When a year ago Thorbjorn was inflating and seemed at risk of erupting, we put out a post to describe the volcanics of the Reykjanes peninsula. The front picture of the post was of a small cone no one had ever heard of. And now, a year later, that cone is at the centre of attention. Keilir.
Reykjanes is the dark horse of Icelandic volcanism. It is far from the hot spot, and has not erupted since the middle ages. It has been 800 years since the last eruption. It seemed safe enough to build both Iceland’s capital and its major airport here. But there is power hiding here. 3 km3 of holocenic lava is located in between Reykjavik and its airport, Keflavik. I should say that neither of these two are in danger from the current activity. But Reyjavikonians may now find themselves with a front seat to an eruption – and hope that the wind blows the sulfur away from the capital. The prevailing wind here is easterly – putting Reykjavik downwind of Reykjanes.
The Reykjanes Peninsula contains a short section of the main fault/spreading rift that affects Iceland. (This section is called RPTZ on the map above.) It is an unusual section though: it is the only transtensional fault found on land in Iceland. ‘Transtensional’ means that it is a combination of a transform fault and a rift. The spreading rate here is about 8 mm/yr. Many of the quakes of the current series are associated with a section of this fault.
But the Reykjanes peninsula contains many more faults here, which are attached to the main fault but act independently. Some of these have left us fissures, which run to the NNE. The fissures are mostly halyoclasts, which erupted during the ice age. The icecap contained the lava, and the lava formed a ridge rather than a flow.
The fissures indicate three main areas of activity, from west to east Reykjanes, Krísuvík, and Brennisteinsfjöll. (Hengill is part of the wester volcanic zone.) Brennisteinsfjall and Krisuvik are the most productive. Reykjanes contains several sub-centres, including Fragadalsfjall, Svartsengi and Reykjanes.
But there is more. Many small fractures run north-south from the main Reykjanes fault. These NS faults become very long south of Reykjavik, where they can extend as fas as 10 km. An example is the Kongsfell fault, south of Reykjavik (‘K’ on the image above). They are much shorter in the Fagradalsfjall and Krisuvik regions. They are strike-slip faults, and become activated when the main fault moves.
A lot of the current activity is happening at these north-south faults. When one gives, the next one comes under stress and is likely to give as well. Like books on a bookshelf, they topple one after the other.
The largest earthquake of the current series reached M5.7 . That suggests a rupture length of order 3-5 km, based on scaling from quakes in South Iceland, down to a depth of 8 km. A series of parallel NS faults have failed, with a combination of large events and small ones that break a small section/depth of the fault.
The faults are described in Páll Einarsson, et al,
The structure of seismogenic strike-slip faults in the eastern part of the Reykjanes Peninsula Oblique Rift, SW Iceland, Journal of Volcanology and Geothermal Research,
Volume 391, 2020, 106372, https://doi.org/10.1016/j.jvolgeores.2018.04.029. The images above are from that paper.
The faults can provide pathways for magma to come up. The Kongsfell fault is a good example. It is the source of a lava flow that has been dated to the 10th century. It happened shortly after the Settlement ash layer, and occurred on the northern tip of the fault. A number of segments erupted, but only one generated a significant amount of lava. That one was quite significant though, with an 8-km long flow.
This eruption was the start of a 400-year long era of Reykjanes eruptions, from about 940 to 1340. The series began in the east, in the Brennisteinsfjöll volcanic system, with a series of eruptions, a few decades apart and ending before 1200. After 1150 eruptions began further west, in Krisuvik, also lasting until about 1200. Now the Svartsengi and Reykjanes systems joined in with a series of eruptions, from 1211 to 1240.
The most famous eruption of these was in 1226, perhaps starting a few years earlier. It left the tip of the peninsula a volcanic waste land. Icelandic sagas mention ‘fires in the sea’ at the Reykjanes Peninsula for 1223 and again 1225-1227. A violent explosion in 1226, spread 0.1 km3 of tephra across the area, as far as Reykjavik. The sagas mention that it caused ‘darkness in the middle of the day’. The winter of 1226/1227 was called a ‘sand winter’. This word is normally used for the year after an eruption, and indicates the effect of the volcanic ash rather than the ash fall itself. It causes mortality in the farm animals, and famine. Fluorine may be to blame, but there are other possibilities. Ash and tephra are unhealthy for cattle when digested with the gras they cover. Sulfur emissions may also have killed vegetation, leaving insufficient fodder to last the winter. Farming and volcanoes make for an uneasy partnership.
This behaviour of progressive eruptions turned out to be typical. There had been a similar series of eruption 1000 years earlier – and the same another 1000 years before that, and again before that one. In this region, eruptions move from the east to the west over several hundred years, terminate, followed by 700 years of volcanic sleep before it restarts.
But why do the fissures tend to run at a different angle than the NS faults? Strike-slip faults may allow magma to come up, they are not ideal for magma to flow sideways. Magma likes to form dikes. These dikes push themselves along the direction of least stress. The faults carry stress. Least stress is perpendicular to the direction of plate spreading, and the fissures take approximately that direction. The dikes cause eruption within a few kilometers of the main Reykjanes fault. The dikes can actually extend much further, up to 50 km, but those dikes stay underground and do not cause eruptions.
The earliest eruptions, shortly after the ice went, were larger than they are now. As the land rebounded from the ice age 14000 years ago, it may have generated more magma. Large shields formed: the largest one covers over 20 km2. Over time, the eruptions became smaller. But they did not fade quietly into the volcanic night: The thickest tephra layer dates from 6100 BP, up to 1.5 meters thick.
The current earthquake activity has focussed on a surprising area: Keilir. In spite of its anonymity, it is actually a well-known landmark, an easily recognized and widely visible, 378-meter tall cone.
The northwestern part of Reykjanes Peninsula has three main lava shields, from east to west, Hrútagjá, Thráinskjöldur and Sandfellshaed. They are half-shields where the lava flowed (as pahoehoe) north but not south. They summits are some 200 meters above sea level and the slopes are very shallow. Thráinskjöldur covers 20 km2. It is dissected by many faults, including those currently active. Keilir sits on the lava field of Thráinskjöldur.
The three shields formed about 9,000-10,000 years ago. Together they contain 15 km3, which is 75% of all magma erupted in Reykjanes in the past 10,000 years. The magma production was associated with decompression when the icecap melted.
Keilir is sitting on this shield. It is however older than the shield. The cone formed from an eruption underneath the ice cap, which melted through the ice only at the very end. It must have been taller than it is now, when it formed between 10,000 and 100,000 years ago. The shield came later and covered some of it up.
Volcanoes here are monogenetic. It is very unlikely that Keilir will erupt again. But the current activity may result in an eruption in its environment. As shown by Kongsfell, eruptions tend to go for the end point of the fault, and the fault is currently ending just 1-2 kilometers east of Keilir.
Will it erupt? This is the million
dollar kroner question. It is trying hard. But that is never a guarantee. As Carl said, get the popcorn ready. You may need it.
Albert, March 2021