Like minds and all, both me and Albert set out to write an update article unbeknownst to each other. I guess that Albert has not yet fully come to grips with me returning back to “life”. But, this is a good thing for you as a reader, you get twice the fun from two different minds as a nice weekend surprise.
Albert is the top part of the article, and I will be down far into the bottom.
Thorbjörn Update by Albert
O what a night.. At the last count, there were 252 earthquake of magnitude 3 or more, the largest at M5.2. That is 252 stars on the map, scattered all over the place as the automatic locator cannot cope with this amount of shaking. It is Christmas come early, with 252 stars of Grindavik. The evacuation of Grindavik adds another semblance of that first Christmas story. But so far, it is a lot of noise but no action. The magma has (so far) remained underground.
To recall, this all started with inflation over an area centred roughly on Thorbjorn, ‘child of Thor’. Thor, according to wikipedia, is a “hammer-wielding god associated with lightning, thunder, storms, sacred groves and trees, strength, the protection of humankind, hallowing, and fertility”. We get our name for Thursday from it. There is certainly a lot of thunder and hallowing going on at the moment – but the protection of humankind has been deferred to the lesser gods of the Icelandic authorities. They are rather good at it and overnight they evacuated both Grindavik and the power plant. Work to protect both against future lava flows has been started, by building embankments of several meters high. It worked for the flows two years ago, although in one case only because the lava did not actually reach the wall. The sign ‘lava – no access’ appeared to be sufficient. The risk, of course, is that a wall may deflect lava onto some other property, raising questions of liability. But not knowing where lava will surface makes planning more difficult.
The following map was uncovered by one of our commenters. It shows how far lava needs to travel to get to Grindavik from various locations. At the moment there is no ‘most likely’ location along the rift. The further north, the better, obviously.
It started mid October when inflation took off. A sill formed with an inflow of 5 m3/s. Over three weeks, that amounts to 10 million m3, or 0.01 km3. It added to previous intrusions which had been happening since 2020.
Why a sill? A sill is a horizontal magma layer, which pushes up the layers above it. This is what is causing the earthquakes and the inflation. The earthquakes happened at 5-6 km depth, suggesting the sill was just below this. How does it form? Magma in rock has to find a weakness, a crack where it can insert itself. This can happen at a place where two different layers connect, for instance where the lava pile which has build Reykjanes lies on top of the oceanic crust below. The two are already a bit separate and magma can prise them apart. This happened over an area of up to a few square kilometer: the sill would have been 10-20 meters thick, in my estimation. There is minor magma around than is in the sill: it has a feeder system below although we don’t seem to know where this feeder is.
Yesterday the sill found another weakness. To move up, it has top break rock and that is not easy in such an old, cold lava pile. So it looks for another connection which goes up. This can be a fault, but in this case it found something else: an old fissure system. The fissure sits on top of an ancient dike, and this dike means there is a vertical discontinuity between the rock and the dike. The first earthquakes along the fissure were several days ago, but yesterday it took off. The magma began to build a new dike along the side of the old one.
The magma moved up by 1-2 km but mostly moved sideways. The dikes here run along the direction of the rift, which in Iceland is SSW-NNE. Because of the spreading, it is easier for magma to insert itself in this direction. And now it went fast. As of this morning, the dike extended over 18 km, from a few kilometer off short to far in-land. That does not mean the magma has traveled all this distance. It means that the old fissure dike is breaking away from the surrounding rock.
So where is the magma? The GPS shows an indications. As this morning, Thorbjorn has sunk by 40 cm. This happened because the dike is forcing the rock apart. That the sinking is so much indicates the magma has shallowed. How shallow? My guess is 2 km but it is only a guess. Both Grindavik and Thorbjorn are moving northwest, but that is harder to interpret. Where would an eruption be? Impossible to tell. It can be anywhere along the line. The highest probability is in the original location which is east of the Blue Lagoon. There is a bit of a gap in the earthquakes and that can be a sign – or not. The second probability is indicated by the deflation – that would put the eruption in the harbour of Grindavik!
RUV put out the following (again taken from a comment)
“The signs of magma movement include significant subsidence in the Sundhnúkur craters, indicating that the magma might be shallow below the surface.
The southern end of the crater row is about 1 km from the nearest buildings in Grindavík and approximately 1500 meters from the Svartsengi power plant.
