The Icelandic Met Office (IMO) has updated the aviation alert for Grímsvötn from green to yellow today (30.09.2020) because the volcano’s activity is above background level, now at a level comparable to that which preceded previous eruptions. They note:
Above average seismicity for September 2020;
Deepening cauldrons in the ice-cap round the caldera from geothermal activity;
Surface deformation exceeding that which preceded the 2011 eruption;
Magmatic gases detected in the summer of 2020.
An eruption is not considered imminent.
Water levels in the sub-glacial lake are high indicating possible jökulhlaups in the coming months. Draining of the lake by a jökulhlaup depressurised the system before the 2011 eruption, so an eruption is considered possible in the event of a jökulhlaup.
Activity may decrease without an eruption in this instance; only time will tell.
Jumping the gun a bit on our next post in the volcanic risk mitigation series, the IMO’s alert is an example of using alert levels to highlight the increased risk of an eruption to those who need to know, without being unduly alarmist – a straightforward statement of the facts supporting the current status. For the exact wording of the alert, please follow the link below.
Grímsvötn is located under the Vatnajökull ice-cap in an active rift zone of the Eastern Volcanic Zone, Iceland. She erupts frequently; her last in 2011 was a large VEI 4, which impacted local farmers and livestock and aviation in Europe.
Googling around a bit more, I note that Iceland’s Department of Civil Protection and Emergency Management, Almannavarnir, have reported in their 25 September 2020 bulletin that an eruption is considered likely this Autumn (use Google Translate or other tool, if you need to, as it is in Icelandic).
In this post we continue our journey round Iceland’s many volcanoes. We have reached the mighty Barðarbunga at the northwest corner of the Vatnajökull icecap.
Barðarbunga volcanic system lies in the Eastern Volcanic Zone, Iceland, near where the head of the mantle plume is thought to be. The system comprises a 2,000 m high central stratovolcano with a 65 km2, 700 m deep caldera, the Veiðivötn fissure swarm running NE to SW, and the Tröllagigar and the Dyngjuháls – Holuhraun fissure swarms running NE; the entire system is c. 190 km long and 25 km wide.
There is second central volcano in the system, Hamarinn, 20 km south west of the Barðarbunga central volcano. Hamarinn may be younger, indicated by the absence of both an intrusive complex and a caldera.
There are geothermal areas near the caldera rim of Barðarbunga and the east of Hamarinn, the latter is the source of jökulhlaups.
The area is tectonically very active: the Eastern Volcanic Belt accommodates much of the separation between the North American and Eurasian Plates. The area is close to the junction with the Northern Volcanic Zone, where Barðarbunga’s neighbours, Askja and Herðubreið, can be found.
According to GVP, there are 55 identifiable Holocene eruptive periods for the Barðarbunga system. Some of the eruptive history has been hidden by the ice-cap. However, lavas and tephra deposits on the ice-free sections of the fissures are more accessible.
Barðarbunga Central Volcano
The central volcano has had around 22 eruptions in the last 1000 years, most occurring between 1200 -1500 and in the 18th century. The last known subaerial eruption was in 1910.
Barðarbunga’s lavas are mainly basalt/ picro basalt. Her eruption types are explosive, phreato-magmatic with jökulhlaups, reflecting the impact of the ice-cap. The central volcano produces eruptions in the order of VEI 3 to 4, producing tephra – both airborne and waterborne. There is a silicic tephra layer in the ice-cap dating to the early 18th century but it is not clear that this came from Barðarbunga. If it did, any rhyolite would have come from partial melting of the basaltic crust.
Magma is sourced from a depth of 10 km or more below the caldera; above this source is an intrusive complex and a lower density region, probably of caldera in-fill. Magma may also be sourced direct from the mantle in the fissure swarms.
Fissure swarm eruptions are basaltic in the order of VEI 1 to 2, with a maximum of VEI 6 on the Veiðivötn fissure.
The last three eruptions on the south west fissure swarm were the VEI 4 at Vatnaöldur in 877, the VEI 6 at Veiðivötn in 1477 and the VEI 2 at Tröllagigar in 1862-1864. The first two of these were explosive tephra eruptions, producing 5 km3 to 10 km3 of tephra and small lava flows. Both the Vatnaöldur and Veiðivötn fissures cut into the Torfajökull volcano, causing it to erupt with silicic tephra and lava. The largest known effusive eruption on the SW fissure swarm is the Great Þjórsá lava which covers 900 km2 and reached the south coast via the Tungnaá and Þjórsá river valleys.
