methodologies gave us slightly different results.
I ended up with the most likely time being between December 2018 and December 2019. Albert got a best fit for 2021.
We wrote our parts as a very friendly competition against each other, but perhaps our real antagonist is nature itself. Foremost it was, and is, an exercise in showing that it is indeed possible to forecast upcoming eruptions to a fair degree. Obviously, it is not possible for all volcanoes, but for those that have regular eruptions, and that are showing clear signs, it is possible.
As we revisit the volcano more than a year later, we once more pit our wits against a sea of data to wrangle out the elusive fish of a Grimsvötn eruption.
Since I haven’t read Alberts part as I write this, I will make another forecast. Albert will employ mathematics as his main tool, and I will try once more to find signs of pressure increase factors in GPS and Cumulative Seismic Moment data in the area of Grimsvötn.
Who will be right? Me? Albert? Or the pesky nature of a volcano? The game is on, but I know that both me and Albert would love to win in an ever so friendly way.
Albert, should we bet the first round of beers? Problem is just how we bill Grimsvötn if it throws us something unexpected.
There are two things that have been the same for the eruptions of 1996, 1998, 2004 and 2011. The first of those are that the pressure increase as shown in the Cumulative Seismic Moment (earthquake energy release) is surprisingly consistent, and the same goes for the GPS-data showing inflation.
This is caused by two factors. One is that Grimsvötn has the highest known sustained influx of magma into the system of any volcano on the planet. The influx ratio is staggering, between 0.5 and 0.8 cubic kilometres per year.
That is roughly 4 times more than the combined input of all Hawaiian volcanoes, and a whopping 10 times larger than the most productive supervolcanic system, Uturunku. Obviously Grimsvötn is not a supervolcano, it is not even a supererupter by the common definitions.
Still it has produced more ash and tephra than any other volcano on the planet in the last 12 000 years. It has also produced prodigious amounts of lava in the form of distal systematic eruptions in its two-pronged southern fissure swarm, around 30 cubic kilometres or so of erupted lava.
The reason behind the almost ridiculous numbers are that the highly elongated system of Grimsvötn take up more than half of the tectonic spreading in Iceland. Or in other words, the bulk of the lava is back-filling the spread at depth, what we see erupting is just the over-spill.
The amount of over-spill is though on a mammoth scale. If we combine the 150 cubic kilometres of the Saksunarvatn Tephras, The 30 cubic kilometres effused in the 3 known rifting fissure eruptions, 10 km Skaftár Fires ash, and all the other central volcano eruptions throughout 12 000 years, we end up with a conservative number of 250 cubic kilometres of total ejecta.
That equates to an average of a VEI-3 eruption every single year since the end of the last glaciation. Not bad for an over-spill, not bad at all.
The deep magma reservoir is open-ended downwards to the mantle, explaining the very small number of deep earthquakes. This deep reservoir can be visualized as the over-turned hull of a sailing boat.
The hull’s stern is situated about 20 kilometres NNE of the Grimsvötn volcano, roughly at a spot of intriguing deep earthquake activity that we have named Greip since it may be a formative central volcano.
The hull then continues for 120 kilometres SSW, continuously tapering off until it dies out around where the Skaftár Fires erupted.
From this hull-shaped reservoir conduits go upwards into the central-volcanoes above, these are from north to south, Grimsvötn, Háabunga, Geirvörtur, Thordharyrna, Hágöngur and Eldgigur. In a sense of it Grimsvötn is the deep reservoir, with all these central volcanoes being sideshow Bobs.
And what glorious sideshow Bobs they are. Not counting Grimsvötn we have the serial VEI-4 volcano of Thordharyrna and the enigmatic Hágöngur responsible for the 536 VEI-6 that had a very serious impact on the climate (Katla topped it off in 540 with another big VEI-5). But that is something for a rainy day to write about.
The pertinent point is that at least Grimsvötn has open conduits from it’s deep 500 cubic kilometre reservoir up to 3 upper reservoirs in the central volcano.
It would be simple to believe that these reservoirs are situated under the 3 intersecting calderas that create the Mickey Mouse shaped caldera-system. This is though wrong, since only 2 out of the 3 calderas have a current chamber.
The North-west caldera (1) has a reservoir running up towards Gjálp (where the Gjálp lava originated), the central caldera (2) has the most active chamber responsible for the 1998, 2004 and 2011 eruptions.
The third reservoir (3) lies SSW of GFUM GPS-station in the direction of Háabunga, but it is clearly separated from the magma-chamber of that volcano as evidenced in earthquake data.
And here comes a bit of a conundrum, because the pattern has changed at Grimsvötn. And we will need to go through what that is.
Having several large volcanoes in close quarters has it’s effects. Especially since Bárdarbunga and Grimsvötn are the two largest volcanoes of their type on the planet.
Gjálp and Holuhraun showed the effects of this amply. In the first case a large earthquake at Bárdarbunga caused a pressure differential at the North-west magma reservoir of Grimsvötn that ripped it open in a fissure eruption.
In the second case the magma moving out of Bárdarbunga bounced against the pressure of the northern end of Grimsvötns deep magma reservoir and turned north.
Now it is analogy time. Imagine that you have two balloons. These are a bit of special balloons since both balloons are constantly being inflated until a valve brakes and a bit of air goes out. If the balloons are in different parts of a room, they will not affect each other in any meaningful way.
