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.
Fig 2: Iceland Total Earthquakes by Region and Yearby the author, using data downloaded from IMO (see Sources below). Note: not all regions
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.
Continuing our theme of seismicity in Iceland, we have now reached the Mýrdalsjökull Region and are heading towards the Fire Districts in the Eastern Volcanic Zone.
We took a slight detour to set up a Glossary to explain some of the terms used here to help out. You can find it on the Menu bar.
Mýrdalsjökull lies at the southern end of the Eastern Volcanic Zone, near its junction with the South Iceland Seismic Zone. The South Iceland Seismic Zone is a transform fault system that links the West and East Volcanic Zones. The Eastern Volcanic Zone accommodates 40 to 100% of the spreading between the North American and the Eurasian Plates; the Western Volcanic Zone takes up the remainder. Active rifting on the Eastern Volcanic Zone terminates at Torfajökull volcano at the rift’s southern end. Katla, Eyjafjallajökull and more southerly volcanoes are on the Eurasian Plate.
Fig 1: Katla 1918 eruption. Image by RicHard-59 Public Domain
We updated our earthquake dataset so we are now looking at the period from 1 January 2016 to 31 May 2020. Apart from the continued swarm on the Reykjanes Peninsula, there has not been any unusual activity (to the untrained eye, at least). We used IMO’s latest earthquake map for Mýrdalsjökull as an indicator for the coordinates to extract the data for the region. This picked up five seismically active volcanic systems (Eyjafjallajökull, Hekla, Katla, Torfajökull and Vatnafjöll) and three inactive areas (Krakatindur, Þórólfsfell and Tindfjallajökull).
Let’s start by taking a closer look at Katla; the other volcanic centres will be covered in later posts.
The Katla Volcanic System
Katla is one of Iceland’s most active volcanoes. The volcanic system is 80 km long, made up of a 30 km wide central volcano and fissure systems. The central volcano has a 10km by 14km wide, 600m to 750m deep caldera with a 5km wide magma reservoir at a depth of 1.5 km. The Katla fissure, Kötlugjá, is located in the caldera. At the north west of the system is the Hólmsá fissure and to the north east, the Eldgjá fissure. There are also inactive fissures to the south. The Mýrdalsjökull ice cap covers most of the central volcano.
Fig 3: Images of the Katla Volcanic System retrieved from the CatalogueofIcelandicVolcanoes (see Sources below) showing the caldera rim, the outline of the central volcano and the north east fissure system. EYJ, TIN, HEK and TOR are other volcanic centres which may be covered in later posts.
Katla’s lavas are basalt/picro basalt, rhyolite and dacite, with a few intermediate hybrids, andesite and basaltic andesite. The basaltic eruptions are the most voluminous , sourced from the mantle via a spreading rift. She has also had many dacite eruptions. Lavas from the Eldgjá fissure are basaltic.
Eruptive History
Volcanism at Mýrdalsjökull began over 800,000 years ago and at Katla, 200,000 years ago. Studies of tephra have identified 200 basaltic and 14 silicic eruptions in the last 8,500 years; unfortunately, no more is known about what happened before the end of the last ice age.
Katla’s largest known eruption was a rhyolitic VEI 6 in 10600 BC which produced more than 10 km3 of rhyolite in pyroclastic flows and airborne tephra that reached 1,300 km from the volcano. The Sólheimar ignimbrite formed from the pyroclastic flows; and the tephra is referred to as the Skógar tephra (Iceland) or Vedde Ash (Norway, after the place where it was discovered).
GVP notes 132 Holocene eruptions for Katla, which range from VEI 3 to VEI 5. All Holocene eruptions occurred in the caldera, except for the 934 AD to 940 AD eruption of the Eldgjá fissure to the north east and the Hólmsá Fires in 6600 BC. Her recent eruptive style tends to be explosive basaltic eruptions from the caldera with tephra volumes up to 2km3, accompanied by jökulhlaups (glacial outburst floods). Water from melting ice cap contributes to the explosivity of the eruptions.
The last eruption to break the ice-cap was a VEI 4 in 1918 which produced an ash column up to 14 km in height, 0.7 km3 of airborne tephra, 1 km3 of debris from jökulhlaups and a small volcanic fissure; no lava emission was reported. The 1625 and 1755 eruptions, both VEI 5s, produced more tephra which reached further than 1,000 km from the central volcano.
The average time between eruptions has been cited as between 40 and 80 years on average. On that basis, Katla is expected to be gearing up for another eruption in the near future.
A period of unrest started in 1999 with a jökulhlaup, seismic tremors, geothermal activity and cauldron formation. There have been more recent subglacial eruptions: a jökulhlaup occurred in 2011, accompanied by a harmonic tremor and the formation of several ice-cauldrons, was thought to be indicative of a sub-glacial eruption; and the most recent jökulhlaup was in 2017, it is not clear if this was accompanied by a harmonic tremor.
Recent Seismicity
We extracted the earthquakes for Katla from the above data set using the coordinates 63.785°N, 19.4987°W to 63.4547°N,18.6608°W. This produced 6,505 earthquakes for the period 1 January 2016 to 31 May 2020.
Our plots show most activity in the caldera, some at the Goðabunga cryptodome and a low level of activity to the south and east of the caldera. Activity in the caldera is fed from a depth of 32km.
So what is causing the hot spots? We looked for earthquake swarms to see whether they are the cause of the hot spots in the geodensity plot. We used the criterion of 30 or more earthquakes in one day or in two consecutive days, which is a higher level of activity than Mýrdalsjökull’s “normal” activity. This showed 16 swarms (ref Appendix), of which five had more than 100 earthquakes; four of the five swarms were clustered (Swarms 4, 6, 7 and 12, shown below) and one was more spread out. Note: our analysis of swarms is not intended to pick up any small magma intrusions that have fewer than 30 earthquakes attached to them.
We see that Swarm 4 and Swarm 12 contributed to the hotter spots in the geodensity plots; the remaining hot spots seem to be caused by an accumulation of activity over four plus years in the data set. Without a clear map of the fissures within the caldera (Googling around did not find one), we cannot tell if the swarms coincide with known fissures. However, swarm 12 coincides with the 2017 jökulhlaup. Most of the swarms in the set appear to be part of the run up to a potential subglacial eruption. It is interesting to note that there do not appear to be any larger swarms after July 2017.
Earthquake swarms are precursors to eruptive activity. Unfortunately, as the last eruption to break through the ice-cap preceded any modern volcano monitoring, there is less certainty over what would precede another subaerial eruption, notably in respect of the intensity of swarms, magnitude of the earthquakes, jökulhlaups, and the time-frames.
It has been a while since Katla produced a large eruption. Let’s hope she sleeps for a bit longer; the world has enough to contend with at the moment.
Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu/
“Iceland”, Thor Thordarson & Armann Hoskuldsson, Classic Geology in Europe 3, Terra Publishing, Third Impression, 2009
“Katla”, Guðrún Larsen and Magnús T. Guðmundsson (2016 March 7). In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=KAT
Fig 3: Map: After Jóhanneson and Saemundsson (1998a), Björnsson et al (2000) and Larsen (2000), Base data, Iceland Geo Survey, IMO, NLSI | Base map: IMO. In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=KAT
“Katla and Eyjafjallajökull Volcanoes”, Erik Sturkell, Páll Einarsson, Freysteinn Sigmundsson, Andy Hooper, Benedikt G. Ófeigsson, Halldór Geirsson and Halldór Olafsson, Developments in Quaternary Sciences, Volume 13, ISSN 1571-0866
An Armchair Volcanologist 