The Icelandic Meteorological Office notes a substantial change in seismic activity, moving south towards the town of Grindavík. According to the Icelandic Meteorological Office, there is a likelihood that magma movement has extended beneath the town of Grindavík.
The volume of magma involved is considerably larger than seen in the major magma intrusions related to the eruptions at Fagradalsfjall.”
Here is the onset of activity, with M2+ magnitudes twice per minute at one point.
An interferogram (thanks Gaz!)
And here is a list of cameras mentioned by our contributors
Live from Iceland has views all over iceland, including:
Albert, 11 Nov 2023
Grindavik update by Carl
I promised everyone an update today, so here is an update about why I and every other volcanologist should go and drink some tea and ponder why we underestimated a volcanic risk. I will get back to this below.
This will mainly just be an update that I will edit as things unfurl, and I will throw in a little prognosis of what will happen, and where it will happen, based upon the situation at 11.00 Icelandic Time on Saturday.
Roughly two weeks ago Thorbjörn started to show signs of an eruption being possible as a secondary intrusion started, and a sill formed where magma accumulated.
On Sunday evening I felt confident that the prerequisites for an eruption was there based on GPS-data, spectrography over seismic data, earthquake locationing, etcetera.
And based on that data I pointed at a possible location for the eruption to break out, if activity had continued like that I would probably not have been far off, nor would IMO either, we are after all talking about as little as 800 meters between assumed centroids from our individual models.
IMO weighted GPS-data higher than I did, I tend to weight Earthquake-locationing higher. Both methodologies are solidly based on science, and give similar results most often, well similar enough to give an 800-meter difference. Something that in the greater scheme of things is not a lot.
Yesterday at around 12.00 Icelandic time, Iceland decided to kick those models in the proverbial nuts and a dyke started to form that was NNE/SSW trending, putting the most likely location of breakout at Sundhnukagigar, East of Bláa Lonid (Blue Lagoon) and the Svartsengi Powerplant.
This later led to the powerplant evacuating the workers and going into remote controlled operations after some sort of minor accident that was probably caused by strong earthquake activity.
That part of the formative dyke rapidly stalled out, and the bulk of the activity switched down the other dyke leg towards Grindavik.
This forced the evacuation of Grindavik Town itself, a decision that was proven to be a very good idea, but perhaps not for the reason it was based on.
Right now, the dyke is hammering away South of Grindavik in under the sea.
What We Forgot
Let us leave aside the risk for an eruption, and the earthquakes, and even the dyke, and instead look at another side-effect of the earthquakes and the propagating and dilating dyke.
Iceland is truly spectacularly good at moving about, it is extending itself in all sorts of interesting tectonic ways. Faults spread and drift apart, micro-plates dance about and rotate… it is a mess of simultaneous movement.
One of those movements have a scientific name, and that is Graben-formation. It is when due to spreading-movement a lineament of land is dropping down into the void created by the spreading land.
Some Graben like the Eldgjá-Graben form sharp sides, and if you have your house on the exact spot where that side form your house will start to lean, or even fall on its side. But if your house is in the middle of the forming graben you might end up with your house remaining perfectly straight if you are lucky.
This happens in large caldera volcanoes; I saw a house topple over in Amatítlan in Guatemala in a single night from this phenomenon. So, obviously it is well known thing among volcanologists, and something we take into our risk-calculations.
This time though it had an effect that I am pretty certain that nobody had considered, but that we will do in the future when applicable.
Let us phrase it as a question. What happens if a Graben form under a town adjacent to the ocean, and the Graben continues extensionally into said ocean?
Let us begin with numbers as of 11.00 Icelandic time, the Graben has dilated 120cm and the Graben floor has dropped a whopping 96cm (I will edit this as often as I can).
This means that conversely the ocean level has risen permanently 96cm in Grindavik. Obviously, it is not the water level that has risen, it is the town that has dropped.
Grindavik is built so that 96cm is not drowning houses, there is a margin to account for storm surges etcetera, but that margin has shrunk with said 96cm, meaning that storms are more likely to cause flooding along the shoreline and in the port.
Obviously, this is now. If the Graben continue to widen and drop it will get worse.
My favourite Graben in Iceland is Eldgjá, it is 8.5km long, 600 meters wide and 150 meters deep. Obviously the Grindavik-Graben will not become nearly that wide and deep, but a meter or two more depth is obviously not out of the question.
