As I am writing this I can’t help myself from remembering all the stuttering news-anchors trying to pronounce Eyjafjallajökull. Something tells me that they will not have an easier time with Eyjafjarðar at Gjögurtá.
Yesterday I decided to put in a short piece about the earthquake swarm at Eyjafjarðar at Gjögurtá since I noticed how the frequency of the earthquakes abruptly turned from low frequency to high frequency. Normally I never write about the earthquake swarms at the Tjörnes Fracture Zone since the swarms there are purely tectonic almost to a fault (pun intended). Also, they happen so frequently that I would be doing them over and over again. Here is a link to yesterday’s post:
Early in the morning Icelandic Met Office released a bulletin via RÚV1 (Icelandic News) that the earthquake swarm indeed was believed to be magmatic in origin and that it was a probable sign of magma forming at root level depth (10 to 15km), and that it probably is intruding upwards to newly formed fractures.
The Tjörnes Fracture Zone is one of the most tectonically active rift systems in the world, it is continuously suffering from earthquakes, and between 5 and 10 earthquake swarms occur per year (guess why I do not write about them).
It is the area where the MAR leaves the Icelandic sub-aerial part and goes into the Arctic Ocean. To compound the problem it is not only a rift between the North American continental plate and the Eurasian continental plate, the Tjörnes Fracture Zone is a divided rift. And in between the two great rifts we have a micro-plate, a miniscule continent that is neither America, nor Eurasia.
This Micro-continent has been known to move since at least 2008 (when measurements started to be taken on a grander scale), turning in a clock-wise fashion. This movement creates enormous strain and charges the Tjörnes Fracture Zone even more.
Tectonically speaking The Tjörnes Fracture Zone is fully capable of producing earthquakes in excess of 7M, and has done so repeatedly in historical times.
The area is known to contain 4 active volcanoes, The large sub-aerial Theistareykjarbunga Volcano that sits on top of a triple junction where the MAR meets the Tjörnes Fracture Zone, it is also the spot around which the miniscule continent hinges. Out at sea in the fissure swarm of Theistareykjarbunga we find a sub-aquatic volcano. The last two are also sub-aquatic and are located east and north of Grimsey.
Volcanic background of Eyjafjarðar at Gjögurtá
The Eyjafjarðar has step cliffs on both sides. These are remnants of an old large fissure volcano that is believed to have been active before the last ice age. Unlike all the active volcanoes today in Iceland this volcano erupted large flood basalts in layer upon layer constructing a small localized “trap-formation”. In a trap-formation the layers of lava often create stair like steps that is quite visible. The volcano was later ripped apart by the same rift that once fed it with basaltic magma, and is now visible on both the eastern and western side of Eyjafjarðar.
Check out this link to a stunning photograph of the trap-formation:
No activity is known from this long dormant volcano, and no magmatic emplacement is known to have happened before today. The only thing we really know is that once upon a time this system was able to produce flood basalts larger than what has been seen in Iceland after the last ice age. Due to the long dormancy it is highly unlikely that if an eruption occurred it would be even remotely as large as the ones in the olden days.
Rifting Fissure Eruptions
Even though it is still unlikely that an eruption will occur I would like to point towards details that is in common with how we believe a new eruption would occur down at the fissure swarms leading into the area between Bárdarbunga/Grimsvötn and Katla.
A rifting fissure eruption is believed to start with an episode where the crust is pulled apart until it thins out enough for the topmost layer of the mantle to decompress. This will cause magma to acreate in the thinned out area causing additional strain on the thinned out crust. As this process goes on it will reach a point where earthquakes start to take place at the bottom of the crust (and perhaps further up as well). The magma will then start to move up into the newly formed cracks and this will both increase stress loading on the crust, and further decompress the top mantle layer creating more magma. If this process is large enough it will after a while reach a point of no return where magma is explosively formed pushing more and more magma to the top of the fissure. This is believed to be the process behind the Lakí eruption.
If we now make a comparison with what has happened at Eyjafjarðar at Gjögurtá during the last few days. We know that it is an area that is consistent with thinning crust, and this could have caused decompressing melt forming a pool of liquid magma to form in the thinned out part. We know that there was a large amount of deep earthquakes for several days down at the edge between the crust and the upper mantle. We also know that about 36 hours ago the frequency shifted and we started to see brittle earthquakes as magma started to move upwards shattering rock as it went.
The big question is of course if this emplacement will reach the threshold and start a runaway process. Or if it will be contained within the crust forming a dyke emplacement. And, even if it stops today as a dyke emplacement, it could still become a weak point that later on erupts. Or, it could go the same way the sad Geirfugl did and die out in a lonely squawk deep down in the bedrock.
Check out this video by DFM of the earthquake stack as it forms and intensifies: