While we are waiting to see if there is any volcanic activity at Grímsvötn or her neighbours, let’s take a look at how the current seismic activity in Iceland compares to previous years.
Our database, comprised of earthquake data downloaded from IMO (see Sources below), now goes back to 29 December 2008 and extends to 23 October 2020, although we may reupload the last week soon because IMO may have been in the process of updating the record when we downloaded the data.
From our graph, we can see that the total number of earthquakes was up in 2010, 2014, 2017 and 2020. In 2010, Eyjafjallajökull erupted, in 2014, Barðarbunga erupted at the Holuhraun fissure; in 2017, there was no subaerial volcanic activity, although Katla may have had some subglacial activity, and, in 2020, there is rifting both in the Reykjanes Peninsula and Tjörnes Fracture Zone but we have yet to see what, if any, volcanic activity emerges. Interestingly, the 2011 eruption of Grímsvötn did not push the total earthquakes up in 2011.
Let’s take a look at activity by region. Here we look at the four main volcanic regions: the Tjörnes Fracture Zone, Vatnajökull, Myrdalsjökull and the Reykjanes Peninsula, where there is most seismic activity.
We see that the eruption of Eyjafjalljokull increased the number of earthquakes in Myrdalsjökull but not to the extent that the eruption of Barðarbunga at the Holuhraun fissure and subsidence in the caldera pushed up the earthquakes for Vatnajökull in 2014 and subsequent years. In fact, the total number of earthquakes in all regions, except Myrdalsjökull, has remained elevated since Holuhraun.
Our database does not extend back far enough to draw any firm conclusions, however, it would appear that rifting events such as Holuhraun produce far more earthquakes than volcanic activity on its own.
Tjörnes Fracture Zone and on the Reykjanes Peninsula
This rather begs the question as to what is happening this year with the large earthquake swarms both in the Tjörnes Fracture Zone and on the Reykjanes Peninsula. Unless our database does not go back far enough, neither the 2011 Grímsvötn nor the 2014 Barðarbunga eruption was preceded by such large earthquake swarms in other regions.
The fact that there is significantly raised activity this year in both regions suggests to us that we may be witnessing the normal plate separation on the Mid Atlantic Ridge (it is not entirely smooth), possible local magma ascent, and / or the plates accommodating an ascending blob of magma from the mantle plume, which is believed to be under the Vatnajökull icecap. There is ground deformation at several spots under the Vatnajökull icecap which points to magma ascent.
IMO have reported that there could be a magmatic intrusion at depth on the Reykjanes Peninsula; this is supported by ground deformation.
Gas measurements, ground deformation and recent seismic activity at Grímsvötn (not enough to show in our graphs but above the background levels for the volcanic system) have led to the Icelandic authorities to consider that an eruption is possible there in the not too distant future and to raise the alert level a notch (see earlier article).
It is too soon to tell whether or not the rifting events, themselves, will result in eruption(s); it is possible that it is just a coincidence that we are seeing two large rifting events at roughly the same time – both areas are seismically active. Time will tell.
It’s back to Iceland to finish off a post I started before being diverted by the earthquake swarm in Nevada.
Having looked at the recent activity at the Reykjanes Peninsula, let’s now look at the Tjörnes Fracture Zone, where the Mid Atlantic Ridge leaves Iceland to head northwards. Here, current seismic activity is predominantly tectonic. Our study is based on the same data set used for the introduction to Iceland and the Reykjanes Peninsula (earthquake data downloaded from the Icelandic Meteorological Office(1) from January 2016 to April 12, 2020, updated to May 3, 2020).
The Tjörnes Fracture Zone (TFZ) is a complex area of transform and extensional faulting connecting the Kolbeinsey Ridge, the Western Volcanic Zone and the Northern Volcanic Zone. The Kolbeinsey Ridge, itself, is slow spreading at a rate of 10mm per year. The main faults in the area are: the EyjaFjarðaráll Rift, the Húsavík-Flatey Fault (the TFZ, itself), the Grímsey Oblique Rift and the Dalvik Fault. Both hydrothermal and seismic activity cluster on the faults. The Húsavík-Flatey Fault has produced earthquakes with magnitudes in the region of 7.0.
Grímsey is an inhabited island on the Arctic Circle. Its main industries are fishing and tourism(2).
Flatey is a small island in Skjálfandi Bay in northern Iceland. It is inhabited in the summer for the tourist season, being home to puffins, terns whimbrels and plovers, amongst others(3).
According to GVP(4) a submarine eruption occurred in 1868 on the Manareyjar Ridge, north of Manareyjar Island, at the south eastern end of the system; the lavas were basalt / picro basalt. A submarine eruption or dyke intrusion in 1999 caused an earthquake swarm 180km north of Grimsey and 100km north of Kolbeinsey Island on the Southern Kolbeinsey Ridge. Volcanic activity occurred in 1372 and 1755, but its whereabouts is unclear.
