A re-post of a Carl-special. With current reports of increasing activity on the Reykjanes peninsula, after 800 years of quiet, this is a post well worth recalling. And remember that in the few hundred years before the current calm, roughly 900-1300, every major volcano on the peninsula erupted. These were amazing years which also included Eldgja. Now the peninsula is a UNESCO global geopark with 55 separate geosites. Let’s see why.
This time around I am not going to write about a single volcano, instead I am going to write about one of the most interesting corners of Iceland, namely the volcanism on Reykjanes Peninsula and Reykjanes Ridge.
Here at Volcanocafé we regularly write about two types of volcanism, subduction driven volcanism or hotspot driven mantleplume volcanism. Rarely do we write about the third type of volcanism, namely plate-tectonic rift volcanism. In a way we should write more about it since it stands for more than half of all volcanism on the planet, but then on the other side, it rarely is noticed since 99.9 percent of it takes place under the ocean at such depths that you need a hydrophone to even notice it. Only places where you can find it above the surface is at the topmost part of the Great Rift volcanism, or in Iceland.
As we leave the triple-junction volcano of Hengill behind us we venture beyond the volcanism that is driven by the Icelandic hotspot. Instead the volcanoes from here on is driven by the spreading of the Mid Atlantic Rift (MAR) only. As the American and Eurasian continents are drawn apart the pressure is lowered at the top of the mantle and mobile magma is released due to decompression melt. That magma in turn moves into the thinning crust, often at the latter end of an earthquake swarm.
Normally we spend a lot of time trying to understand if an earthquake swarm is tectonic or magmatic, but here the swarms often go rather seamlessly from a tectonic entry-stage to a magmatic end stage. Often these types of swarms are called magma-tectonic just to lessen the confusion. Most of these swarms never amount to anything, but on rare occasions swarms happen at the same place and magma start to build up in the thin layer of crust and at even rarer occasions you get an eruption. But one thing is certain; sooner or later you will have an eruption at pretty much every point along the rift zone.
In no place in Iceland is the question about what is a central volcano as infected as here, and to be honest, volcanoes worthy of the name central volcano is far in between, but they do exist if you look for permanent magma reservoirs. Problem is just to know if one exists or not. I will try my utmost to be clear when we know or not.
The rest of the volcanism is from rift swarms, these are permanent weaknesses that lack a permanent known magma reservoir, but they have suffered from repeated eruptions. Now we can start our tour.
Between 875AD and 1371AD Brennisteinsfjöll suffered from 6 eruptions, or on average 1 every 78 years. Before that the volcano had mainly been inactive for a period of 2 000 years. The eruptions from Brennisteinsfjöll are mainly effusive lava floods with a small explosive component caused by access to water.
The magma from Brennisteinsfjöll is high in sulphuric content giving the mountain its name, in translation to English it would be Mount Brimstone. So expect any eruption from here to be slightly on the stinky side. During the 1 000AD eruption the stench was so bad that it disrupted the parliament that was held outdoors in Thingvellir.
Currently the seismic activity is low at Brennisteinsfjöll, but the area is by no means quiet, it just lacks massive earthquake swarms indicating magma-tectonic filling of the volcano. Brennisteinsfjöll is believed to hold an independent magma reservoir.
The central volcano of Krísúvik is heavily tectonically active, and has also suffered from a recent long term magmatic intrusion at depth. The volcano has a large field filled with solfataras, fumaroles, boiling mud pots and hot springs. There are also large sulphur deposits that previously was mined.
Due to the volcano running in under Kleifarvatn Lake the area is prone to suffer from explosive Maar-formations and the area is also very hydrothermally active.
After the 2 000 earthquake cracks opened up at the bottom of the lake and the water level dropped so much that more than 20 percent of the surface area disappeared, in 2008 the water level had returned to its previous level.
The water level though dropped again when an inflation period started in May 2010 that lasted all the way to April 2012. During this time new hot springs came to the surface as the water level dropped and scientists recorded a lot of other signals associated with a volcano nearing an eruption.
But in May 2012 everything calmed down and the volcano started to slowly deflate and the risk for an eruption diminished. One should though recognize the very tangible risk for a new intrusion happening, which might set off the volcano in a fairly limited amount of time.
Krísúvik might be seen as a bimodal volcano, in and of itself it is not explosive at all. But if you factor in the ready access to water and previous history of Maar-forming detonations, well then you have a high risk for explosive bursts during an otherwise mild effusive eruption, so it might be wise to keep a good distance from the volcano when she erupts. The volcano is last believed to have erupted in the year 1340.
