It is sometimes hard to understand the size of the Icelandic volcanic systems. We often read statistical things like “Half of all the ash in Europe” and “One third of all basalt produced in the world” and we still do not really get it because we lack a point of reference.
Instead we time after another get stuck with what looks big on the surface, such as large stratovolcanos in the Andes or big caldera formations like Yellowstone. But, by looking at what we see we miss the big picture.
Let me start all over again at the beginning. What is a volcano? A volcano is in one aspect like an iceberg. Most of it is hidden. When we think about volcanoes we have to start at the beginning of them, and that is where the magma comes from that ultimately forms the small part that we can see.
In Iceland there are two processes that produce magma. One is basalt formed as the MAR (Mid Atlantic Rift) is spreading. That is called MORB-formation (Mid Ocean Ridge Basalt). It is basically decompression melt that occurs as the plates are pulled apart at the MAR. This is what creates most of the magma on the Reykjanes Peninsula and north of Theistareykjarbunga.
The other process is the nascent mantleplume that is burrowing ever downwards into the mantle. How it started is not well known, all we know from petrochemical evidence is that it over time has increased in flow-mass and depth. And that it is fairly stationary at the center of spread in Iceland.
Basalt formed from the mantleplume is hotter and petrochemicaly different compared to the MORB derived magma. This kind of basalt is typically found in relatively pure form in the Bárdarbunga and Grimsvötn volcanic systems. There is a general trending away from those two towards the outer edges of Iceland where gradually MORB increases and plume derived basalt decreases.
The deep reservoir
Grimsvötn volcanic system consists of 6 en echelon central volcanoes and a fissure swarm that is between 150 and 190 kilometers in length. Here the fissure swarm is imperative for understanding the deep magma reservoir. As Iceland is pulling apart with 0.9 centimeter per year at Grimsvötn an upside down canyon is formed from the mantle and up under the volcanic system. As it spreads the canyon is filled with magma that enters the system mainly via the mantleplume, but partially from MORB-processes.
This process explains how a 400 cubic kilometer deep reservoir can form under the Grimsvötn volcanic system. The same would obviously also be true for the Bárdarbunga system. So, under an area of 190 by 50 kilometers we have roughly 800 cubic kilometers of magma.
The magma reservoirs
From the deep reservoir conduits lead up into shallow magma reservoirs that are commonly called magma chambers. In the case of Grimsvötn the high rate of eruptions has led to these being constantly open, so there are few and small earthquakes occurring as magma move upwards. We also know that there is more than one shallow magma reservoir under Grimsvötn from GPS-trajectories during eruptions. In other words, Grimsvötn often alternates between different magma chambers during eruptions.
Now we have arrived at what we see as Grimsvötn. It is a complex volcano that has little resemblance to what we would think of as a strato-volcano. In fact it is a strato-volcano with Tuya-formations, shield-formations and dome-formations all at the same time. But, it is also a caldera-formation. Currently we know of 3 separate calderas at Grimsvötn and that is the reason for the peculiar form of the caldera-system.
As the last ice age wound down the isostatic pressure relief caused a marked uptick of eruptions in Iceland. The pressure from the ice-cap diminished the influx of magma and few eruptions occurred. This caused magma to pool for a longer time in the deep reservoirs, but also in the shallow magma chambers under the central volcanoes.
As the ice melted and the ice-age ended this caused rapid formation of new magma at depth and hot fresh magma to intermingle with old stale crystallized magma. It also caused lowered pressure in the shallow magma chambers. In a few thousand years some of the largest effusive eruptions occurred on the planet during all of Holocene. It was for a time even believed that one or more VEI-7 eruptions happened in Iceland.
The last part has though been hotly debated. The opponents point to the crust being too thin to be able to withstand a large enough shallow magma reservoir, and the proponents shouted back that the amount of ash released in some eruptions was so big that they had to come from a VEI-7 eruption.
In this fight the biggest stick thrown around was the Saksunarvatn Tephra with a face value of 150 cubic kilometers (DRE)*.
The Saksunarvatn Tephra
First of all, the Saksunarvatn is not to be found in Iceland. Instead it is a lake on Streymöy in the Faroe Islands. The tephra was not found by geologists, instead it was found in 1968 by Waagstein and Johansen.
The ash layer has since been found in bogs and lakes all over Scandinavia, Denmark, Germany and in ice-cores on Greenland and Spitsbergen. It is nowadays used as an important stratigraphic layer when dating eruptions, archaeological sites, paleobiological sites and for climate research.
The problem is that there has been a long standing state of confusion caused by the interdisciplinary nature of the find. Basically it has been biologists and archaeologists that have studied the Saksunarvatn layer, and they have just said “look at the pretty sand, it looks like one big layer”.
It took all the way until this decade before a few volcanologists took a look and said “wait a minute, something is funky here”. And then some of the big guns of Icelandic volcanology were on it like bees on honey.
Now, go and get something to drink and sit down and relax. We are only going to rewrite volcanic history and partially volcanology itself. So, no big thing at all.
The Saksunarvatn Tephras
I read a couple of hundred scientific papers a year in the field. The vast bulk of them are produced so that various researchers can aid their careers to plod onwards to Professor-hood. They tend to be unimaginative, technical and most often debates ad nauseam some obscure detail. Then there are those papers that actually move science a bit forward and make me happy to read.