In the period from January 2016 to May 3, 2020, there were 26,762 earthquakes reported by the Icelandic Meteorological Office (IMO)(2) for the region. 131 earthquakes had a magnitude of 3.0 or more; 67 occurred in month 26 (February 2018) on the Skajálfandadujúp Rift, 52 miles ENE of Grímsey, the largest of which was 5.21M.
A depth plot of the February 2018 swarm shows that most earthquakes over 3.0M occur in the lithosphere.
According to IMO, these swarms have occurred before; the most recent being in May & September 1969, December 1980, September 1988 and April 2013. The data for most of the earlier swarms is not publicly available on IMO’s website, but we can get data for the April 2013 swarm. In that swarm, there were 84 earthquakes with a magnitude of 3 or more; the largest of which had a magnitude of 5.37.
A depth plot of this swarm shows that most earthquakes over 3.0M also occurred in the lithosphere.
How does this compare to the activity on the Reykjanes Peninsula?
In the Tjörnes Fracture Zone, most seismic activity is occurring in the lithosphere. There is no reported volcanic activity associated with the two swarms we looked.
Apart from the recent large swarm, the Reykjanes Peninsula shows much less activity in the same period; again, most activity was in the lithosphere. The recent swarm, itself, was atypical (still ongoing at the time of writing, but at a reduced rate) and accompanied by ground uplift – hence the increased monitoring put in place there.
I am not Icelandic so apologies for any typos in Icelandic names.
There has been a large earthquake swarm on the Reykjanes Peninsula over recent months, still ongoing at the time of writing, albeit with reduced intensity.
So let’s take a look at what’s been going on.
The Reykjanes Peninsula lies at the south west tip of Iceland on the Mid Atlantic Ridge, the boundary between the North American and Eurasian Plates. The North American Plate is moving westwards in relation to the Eurasian Plate; transform and extension faulting accommodate the relative Plate motions. The Reykjanes Volcanic Belt lies on the Reykjanes Peninsula, comprising five north east trending volcanic systems: Reykjanes, Svartsengi, Krýsuvík, Brennisteinsfjöll and Hengill. The volcanic systems are fissure swarms.
The line of earthquake epicentres in Fig 1 shows the path of the Mid Atlantic Ridge. These earthquakes were extracted from the data set used to generate the plots in the earlier post introducing Iceland. The raw earthquake data is publicly available data downloaded from the Icelandic Meteorological Office for the period January 2016 to 12 April 2020. The plot above is still data-heavy (too much data to see what is happening) so I have extracted the earthquakes by month to see where and when most activity occurred.
In the plots there is an impression of seismic activity trending along the Peninsula from the east to the west. This impression is born out most strongly in the activity from month 48 onwards. Although it should be noted that the level of activity for these months is unusually high.
There are several possible reasons for the east to west trending of the earthquake swarms:
the plates do not move smoothly past each other, so friction generates faults and earthquakes;
rifting is occurring to accommodate the upward motion of land further to the east, generated by the mantle plume in the vicinity of Vatnajökull; and /or,
magma intrusion in local volcanoes.
Let’s take a look at depth plots of earthquakes under the Reykjanes / Svartsengi area. This shows that the earthquakes over 3 M are largely in the lithosphere.
Plotting depth against longitude, effectively looking northwards through the swarms, also shows that most larger quakes are in the lithosphere but some over 2 M track towards the surface. On its own, this is not enough to draw any conclusions over the likelihood of an eruption; field observations, including gas emissions and ground deformation are required to determine how close to the surface magma may be.
These swarms were accompanied by local uplift, as shown by local GPS stations.
IMO(3) has reported that the most likely explanation for the recent swarms and uplift is a magma intrusion near Mt Thorbjörn at depth; Mt Thorbjorn is located near the Blue Lagoon, Svartsengi. As to whether or not magma will reach the surface for an eruption and where it emerges, we will have to wait and see. In the meantime, let’s look at the recent activity in historical times.
Historic Volcanic Activity
As noted earlier, the volcanic systems are fissure swarms. Activity is driven by rifting which enables magma to reach the surface. The most recent onshore volcanic activity took place between 940 AD and 1340 AD; later activity has been offshore. Onshore lavas from these eruptions tend to be tholeiitic basalts (1). The systems are still active as demonstrated by current geothermal activity.
The Hengill volcanic system is a series of fissure vents, crater rows and small shield volcanoes, with a highest point of 803m. It lies at the triple junction of the Reykjanes Peninsula volcanic zone, the Western volcanic zone and the South Iceland seismic zone. The lava types are basalt / picro-basalt, andesite / basaltic andesite and rhyolite. Hengill’s lavas are more complex that those to the west, reflecting its position at the triple junction. GVP(2) lists 13 Holocene eruptions ranging between VEI 0 and VEI 2; the last known eruption was a VEI 2 in 150 AD.