To the west of Krísúvik you have at least 3 rows of rift swarms running at an angle to the MAR, they are all oriented in a NE direction to the EW direction of the MAR. These volcanic rifts are ill researched, but at least one of them is believed to have erupted during the last 1 000 years, probably around the year 1200. There is nothing pointing towards these rifts having an independent permanent magma reservoir, so as such they are not volcanoes in their own right even though they have erupted more than once.
Up until recently it was hotly debated if the heat drawn by the Svartsengi Hydrothermal Power Plant came from an independent magma reservoir, or if it was fueled by residual heat from the 1226 Illahraun eruption from Svartsengi. Nature though tore that dispute rather decisively in early October of 2013 when a rather dramatic intrusion occurred in the volcano.
The swarm started as a regular tectonic swarm but rapidly became magma-tectonic as new magma rushed in to fill the newly created empty spaces. In effect, the initial earthquakes transformed Svartsengi into a vacuum syringe pulling in the magma as it opened bottom up.
Svartsengi is often in the literature mixed up with the Reykjanes volcano, but it does not in any way share the same magmatic reservoir and is as such different volcanoes. An eruption from Svartsengi would mainly be effusive in character.
The real Reykjanes Volcano might be the smallest volcanic feature of them all, it last erupted in 1 211AD in an eruption that transected both land and water, and as such it was the largest explosive eruption in both the Reykjanes Peninsula and Reykjanes Ridge areas. Called the R-7 Tephra it ranked in at a respectable VEI-4.
There is no sign of an independent stable magma reservoir under the volcano and it is doubtful that it is a central volcano, but an eruption here would be a danger to anyone living close by due to the explosive nature caused by the readily available water.
Eldey and Eldeyarbodi
It is not known if this is one volcano, or if these are two different adjacent volcanoes. I am here going to avoid that discussion and leave that for others to decide. Eldey had its last eruption in the beginning of June 1926, it was a small eruption not assigned any VEI-number.
The last eruption at Eldeyarbodi was in March 1830 and ranks in at a respectable VEI-3. This eruption caused the start of the end of the Great Auk as the entire Eldey and Eldeyarbodi colony moved to Geirfugl where they dwindled rapidly until the last couple was strangled and the eggs stomped asunder.
The area is highly tectonically active with frequent recurring earthquake swarms. The area is believed to house a permanent stable pocket of magma, and from time to time ephemeral islands have sprouted forth. There is also a trace of young magma that has been sampled from the ocean floor pointing to volcanic activity as late as in the 1980s.
In May 1783 an island was born near Eldey, it was christened into Nyey (new island), but it had a short lifespan and rapidly sank due to the harsh wave conditions in the area.
Geirfuglasker and Geirfugladrangur
The remote rock of Geirfuglasker has come and gone during the centuries and eruptions have taken place here at numerous events. The same goes for this area as for Eldey, we do not know if this is a larger volcano with 2 individual peaks, or if it is 2 separate small volcanoes.
The last eruption at Geirfugladrangur took place in May 1879 and the last at Geirfuglasker in 1422. But there is good reason to believe that those eruptions are just a tip of the iceberg and that several small eruptions have taken place later on.
The area is today wrought with intense earthquake swarms and it is not hard to believe that sooner or later a new eruption will take place here. It is quite likely that there is a permanent magma reservoir here, but there is no conclusive evidence for that.
There are more volcanic features out in the waters of Reykjanesskaggi, but these are most likely ordinary volcanic rift zone and not individual volcanoes. At least 5 other volcanic features are known to lurk in the waters, but until more research is done we can leave that be.
For the Reykjanes Peninsula we can see that the volcanism tends to be happening in groupings where the region is active for about 500 years and then goes silent for about 1 000 years. This is most likely due the tension buildup being prolonged here and that it takes time for the tension to become strong enough to thin out the crust sufficiently for a rifting episode to start.
There is currently nothing pointing towards such a rifting period being close, but then on the other hand we have seen intrusions happening at two well known volcanoes. Only problem is that we do not know the complete track-record for the area, these intrusions might after all happen every 50 or so years without any eruption happening.
When an eruption happens in the area it will be effusive unless it happens at Krýsúvik or Reykjanes Volcano, there the ready access to water could lead to a severe explosive event. In the end though even an effusive eruption would cut the power from Svartsengi Power Plant and cut the main road to the Keflavik Airport, but Iceland has abundant electricity and even emergency airports that could take over the slack.
In the end the place to expect an eruption is out in the water, and in all likelihood we will see one sooner rather than later. And, in the end isn’t the idea of a brand new Island shooting out of the water outside of Iceland what all of us dream of?