But if you push the two balloons hard against each other things will rapidly become interesting. In the real world there are several balloons massaging each other, and the names of the balloons are Gjálp, Grimsvötn, Háabunga and their common deep reservoir, they in turn massage (and get massaged by) Kistufell, Bárdarbunga and Hamarinn.
During the 1996 eruption of Gjálp the highest pressure in all of these was inside the Gjálp reservoir of Grimsvötn. As the relatively low-pressure volcano of Bárdarbunga suffered that large non-double-couple non-volumetric-change earthquake it lowered the pressure in Bárdarbunga creating the possibility for the Gjálp reservoir to rapidly expand, this in turn opened up the fissure nearby and general squirting ensued.
After 2011 all the reservoirs at Grimsvötn was relatively low pressure, but Bárdarbunga was medium-pressure and an oddball turned out to be high pressured. As magma pushed up from depth into Kistufell that volcano burst sideways towards Bárdarbunga, creating an extra-volcanical dyke running straight into Bárdarbunga insta-flipping it from a medium-pressure volcano into a high-pressure system, Bárdarbunga then cracked, and a new dyke ran off towards Greip at the northern end of the Grimsvötn deep reservoir, there it either connected, or rebounded, and went towards Holuhraun.
Not only did this magmatic pinball machine create a spectacular eruption, it also had grave consequences for the timing of the next Grimsvötn eruption.
As the balloons of Kistufell and Bárdarbunga erupted they lost pressure and shrunk, quite a bit. As they did that, they squeezed Grimsvötns magma chambers less and less. This meant that the Grimsvötn reservoirs could expand in the direction of the two deflating central volcanoes ever so slightly. This expansion lowered the systemic pressures of the magma chambers at Grimsvötn.
This stopped the build-up towards the next Grimsvötn-eruption dead in its tracks, or at least created a 2 year long hiatus in pressure build up as Grimsvötn took up half of the work of refilling the 2 cubic kilometres erupted out of Holuhraun from a pressure standpoint.
99.9 percent of all volcanoes on the planet would by now have been out of commission for a century or more while doing the refilling, for Grimsvötn it took less than a volcanic heartbeat since it’s the grandmaster of magmatic influx.
In July the pressure reshuffling was finished as pressure had gone up in both Grimsvötn and Bárdarbunga to create a return to the normal hug-and-squeeze-fest between them.
But before we go there we have to talk about another profound effect that Holuhraun had on Grimsvötn. Prior to the 2014 eruption it was the central magma chamber that was inflating in the normal fashion, and we where heading for an almost certain repeat of 1998, 2004 and 2011.
But as the pressure fell in that chamber something unexpected occurred. The Nunatak upon which GFUM GPS is situated started to go up and being pushed to the north-east. This indicated strongly that it was the magma reservoir directly south-west of the station that was inflating. This was also evidenced by earthquake data.
Now it is time to see what has happened in the last half year, and what the effects will be upon my August 2017 forecast.
We can safely say that pressure was fully restored in the systems as of early July 2018, at that moment a period of uplift started that so far has raised GFUM 10 centimetres in less than half a year. Total northward motion is 3.5 centimetres, and the maximum east motion was 3 centimetres.
If we just look at those numbers, it would be easy to assume that the next eruption would be south-west of the station. But it is not as easy as that.
Because a month ago the central upper magma chamber caught up and started to affect things quite a bit. The northward trajectory stopped dead in its tracks, and eastward motion reversed completely, and the poor GPS moved a centimetre west in under a month.
Currently, it is impossible to say which of these two magma chambers will take care of the eruption. It does though seem like the central chamber is the stronger of the two and should therefore be most likely to erupt.
On the other hand, those two could be connected and the eruption could utilize the magma from both for the upcoming eruption.
If the eruption would emanate from the southern and less well-known chamber there is a slight possibility that the chamber roof could fail if the ensuing eruption is large enough, and Grimsvötn is quite good at landscaping at times. One should though remember that it is a highly uncommon thing.
Cumulative Seismic Moment
As the pressure increased between the systems the inflation and pressure started to increase rapidly, and it came as no surprise that the amount of energy released by earthquakes started to increase rapidly.
From having had the flattest trajectory ever recorded, the CSM-plot roared back with a vengeance as can be seen on the image above.
If the new trajectory would hold true, we could indeed have that December 2018 eruption, but it is more probably that it will return to a trajectory closer to 2004, and that would mean that an eruption would occur somewhere between April and July 2019. If the trajectory calms down into the 2011 version, we would see an eruption at the end of 2019.
I am here using the assumption that the necessary CSM-value should be between 3 and 4.5, that might be a bit off, but I do not think by that much since we have never seen anything pointing towards that.
Obviously, the trajectory could turn into something flatter than I envisioned above, if so the eruption would come at a later date more along what Albert originally deduced mathematically. And, obviously there could come a large prolonged swarm taking care of things within days or weeks, if so, it would cut the time short with a vengeance.
In the end I find that I do not need to change my original forecast that much given the data at hand. I think that December 2018 might be a bit optimistic, but at the going rate anywhere between April and December 2019 seems to be a good bet for a bit of volcanic tourism for those of the ashy persuasion.
Next week we will have Alberts version. And I for one am highly curious where the mathematical approach will lead. Upwards and erupwards I am sure!