Iceland is filled with active volcanoes and normally we gawk like Auks at the volcanoes that are driven by the combination of hotspot-volcanism and MAR spreading. Especially we “Ooh” and “Aah” at those tormented spots that are hotspot, continental-spread and triple-junction driven at the same time. There is after all a good reason for that, they are stupendously spectacular when they erupt.
By being that narrow-minded we though miss a lot of volcanic candy on Iceland. After all, half of Iceland’s volcanoes are barely or not at all amplified by the hotspot, and they still erupt frequently. To the north the boundary of the hotspot/mantle plume volcanism is at the triple-junction volcano of Theistareykjarbunga, and in the south the boundary is also a triple junction volcano named Hengill. Both of these volcanoes are formidable beasts and have suffered among the largest eruptions in Iceland’s history. We know that they are at the boundary of the effects of the mantle plume since they are the last volcanoes whose lavas contain traces of deep mantle origin, as we go further north of Theistareykjarbunga and south/west of Hengill the traces are rapidly diminish or are completely gone.
One could now say that why bother to look further? North of Theistareykjarbunga there is no land to have volcanoes on, but that is to make it easy for oneself. There are at least 4 submarine volcanoes off the coast to the north, and it gets even more complicated south and west of Hengill.
As we move away from Hengill we find Brennisteinsfjöll, Krísúvik, Svartsengi and Reykjanes volcanoes. The last one is a fissure volcano that is partially sub-aerial and sub-marine as the volcanic fissure zone straddles the SW tip of Iceland.
If we then move out into the water we find a staggering 11 known volcanoes that is counted as part of the mighty Reykjanes Ridge volcanic system. All of these are driven by plate tectonics. As the continental crusts are pulled apart fresh magma is created due to decompression melt and rushes up to fill the void. This is not a continuous process, instead years can go between earthquakes indicating plate separation, and now and then earthquake swarms take place.
These earthquake swarms are always caused by tectonics, but often have at least partial volcanic repercussions. As the plates are pulled apart during an earthquake swarm it is ever so slightly thinned and that causes decompression down at the boundary between the crust and the mantle (the MOHO boundary). When the decompression occurs the melting point is lowered of the semi-solid mantle material, sometimes beyond the solidus (melting) point.
As the thinning of the crust continuous points will form that are thinner than others, and there molten magma will pool together. That pool of magma will then start to work as a permanent weakness in the crust and slowly it widens the part above it more than at other points and sooner or later an earthquake swarm will weaken the layer on top sufficiently to allow magma to push upwards and a magmatic emplacement is formed. Normally you have about 10 emplacements that go nowhere and the magma stays down below the surface. But in the tenth it will break through and you have a volcanic eruption. And during an eruption only ten percent on average will erupt above surface, the rest stays down below inside the crust. So, we humans only ever see 1 percent of all magma that goes up into the crust.
If we move on from Reykjanes Volcano we just have to go a few kilometers and we find Eldey Volcano. And as we pass that barren Gannet-ridden rock and continue another 10 kilometers to the SSW we find the currently recalcitrant volcano of Geirfuglasker.
Not much research has been done on these submarine volcanoes out on the Reykjanes Ridge. There have though been a few scientific expeditions to them, in fact one is underway as I write this. And local fisherman and mariners have reported on any activity seen out there.
There are two recorded volcanic eruptions at Geirfuglasker, the first one happened in 1830 and caused the small rock island of Geirfuglasker to disappear. The second eruption caused the island to reappear, but for some reason nobody seems to have written down which year that happened. One would think that somebody would have written down the year an entire Island surfaced, but no… Probably this is hidden somewhere in an Icelandic paper source that I do not have access to where I write this piece. If you know the year, please tell me in the comment field. There was also a probable eruption somewhere in the eighties since one of the scientific expeditions found a fresh looking lava field.
During all of this commotion a part of the Geirfuglasker volcano remained above the ocean. That was a rock pillar named Geirfugladrangur. It stood about 10 meters above the ocean surface, and the Icelandic government had planned to put a lighthouse on top of it since it was the southernmost sub aerial point of Iceland. In the end it was good that they procrastinated on the decision. On the 22nd of March 1972 the rock pillar crashed into the ocean creating a very hazardous reef.
The volcano suffers more than yearly from persistant earthquake swarms that can contain hundreds of earthquakes per day, and some of the earthquakes can reach amplitudes beyond 4M. Normally these earthquakes are between 9 and 0 kilometers of depth and have purely tectonic signatures. The swarms normally take place under either Geirfuglasker island or under Geirfugladrangur reef.
About one week ago a brief small earthquake swarm took place under Geirfugladrangur; it was unusually short for the location and it also had earthquakes that were deeper than normal. The boundary between the crust and the mantle here is close to the surface, it is believed to be situated at between 9 and 12 kilometers depth, and the earthquake signatures seems to prove a depth of around 10 kilometers here.
On Tuesday this week a larger and more persistant earthquake swarm started consisting of about 100 earthquakes during the initial 6 hours. The most powerful earthquake reached 3.1M. What makes this earthquake swarm stand out is neither the size of the earthquakes nor the duration. It was both small in scale and in duration for being at this locality. No, what makes it stand out is that about 1/3 of the earthquakes (30 to be precise) was deeper than 12 kilometers. Two of those earthquakes was at 22 and 22.4 kilometers respectively and are as such far below the crustal rock layer.
Even though IMO has not given out a report the depth makes it clear that those 30 earthquakes are magmatectonic in nature and that they are a sign of magma production. This does not in and of itself mean that we have an upcoming eruption there. Just that the potential is increasing. It could end up as a magmatic emplacement as a conduit upwards opens up, or that might already have happened.
If an eruption occurs it could be so small, or occur at such depth, that nobody will ever notice when it happens. But, if we are lucky enough we might get a Surtsey like event and an ephemeral island might surface from the ocean floor.
Brief history of the Geirfugl (Great Auk)
The Geirfugl lived at the previous Geirfuglasker, but as the island disappeared in the 1830 eruption most of the poor birds died, only about 30 pairs survived and moved to the Island of Eldey. Sadly Eldey was much easier to climb for humans. Since the Geirfugl was so rare by then the museums of old decided that they should conserve the Geirfugl by having them slaughtered and stuffed for their collections.
On 3rd of July 1844 Jón Brandsson, Sigurður Ísleifsson together with Ketill Ketilsson climbed the rocks of Eldey (on commission by the British Naturalist John Wolley). They only found one grown pair; these were strangled by Jón Brandsson and Sigurður Ísleifsson while Ketill Ketilsson crushed the eggs under his boot. Sigurður Ísleifsson reported to a happy John Wolley:
“The rocks were covered with blackbirds and there were the Geirfugles … They walked slowly. Jón Brandsson crept up with his arms open. The bird that Jón got went into a corner but mine was going to the edge of the cliff. I caught it close to the edge – a precipice many fathoms deep. The black birds were flying off. I took him by the neck and he flapped his wings. He made no cry. I strangled him.”
I find it befitting that the memory of the sad Geirfugl is surviving for eternity as the name of a volcano, the greatest sign of nature’s supreme power.
P.S. And on a personal note, in the future I will be writing considerably less than I have done. The reason is that I am going to become a student once more at the ripe old age of 42. For those who for some unfathomable reason enjoy reading my musings, Wednesdays will be the day I write on in the future. For the rest, fear not, there are as you know many more writers who will fill the pages with quality material. D.S.