Once upon a blue moon I get across a paper that changes science, the ones that leaves you sitting in cold-sweat staring blankly in the wall uttering intelligent things like “duh” while longing for a good shot of whiskey since you have to remodel all that you thought you knew.
“The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)” by David A. Neave, John Maclennan, Thorvaldur Thordarson & Margaret E. Hartley is one of those very rare birds.
Be warned, it is not an easy read. It is a highly technical paper on petrochemical analysis. Instead of going into the technical details of this very well written paper I will instead move unto the consequences in an understandable way for the layman.
Let me first start with this, the Saksunarvatn Tephras where laid down in 6 large eruptions ranging from 1 cubic kilometer to 30 cubic kilometers. The figures here are the most conservative estimates. One of these five eruptions travelled in the direction of the Faroe Islands and the other four travelled towards the northwest.
Now it becomes even more interesting. The timeframe for these five large eruptions is 500 years. That means that in a very short geological time-span Grimsvötn suffered two VEI-5 and 4 VEI-6 eruptions with a combined conservative output of 150 cubic kilometers Dense Rock Equivalent (DRE).* This is a rate of large eruptions unheard of in volcanology.
There is more. The petrochemical analysis shows that the magmas formed Chrystal inclusions at very different depths and temperatures. The scientists that wrote the paper very markedly point out that a partial explanation is likely to be found in our models of heat and depth for formation of some of the samples, but this is just a call for more scientific studies in their field. These ambiguities are though comparatively small and we end up with crystals that have formed in different temperature melts at varying depths.
This leaves us with a magma composition that formed inclusions between 1140 to 1300 degrees and at pressures warying from 1 to 7.5kbar. Or in other words, some of the magma was old mush filled with melt inclusions and that had undergone evolvement, and other magmas was hot unevolved magma with less melt inclusions. In even simpler terms, during this set of eruptions magma from 2 to 15 kilometers in all stages from stale to fresh plume origin squirted out intermingled with each other.
This more or less ripped out the shallow chambers and the conduits down to the deep magma reservoir. In turn the deep magma reservoir answered by pushing up magma on an unprecedented scale. Even though the magma production in that part of Iceland is very large it does not explain what happened and the caldera formations are too small to explain it.
One explanation would be that a Graben would have formed all along the Grimsvötn Volcanic System with an average depth of 39 meters, but there is no evidence to be found now for that.
The other possible explanation is that locally plate tectonics switched direction for a while. Obviously I am not talking about this comparatively small eruption pulling Eurasia and North America towards each other. Instead I am talking about the micro-plate that exists in Iceland moving eastwards for a while.
It is likely that both things happened at the same time. By now the depression would have either uplifted or been filled in by Iceland’s large rifting fissure eruptions (Laki for instance). In regards of the micro-plate moving afterwards hypothesis, I can only say that for a while afterwards there was a few rather large eruptions at the western margin of the micro-plate. Largest among those is the Skjaldbreidur shield volcano that started to form 500 years later.
After this eruptive sequence Grimsvötn was left a shadow of itself. The volcanic system was gutted and 37.5 percent of all the magma was spent. Any normal volcano with a normal rate of magmatic influx would now have gone into dormancy lasting tens of thousands of years.
If Grimsvötn erupted in the following thousands of years is unknown, but the first known return eruption happened 3 800 years later (4 550 BC) and it was a big one. It was the 9.4 cubic kilometer Botnahraun that formed the Lakí Mountain among other things.
After that Thordarhyrna had two large effusive basalts in the same size range, namely the Bergvatnsahraun (3 550BC) and the Raudholar/Brunuholar eruption of 1 950BC.
After that small explosive eruptions started at the volcanic center ranging up to VEI-2. This 3 800 long period of small scale volcanism ended with the 15 cubic kilometer Skaftár Fires at the Lakí Fissure.
During those 10 000 years the volcano had not only replenished all of the lost magma during the Saksunarvatn Tephras, it had also replenished the large effusive hraun-eruptions. After Lakí it was supposed that larger eruptions were unlikely, but once again Grimsvötn had a surprise up its sleeve.
In 1996 Grimsvötn started a series of VEI-3 eruptions ending with the even larger VEI-4 eruption of 2011. This means that the system is now fully reformed and this has consequences about how we look at it in the future and this is something that I will get back to in the next part.
*At Volcanocafé we have the good fortune of having Professor Albert Zijlstra as esteemed administrator and contributor. Albert pointed out when he saw the article prior to publication that there seemed to be an error in the figure of 150km3 DRE output in the eruptions.
He came up with very strong arguments, but in the end I used the figures given in the papers up above. I was though intrigued and doggedly dug onwards and in the end I had made Isopac-projections and calculated the volume of the 3 calderas at Grimsvötn. The result ended up between 60 and 120 cubic kilometers DRE, a figure I at least am happy with.
Thank you Albert for forcing me to not be lazy while writing and making me do the calculations myself, in the end the article is much better with this small addendum.
Another point I wish to do, the 500 year timeframe is probably a bit over the top, the ice-core samples seem to indicate a shorter timeframe.