In the previous part we learned that Grimsvötn indeed is able to produce large scale eruptions as we looked closer into the Saksunarvatn tephras. The first part can be found by following the link below.
This time we will look into what this new information might mean for the future eruptions of Grimsvötn. But first we need to look at the cycles that affect Grimsvötn.
The Regional Cycles
Icelandic volcanism is normally governed by two different processes that may be related to each other in various ways that we do not fully understand yet. These are the magmatic pulse cycles determined by increased influx from the mantleplume and the rift cycles as Iceland is being pulled apart. There is also ample evidence that we are entering a third man-made cycle caused by melting of the large glaciers that overlie several of the larger Icelandic volcanoes, a form of man-made isostatic rebound. Let us look at these individually.
It is normally said that statistics is unimportant when talking about volcanism and as a tool for volcanic prediction. This should be taken with a large pinch of salt. If you have a large enough collection of sampling of eruptions and a large enough collection of volcanoes you can sometimes deduce a few things.
On average Iceland suffers from an eruption every 7 to 10 years, but if we look at the historic distribution of these eruptions we will find that there is an ebb and flood to how they are distributed. During the “floods” you will see eruptions roughly every third year and during the “ebbs” there can be as much as 20 years or more between eruptions.
At no volcano it is as marked as at Grimsvötn since it is the most frequent erupting volcano in Iceland. On average it erupts ever 10 to 12 years and about every 3 to 7 years during a heightened period of eruptions. During the low periods of activity there can be between 20 and 50 years between eruptions.
During most of the twentieth century there was a low period. Up until 1938 it was business as usual with frequent eruptions, after that there were only two eruptions that we can be certain about before 1983. Then there was a lapse for 18 years prior to the 1954 eruption and another 27 years until the 1983 eruption. After that the trend turned, both in intensity of eruptions and frequency.
In 1996 the large mainly effusive Gjálp eruption took place (VEI-3), the next one was only two years later in 1998 (VEI-3), then came the 2004 eruption (VEI-3).
After that came a 7 year long lull before Grimsvötn pulled something that was at the time unexpected. It’s most explosive eruption since the Saksunarvatn Tephras. The eruption is rated as a very large VEI-4 or even a small VEI-5 with a combined dispelled 0.8 cubic kilometers of tephra in Dense Rock Equivalent. If we combine that with what is believed to be 1 cubic kilometer of lava emplaced under the central caldera lake it was a whopper of an eruption however you count it. It is to date the largest eruption this millennium on Earth.
The general average for this cycle is roughly 130 years, but it varies slightly so from peak to peak there could be as little as 100 years and as much as 160 years. If we count from the peak of Grimsvötn, Katla and Bárdarbunga eruptions we would get roughly 100 to 110 years between peaks for this cycle (if we count that we right now have the peak of the cycle, but it is probably about 10 to 20 years before the true peak arrives).
It is not likely that we have reached this peak yet since we have not seen eruptions from Askja, Katla and Thordharhyrna as of now, and they normally come out and play around the peak.
The Icelandic Rift Cycles
We tend to talk about the Icelandic Rift Cycle and mean the one that is taking place in the aseismic area between Torfajökull, Bardarbunga, Thordharhyrna (on the Grimsvötn Fissure Swarm) and Katla. This area is sometimes called The Dead Zone since it is so devoid of seismic activity, but we know that as soon as an eruption is on the way there it is very seismically noisy.
But talking about this tiny area as The Icelandic Rift Cycle is wrong. Iceland is filled with other rift cycles that can be equally large in effusive size. We have the 5000 year rift cycle of Theistareykjarbunga, the 3000 year cycles of Fremri-Námur and Heiðarsporðar central volcanoes.
Krafla and Askja also seems to follow roughly 250 year long cycles and The Vestmannaeyjar show at least partial evidence of following the same pattern. The rifting cycles are though offset from each other in time with Krafla and Vestmannaeyar cycles most often happening prior to the Dead Zone rifting cycle.
If we look over at Reykjanes it seems to be running on a 1 000 year cycle. There is one thing that leaps out when you look at the different cycle patterns. There seem to be a line of rifting that is more potent compared to the rest of Iceland that starts at Krafla and runs via Bárdarbunga/Grimsvötn down via Katla to the Vestmannaeyjar. Sometimes parts of this rifting line is skipped over so there is no certainty that a rifting event will happen at any particular area, but the statistical chance is about 75 percent that a rifting episode will take place every 230 to 270 years in each segment.
For The Dead Zone these rifting cycles take place at the fissure swarms of either Bárdarbunga (Veidivötn, Vatnaöldur or Thjorsahraun), Grimsvötn (Lakí or Raudholar) and Katla (Eldgjá). In regards of which of them is more likely to erupt there is just no way to say as of now, all we can say is that there is a 75 percent risk of one of them going off and that if that happens that it will be between 5 and 30 cubic kilometer in size.
After the last Ice Age there was a 2 500 year long period with very intense volcanism in Iceland caused by decompression melt due to the intense isostatic rebound. This period contains the largest eruptions in Iceland, both effusive and explosive.
On the effusive side we have the 30 cubic kilometer Thjorsahraun (the largest Holocone lava flow), The 35 cubic kilometer formation of Skjaldbreidur and the 50 cubic kilometer combined eruption that caused the Theistareykjarhraun and the Theistareykjarbunga Shield Volcano.
We also had the Vedde Ash from Askja and the Saksunarvatn Tephras from Grimsvötn. The later of them we now know is the largest explosive sequence in Europe since the end of the Ice Age.
The reason why this is so important is due to human stupidity. We are now causing our own man-made deglaciation and isostatic rebound due to antropomorphic heating. In about 100 years there will be no more glaciers on Iceland, and the isostatic rebound will cause large scale decompression melting of the mantle under Vatnajökull.
We know from historical eruptive records and tomographic images that the magma reservoirs under Grimsvötn are fully reformed and that by now the volcanic system is erupting all of the influxing magma.
If we combine this with the occurance of two upcoming cycle peaks coinciding we have a situation where there is an increased risk of a larger eruption than normal from Grimsvötn. The magmatic pulse cycle means that during the next decades the volcano will receive more magma from the mantleplume and that the volcano will be suffering from increased strain at the fissure swarm running through the volcano. The increased rate of eruptions and eruptions size increase is also causing an increased risk of magma reservoir failure due to fatigue.
This last part can sufficiently weaken the roof over any of the magma reservoirs causing an increased risk for caldera formation if a large eruption occurs, or if a rifting event rapidly evacuates one or more magma reservoirs.
The antropomorphic isostatic rebound will also add to the mix as additional decompression melt could occur.
If we put these 3 different cycles together and add them on top of the fully reformed system post-Saksunarvatn Tephras we find a volcanic system capable of eruptions beyond the projected VEI-4 roof of possible eruption size.