As Þorbjörn rumbles on it seems ever likelier that it will erupt. So, to explain what is happening I decided to pull out all the stops and produce a more definitive article explaining the news and a possible future now that Albert has put things into context.
So, without further ado, I would like to welcome you to a bonanza of all things Þorbjörn.
A brief background of Þorbjörn
First, I would like to explain how Þorbjörn is pronounced for our Irish-speaking friends around the globe. Scandinavian languages after all tend to cough up words that generally causes sprained tongues amongst our linguistically challenged brethren.
Let us begin with the Þ. It is pronounced as in Thing. Problem arrives right after that since Scandinavian languages has two ways of pronouncing the letter O, in this case it is not pronounced as in London, instead it is pronounced in an open manner as in Ogle. We also pronounce the letter Ö in two ways, in this case it is pronounced as in the Irish word Burn. In other words, you just simply say Thorbjurn and remember to look ogly while doing it, and no need to rush the word, remember that Scandinavians are laidback.
Now that we have gotten the pronunciation out of the way we can return to what Þorbjörn is doing, and how it is doing it.
Þorbjörn is located on the Reykjanes Peninsula, and it is only minutely affected by the Icelandic mantleplume. The lavas are close to MORB (Mid-Oceanic Rift Basalt) and contain just a little of the plume-derived basalt that the rest of Iceland contains.
The Reykjanes Peninsula is where the Mid-Atlantic Rift enters into Iceland and contains fairly thin crust. The volcanoes here form volcanic fields more often than central volcanoes, and the eruptions are caused by rifting causing spread-centre melt instead of plume derived intrusions.
These volcanic fields often transect the continental divide and are elongated at an angle across the divide. In other words, welcome to the land of rift-volcanic eruptions.
Most volcanoes are mono-genetic, in other words, they only erupt once. They are also discrete and do not feed from a central magma reservoir. That being said, eruptions still tend to lump together where the resistance is least, like at Þorbjörn.
Þorbjörn so far
The earthquakes are forming a uniform stack from the boundary between the crust and the upper mantle. These earthquakes are evidence that a conduit has formed, in turn this conduit has been filled with magma moving upwards.
There are also earthquakes associated with rifting, these have among other things caused surface cracking in an area north of the Town of Grindavik.
The intruding magma has so far caused a localized uplift of 60mm. If we put everything together we see that there has been an episode of decompression melt and that a volcanic conduit has formed above, and that it is leading to an area very near Þorbjörn, and that about 5 to 2.5km depth a lateral dyke has formed trending towards Grindavik as tectonic earthquakes crack the ground.
Volcanic features like this has a tendency to be cyclical as they evolve towards an eruption. First you get a marked tectonic earthquake swarm, then things calm down a bit and the inflation calms down. This is caused as the tectonic earthquakes form new fertile ground for the magma to move into, and that in turn causes a drop in magmatic pressure which in turn causes a lowering of the upwards component on a GPS.
But, since magma is constantly moving upwards the pressure will after a while increase again, upwards motion increases, and you will start to see more volcano-tectonic earthquakes and charging tremors (tornillos). At this point there will be few purely tectonic earthquakes.
As the pressure reaches the threshold of what the bedrock can take another tectonic swarm will start. And this is exactly what happened in the last couple of weeks. I love Þorbjörn since it is so well behaved for being a volcanic feature.
Last weeks earthquake swarm
Few who are interested in Icelandic volcanism missed that Friday last week was a bit memorable due to a short, but very intense, earthquake swarm that took place. It was followed by a more prolonged less energetic swarm that is still ongoing.
It came after a day of increasing signs of magmatic pressure increase. The Icelandic Met Office has previously stated that the pent-up energy potential in the area is equivalent to a potential M6 earthquake (or two).
So, it is not surprising that we get fairly large earthquakes for the area. I will concentrate on the two largest, an M4.3 and a follow-up M4.0.
Both indicate that they were of the spreading type of earthquakes, so it is safe to assume that they created new fertile ground for magma to enter down the line. Both they also showed an intriguing low frequency signal that was larger than normal for earthquakes of this size, but that is often associated with spreading faults.
Right after the large swarm there was a marked drop on the GPS-system around Þorbjörn. For several days it looked like the volcano was deflating, but that is not actually true, instead it was the magmatic pressure that dropped as magma found new and fertile cracks to gush into. By now the pressure is back up again and Þorbjörn has risen past its former level.
Signs & Portents for the future
Since the crust is shallow it can’t take a lot of inflation and earthquake activity. This is why eruptions in this area are small for being at Iceland. So, I do not see any need for a that many cycles of magmatic charging causing tectonic earthquake swarms.
Question is more what will happen first? The rift runs out of pent up energy and the swarm’s loose power? Or, will the rising magma find a way up to the surface?
Right now, the topmost M2+ earthquakes are located at 2.1km, but with the bulk at 5km. At full regular speed that means that an eruption would be possible at roughly 2 – 15 days. But that is a figure to be taken with a huge pinch of salt and is entirely depending upon if we will see a return to signals indicating increasing magma pressure, and that there is enough tectonic energy remaining.
I do not think we need to worry about the amount of pent up tectonic energy, but the amount of available magma moving upwards is a big unknown in the equation.
In my opinion we are currently hanging evenly balanced on the fulcrum between everything dying out, and an eruption. It takes quite a bit of magma to fill the voids created by larger earthquakes and swarms. If that happens, we most likely will have an eruption, if not things will slow down in the next couple of weeks.
This is where I play the Devil’s Advocate and do ponderous musings about how the eruption would be if it happens. This is entirely dependent upon how the dyke evolves.
If it mainly progresses upwards slightly west of Þorbjörn all is well and there is ample time to take measures to evacuate people if needed, and to close affected infrastructure. This is the best-case eruption scenario.
Problem is that it seems like the dyke is slowly progressing laterally towards Grindavik, as evidenced by the cracks that has formed in the ground. If this is the case, we may end up with a situation similar to the 1973 eruption of Heimaey in the Vestmannaeyjar.
There is though a third option that is so far the least realistic, and that is that the dyke propagates beyond the coastline. This would create a fairly ashy eruption more akin to Surtsey.
If I lived in Grindavik I would be hoping for alternative 1 and 3 instead of option 2 for obvious reasons. After all, lava gushing forth in your basement would be unnerving even for a mentally sturdy Icelander.
Unless something happens this week, or new and improved data arrives, I will leave Þorbjörn be for a little while. You may see this as a sign that we are in for the long-haul if you wish, because that is what I believe.