The Brennisteinsfjöll volcanic system is a series of crater rows and small shield volcanoes, with a highest point of 610m. Its lava types are basalt / picro-basalt. GVP(2) lists 9 Holocene eruptions ranging between VEI 0 and VEI 2. One eruption, previously attributed to Hengill, occurred during a meeting of the Icelandic parliament at Thingvellier in 1000 AD. The most recent eruption was a VEI 2 in 1341.
The Krýsuvík volcanic system is a series of crater rows and small shield volcanoes, with a highest point of 360m. Like Brennisteinsfjöll, its lava types are basalt / picro-basalt. GVP(2) lists 11 Holocene eruptions ranging from VEI 0 to VEI 2; the most recent of which was in 1340. The Krýsuvík Fires spanned a period between 1151 and 1188, producing 36 km2 of lava.
Reykjanes & Svartsengi
The Reykjanes volcanic system is a series of crater rows and small shield volcanoes, which extends offshore and includes several small islands. Reykjanes highest point is 140m. The Reykjaneshryggur volcanic system is a submarine system which is considered part of Reykjanes. GVP includes Svartsengi as a crater row of Reykjanes. Reykjanes lava types are basalt / picro-basalt. GVP(2) lists 22 Holocene eruptions ranging between VEI 0 and VEI 4, of which the most recent was a VEI 0 in 1970; the VEI 4 was in 1226. The 1226 eruption was part of the Reykjanes Fires which started in 1210 and lasted until 1240.
The area is one that has had a period of intense volcanic activity, so the recent earthquake swarms have generated a lot of interest.
Regular updates on seismic and volcanic activity in Iceland are published by the Icelandic Meteorological Office(3).
The Armchair Volcanologist
14 May 2020
References & Further Reading:
David W Peate, Joel A. Baker, Sveinn P. Jakobsson, Tod E. Waight, Adam J. R. Kent, Nathalie V. Grassineau, Anna Cecile Skovgaard , 2009. “Historic Magmatism on the Reykjanes Peninsula”, Contrib Mineral Petrol (2009) 157:359-382
This is a test post as a starter for my blog. I hope you find this and later posts interesting.
Iceland, home to Eyjafjalljökull and Grimsvötn, whose eruptions in 2010 and 2011, resp., disrupted European airspace, is recommended as a great place to study volcanology, with many types of volcanic activity and relatively easily accessible. It is also a great holiday destination, not that I have been privileged enough to visit.
Iceland, located between 67.2°N 23.0°W and 63.0°N 13.0°W, has an area of 103,000 km2 and a population of 364,000. Lying on both the North American Plate and the Eurasian Plate, it is the only large surface expression of the MAR where its volcanic activity can be easily studied by field volcanologists. Iceland, itself, was formed from magma and accretion.
The Mid Atlantic Ridge in the northern hemisphere is the boundary the North American Plate and the Eurasian Plates. As the plates separate, the ridge widens and allows rising magma to come to the surface. The magma rises under its own buoyancy; hotter magma is less dense than the colder surrounding rock.
In addition to the MAR, the Icelandic Hotspot, a mantle plume, contributes to both plate separation and volcanic activity. The Iceland Plateau, itself, is a large basaltic igneous province. The plume head is thought to be located in the region of the Vatnajökull icecap.
The MAR crosses Iceland in a series of transform and extensional faults, starting at the Reykjanes Peninsula in the south west to the Tjörnes Fracture Zone in the north. Plotting the earthquakes reported by the Icelandic Meteorological Office (IMO) from January 2016 to 12 April 2020 shows the path of the MAR.
If we take a look at a three-dimensional plot of the same earthquake looking from the south, we can see that there are deeper earthquakes under the volcanic regions Myrdalsjökull and Vatnajökull.
A geodensity plot of the same earthquakes, weighted by magnitude, shows most activity (yellow areas) at the western end of the Reykjanes Peninsula and on the Tjörnes Fracture Zone to the north. Other areas of interest are Myrdalsjökull (Katla), Vatnajökull (Barðabunga) and Herðubreið.
Iceland is often touted as having every form of volcanism going but it does not have an obvious active subduction zone; eruptions tend to be associated with fissures. Not to worry, Iceland has many active volcanoes, including: Askja, Bárðabunga, Eyjafjalljökull, Katla, Grimsvötn, Hekla and Surtsey – plenty to offer us in terms of volcanic and seismic activity.
Our plot is data-heavy (in the region of 104,000 earthquakes) so we cannot see much more without further analysis. We will look at each volcanic region in turn in later posts.
Thank you for reading this and I look forward to sharing the next post with you.