So, there is an idea to look at those Volcanic Hazard Maps and see if another town is at risk of Grabening itself into The Big Fishtank in the Ocean.
Remember that this is based on data at 11.00 on Saturday, so take into account that conditions could change with time, it is a snapshot of “now”, and based on future trends that are realistic and congruent with previous eruptions of similar nature in Iceland.
I am writing the above paragraph so that even Charles Gregory can understand, hopefully, that a prognosis changes over time and with shifting data. I do though not have great hope of that.
Eruptions are statistical games based on data, and the length of a dyke is indicative of where it will erupt. Thorbjörns dyke has two dyke-legs based on the breakout point (feeder), the shorter leg is the one running East of Bláa Lonid, and the longer leg is running out into the ocean south of Grindavik.
On a purely statistical standpoint it is more likely that a longer dyke-leg will encounter a weakness that the magma can use to pop up to the surface.
What we know so far is that the Northern leg up to now remains dry (uneruptive), but there are faults that way that may in turn end turn up to be the weakest spot along the dyke.
There are also faults and weaknesses on the longer Grindavik leg of the dilating dyke, but they are a bit more spaced out. There is though one thing that is probably going to affect things more than everything else.
It is that the ground level is constantly dropping as the Grindavik dyke-leg extends and dip its Grabening foot into the water. This means that the distance a breakout needs to travel to the surface is decreasing since the overburden become increasingly thin.
Let us assume that the top of the sheet-dyke is 2km deep at the breakout point from the sill near Bláa Lonid, as the dyke goes south in Grindavik the height of the overburden drops around the point it hits the water and it slowly get deeper until it hits the continental shelf, and it drops below the top of the dyke. But the continental shelf margin is quite some ways out.
On top of that the Graben itself is fertile ground for an eruption to pop up, and on Graben margins you often get a formation of a crater row, see Eldgjá and Lakí just as a couple of examples.
In other words, based on current data a breakout is most likely from about a kilometre north of Grindavik down to a couple of kilometres out into the ocean.
As magma rushed into the dyke, overall systemic pressure has dropped, so for the moment the pressure is probably somewhat to low for continued lengthwise progress, and to form an upwards directed conduit, but the last is depending on how firm the overburden is and how much resistance it can put up.
The diminishing systemic pressure is why we are seeing a drop in the amount and strength of the earthquakes (11.00), it is though still quite impressive.
As magma continue to enter the system from the bottom feeder conduit from the mantle pressure will though go up, and we will get increased activity again, and probably the expected breakout. This can happen anytime from a few hours, up to a few days.
But, at this point it is pretty much a sure thing that something will pop up.
Here I would like to make a point, normally the initial breakout is 10-20 percent from the furthest point of dyke propagation, it is not clearly understood why really, it might have to do with some sort of fluid-dynamic hammer-effect, but why and how is not well understood as I said.
If that will turn out to be true here, we will see the eruption start outside of the port. It would be a tad ashy as it breaks the surface and “rooster-tails” form, but as soon as it is above the surface ashfall will rapidly decrease.
In other words, do not expect the end of the world, well perhaps the fishermen of Grindavik have a different opinion, for them it might end up as the end of the world.
I have seen people “biggying up” the upcoming eruption to proportions it just can’t take. No, it will not be even remotely near the size of Holuhraun. That being said, judging from magma-influx and previous eruptions a likely figure would be a peak average discharge rate doubling that of Fagradal eruptions.
If memory serves that was 40 cubic meters per second during the fountaining phase of Fagradal II eruption, so perhaps as high as 80 cubic meters per second during the peak hour, and then slowly falling to 10 to 20 cubic meters per second.
If we assume the length of the eruptive episode to be somewhat equal of Fagradal eruptions, we end up with an eruption at around 0.1km3, and that is not a bad figure as sizes go. In other words, it will most likely be a fairly impressive tourist eruption, but nothing more.
If the influx is steady and continue, we could end up with a longer eruption and the amount of magma would slowly climb upwards. At the extreme end of probability, we get a small shield formation that lasts for up to a century, but that is a very low probability indeed.
As a final thing I would mention that I seriously hope that Grindavik will be affected as little as possible, and that the lives of the inhabitants soon can return to normal.
P.S. Charles Gregory, “Told you so”. It is now the third time you have been rude and wrong and gotten “I told you so’d”. I suggest pulling in your attitude and stop trying to bully people. I am definitely unbullyable if nothing else.