We are waiting for developments! Volcanoes can’t be rushed, or forced, and they can sit there seemingly quietly while in reality they are raging on the inside waiting for any trigger that will cause an eruption. Be careful with volcanoes: they will sit there innocently and then suddenly explode in your face. Or not. If there was a geology mental health service, volcanoes would be queueing outside for a bit of support and counselling.
The Reykjanes peninsula was due a significant quake, with many decades of unresolved transform motion. It decided to give way in a segmented way, with several M5 quakes since last year on different parts of the fault. This culminated in a double quake last month. But after this had happened, the earthquakes kept coming. You expect aftershocks, which continue for some days and declining with time. The largest aftershock is typically 1 magnitude lower than the main shock. But this was not like that. The shocks kept coming, in small clusters. This suggests that something more was happening.
The main fault that runs along the Reykjanes peninsula is not a pure transform, but also has a spreading component. That is important. The South Iceland Seismic Zone, further east, is a pure transform fault. It has less volcanic activity. In contrast, the Reykjanes peninsula is packed with volcanic features. In fact, that is how it formed. The peninsula is a long volcanic rift feature, a bit like Puna. The spreading component of the fault allows magma to come up by creating space for it. Pure transform faults, such as the SISZ, are much less efficient for magma transport.
The density of volcanic features shows that the Reykjanes peninsula lacks a central volcano. Features here tend to erupt only once. There is no major magma chamber, just distributed magma pockets with a variety of pathways for heat to come up. The brittle crust in Reykjanes is as thin as 5 km. Reykjanes earthquakes are normally not much deeper than this: they take place in the brittle crust. A guess would be that magma pockets can form near the bottom of this brittle crust.
The magma pockets are fed from the depth. But the peninsula is far from the hot spot in Iceland, and the magma supply is not high. Magma pockets will take a long time to grow to an eruptible size. This is probably the reason for the long periods of quiescence here. The magma pockets do not coalesce into a single chamber, but in a series of smaller ones. Therefore eruptions happen over a range of locations, with many small lava flows rather than one big one.
That is what was happening over the past weeks. The earthquakes created pathways, and some of the magma pockets started moving. Did magma pressure cause the earthquakes? Probably not initially, but the continuation was due to this added pressure, and the moving magma.
Where did the magma move to? There are two important directions. An obvious one is along the main fault. But many of the earthquakes happened on short faults at an angle to the main transform fault. They acted as book shelfs, toppling one after the other. They provide the eventual conduits. These faults run mostly north-south. Further from the fault, they turn northeast, and run along the main Icelandic spreading axis.
Magma moves along the main fault until it feels its way into one of the perpendicular NS fault. Here it will coalesce. It took some time for that to happen, but eventually the magma found the Fagradalsfjall region as the ideal central location. The earthquakes elsewhere died down, but in this area they kept coming.
The strongest earthquakes were at a depth around 5 km where the two faults connected. Magma pushed its way into the perpendicular fault. Now it formed a dike. Dikes are independent structures. They don’t follow a particular fault but take a direction of least resistance. In this particular case, that was northeast, towards Keilir. As the magma reached higher up, the crust breakages were frequent but small. This process is still continuing: the M3 earthquakes are at 5 km, the weaker earthquakes are at a range of depth. (Do be aware that locations of weak earthquakes are often uncertain.)
The current dike is reported to be 1 meter wide and extending up to 2 km depth. The dike follows the perpendicular faults, but at a fairly steep upward angle. The particular fault that is leading the magma is pointing approximately at Keilir. It reached that region, but not much further.
The tremor event a few days put everyone on edge and indicated an eruption was imminent. Even IMO thought so. The tremor was caused by magma flowing along the dike and filling it up. But the magma didn’t cooperate, and it stopped flowing.
In recent days the magma has stopped pressuring the tip and it is now pushing upward closer to Fagradalsfjall. At some point, the dike may find a weakness and comes to the surface. That hasn’t happened yet, but it could happen anywhere between Fagradalsfjall and Keilir. Or it may not happen!
What would it take for Reykjanes to erupt? The region remains primed for an eruption. The earthquakes are continuing. But the main pathway is currently blocked. The likely outcome is still a small eruption, within weeks. This may be triggered by the slow build-up of pressure in the magma chamber reaching some critical point, by an earthquake that breaks the blockage, or by a new inflow of magma into the forming chamber. Renewed tremor will be a sign that this is happening. But if none of these happen, we will be waiting in vain. That is a perfectly possible outcome. Disappointing it would be (for us), but you can’t force volcanoes. Eventually, something will happen somewhere. You just wait. Our sleeping beauty may have gone back to sleep, but one day the prince will come. One day.
Albert, March 2021
Drumplots. Use KRI or ISS for ones close to the activity. (ISS is not the space station (where tremor would not be welcome) but is near the coast, a few km east of Grindavik.)
Some useful Icelandic words which you may come across in names of locations. Accuracy is not guaranteed.
|botn||head of a valley|
|dyngja||dome or shield|
|fell (or fjall)||mountain|