I had intended to write either an angry op-ed, or a piece about Guatemalan volcanism. But, as usual Iceland had to butt in and grab the attention, so here we go (again).
Seismic unrest at Herðubreið
Before I write about what has happened in the last few days and about what is going to happen in the near geological future we need a geological background.
The Herðubreið volcano formed during the last glacial-period when Iceland was covered by a 2km thick layer of ice. When the eruptions took place that formed Herðubreið they happened under the ice, melting a circular cavity in the ice. During the eruptions, a lake of water formed above the eruption and a circular cauldron formed with walls of ice.
This lead to lead to layers of pillow lava forming, pretty much as they would during a sub-aquatic eruption, but with a small twist. Due to the ice-walls of the cauldron the sides of the volcano became very steep, almost at a 90 degree angle and the top became flat.
This type of volcano is fairly rare and is called a Tuya, or a table-top mountain. And no volcano on earth is a better example of this process than Herðubreið. At the end the mountain became too tall for the diminishing ice age glacier and the final eruption occurred above the ice and a small spatter cone formed at the top.
This makes us fairly sure that the volcano has not erupted during the last 12 000 years since we have a glacial timeline for it.
Herðubreið is a bit of an oddity for Iceland since it is not situated on a known fissure swarm and seems to not have a fissure swarm of its own. It is therefore not a part of the regional east-west extension of Iceland and should therefore be relatively seismically inert.
We do though know that it is a central volcano of its own and that it used to have a developed magmatic system that was among the more active during the glaciated period prior to Holocene. If left to its own devices that magma system would after 12 000 years have become solidified and Herðubreið should have become a former volcano by now.
In 2008 a large seismic unrest started at the volcano of Upptyppingar to the south of Herðubreið as about 1km3 of magma intruded into the crust at 14km depth and there was a scare that the hilariously named Upptyppingar would erupt. If that had happened we can only hope that CNN would not have translated the name themselves, in the end the name Viagra Hill was suggested to the international press. And yes, the actual name is far worse…
That being that, in the end the bedrock above the intrusion was to solid for the magma and a northward trending dyke formed. After that we have seen between 1 and 4 vigorous earthquake swarms per year as the dyke propagated slowly northwards at an upwards angle. As the dyke arrived at Herðubreið it was at a depth of 7km.
What happened as the magma entered the old system of Herðubreið was so fantastic that it made me do splits while playing lip-banjo out of joy. I have several reasons for my joy. The first one is that something almost magical happened, as the magma entered the old system it started to form earthquake stacks towards the surface indicating reformation of the old volcanic system. This in turn caused the second reason for my joy, science have never before been able to follow in detail how it looks as a long dormant volcanic system comes out of dormancy. And the third reason is that the system must have stayed reasonably active, otherwise the volcanic system would have solidified beyond the ability to form new magma tubes towards the volcanic edifice.
During the last 3 years we have seen earthquake stack after earthquake stack form as the magma has tried to find pathways towards the surface, and every time the magma has come a little bit further up. The image that is starting to build of the volcanic system from these earthquake stacks are intriguing and gives us a very good image of how a reforming volcanic system functions.
The current seismic swarm
It is easy to get waylaid by small and badly located earthquakes. There has been quite a bit of talk about earthquakes at 2km. The reason that people bring up earthquakes at that depth is that it is thought of as the point of no return for buoyancy driven eruptions.
Above that depth the magma will start to rise upwards due to the magma being lighter than the surrounding bedrock due to the heat energy having expanded the magma. This is in most instances quite true, and at depths above that volatiles will start to nucleate out of the magma causing rapidly increasing pressure levels.
Only problem is that there has been few earthquakes at, or above, 2 kilometers.. If you are looking at earthquakes it is good to remember two basic rules. In all Icelandic volcanoes except Katla and Hekla any earthquake of less magnitude than M1.5 will be too small to have been located accurately. So, look at earthquakes above that size and you will get decent depth estimates. Second rule is to wait for the manual check of the earthquakes. The automatic system is giving unreliable depth locations.
If I look at manually corrected earthquakes above M1 there has been a few at 3.1km depth and one M2.1 at 1.2 kilometers depth. This is pretty much to be expected since each earthquake swarm has gotten about 0.5km closer to the surface. At the moment the swarm seems to be dropping in intensity, so it is unlikely that something will happen right now.
I have seen a couple of comments around the internet that this earthquake swarm is not caused by magma, all of them based on the IMO specialist remark about the swarm stating that there is no fluid signature detected. The IMO comment is of course correct, but slightly taken out of context.
The earthquakes are caused by the general pressure that the magma intrusion has built up under the volcano. This causes strain that fractures small rock faults and in time magma will move into them causing renewed stress. But this process does not give the telltale seismic signal of freely moving fluids.
To be able to see small quantities of magma moving distances of a few meters at a time you would need equipment that is both close by and that are applied in large quantities. And that is not the case right now.
Instead you look if the earthquakes form a distinctive earthquake stack that might be a magma conduit, and in this case, we do see one of those forming. In fact, it is a rather handsome little stack.
A peek at future possibilities
The Upptyppingar to Herðubreið intrusion is one of the most well document in the history of mankind. It has given loads of scientific data about reformation of volcanic systems that are long dormant. This wealth of data also gives us the possibility to coax out metadata such as rate of upwards progression per earthquake swarm.
We know for instance that the rate of earthquake swarms is between 1 and 4 swarms per year, and that each swarm averages at 0.5km in upwards motion and that the initial northward motion has stopped in the Herðubreið area.
We also have ample evidence that the volcanic system of Herðubreið is rapidly reforming in the form of sills and dykes with a possible magma reservoir at 7km depth.
We also know that the bulk of intrusions in Iceland putters out before reaching the surface in Iceland. But, in this case the rate and intensity of the earthquake swarms does not indicate that it is about to putter out at Herðubreið.
All of this gives us a logical conclusion based on the evidence at hand. It is currently unlikely that Herðubreið will erupt from this earthquake swarm unless it continues for several more days, or even weeks, with the same amount of vigour. Instead it is likely that the earthquake swarm will slowly loose vigour and putter out. For now.
If we instead project what we know into the near geological future, we see that it would take anything from 1 to 5 earthquake swarms before we reach a point of no return depending on duration and size of the earthquakes in the swarms. And as that point of no return is reached an eruption will occur. So, let us say that an eruption at Herðubreið is likely to occur within 6 months to 4 years at a significant percentage rate of probability.
An upcoming eruption would occur within a few kilometres of the original volcano, or perhaps even up through the original volcano. Unless there is a pocket of evolved magma somewhere down there the eruption would be from a distinct vent and create a spatter cone and freeflowing lava. The eruption could be cyclical like at Krafla or be in one long go like at Holuhráun and the size would probably be somewhere in between those two eruptions.
An upcoming eruption would probably be more scientifically interesting than it would be interesting for the general population, but then on the other hand. People love Icelandic eruptions a lot so I may be wrong about that.
I must though admit that it will be a bit sad watching one of the most beautiful volcanic edifices be destroyed, but mother nature is after all not about aesthetics.