The Gjálp eruption in 1996 occurred on a subglacial fissure that links the Barðarbunga and Grímsvötn volcanic systems. While it is thought that the magma was sourced from beneath Barðarbunga, based on seismic and geodetic data, the magma erupted subaerially was characteristic of Grímsvötn’s lavas.
The frequency of eruptions on the northern fissure swarm is not known; the last eruption was at Holuhraun which started on 29 August 2014 and lasted until February 2015. Precursors to this eruption were a build up of seismic activity at Barðarbunga over seven years, which stopped immediately after the Grímsvötn 2011 eruption but resumed soon afterwards. The largest known effusive fissure eruption north of Vatnajökull is the mid Holocene 15 km3 Trölladyngja lava shield.
Holuhraun Eruption 2014 – 2015
The subaerial eruption started on 29 August 2014 at the Holuhruan vent 45 km NE of the Barðarbunga caldera; the eruption ended on 28 February 2015, having left an 85 km2 lava field and a 65 m deep depression in the Barðarbunga caldera’s ice cover. The eruption was a large SO2 and other volcanic gas producer, however there was little ash or tephra.
The central volcano, Barðarbunga had inflated prior to the eruption then deflated during the eruption as evidenced by subsidence in the ice covering. The volume of the subsidence was consistent with the dyke intrusion and the lava erupted at Holuraun, although there is seismic and geochemical evidence that some of the lava erupted at Holuhraun was fed direct from the mantle. It is estimated that 1.6 km3 lava was erupted.
Since September 2015, seismic and GPS data show that the volcano has started to refill at a depth of 10 to 15 km.
Recent Seismic Activity
We looked at earthquakes in the Barðarbunga, Askja, Herðubreið and Holuhraun area (64.56°N, 17.65°W to 65.3°N, 16.1°W) for the period 1 January 2009 to 28 June 2020. Not much activity had been noted in the area to the west and south west of Barðarbunga in our earlier plots; however, we had noted that heightened activity at Askja and Herðubreið had preceded the 2014 eruption at Holuhraun which lies between the three volcanoes, hence we included them in our plots. The link between the centres is rifting in the crust to accommodate the separation of the North American and Eurasian Plates.
There were 70,128 earthquakes in the period, of which 16,573 occurred before the 2014 -2015 eruption, 19,061 during the eruption and 34,494 post eruption; the average per calendar month was 247 pre eruption, 2,723 during the eruption and 539 post eruption; the maximum magnitude earthquake pre eruption was 3.9 M, during the eruption 5.5 M and 4.9 M post eruption; and, the deepest quakes had respective depths of 33.9 km, 31.0 km and 33.9 km. These numbers include activity at Barðarbunga, itself, the Holuhraun fissure, Askja and Herðubreið. The larger magnitude earthquakes occurred near the north and south caldera rim during the eruption. Since the eruption all four centres have had elevated seismic activity.
Seismicity during the 2014 to 2015 Holuhraun eruption
Three months prior to the eruption, there was an earthquake swarm at Herðubreið, noted here because the rifting event that preceded the Holuhraun eruption occurred on the same plate boundary. Seismic activity at Herðubreið or Askja may be precursors to activity at Vatnajökull, if they, themselves, are not the main event or brewing something. Something to watch out for bearing in mind the recent large swarms in the Tjörnes Fracture Zone and on the Reykjanes Peninsula.
The earthquake plots for August 2014 and November 2014 show the intense swarms from caldera collapse and also the rifting event.
We will look at Askja and Herðubreið in future posts.
“Barðarbunga”, Guðrún Larsen and Magnús T. Guðmundsson (Institute of Earth Sciences – Nordvulk, University of Iceland). 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=BAR
Fig 2: Map. After Björnsson (1988), Gudmundsson and Högnadöttir (2007), Jóhanneson and Saemundsson (1998a & b), Sigurgeirsson et al (2015). 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=BAR
While browsing IMO’s website a few days ago, I saw that signs have been detected that Grímsvötn is getting ready for another eruption, IMO ; a team of scientists noted large sulphur dioxide emissions near the south west caldera rim, indicating that magma is close to the surface. At the time of writing, the alert level for Grímsvötn remains at green.
Grímsvötn is Iceland’s most active volcano, erupting every 10 years and last erupting in 2011 with a VEI 4.
Grímsvötn is one of six active volcanoes under the Vatnajökull ice cap: Bárðarbunga, Kverkfjöll, Grímsvötn, Esufjöll, Þórðarhyrna, Öræfajökull. Apart from Þórðarhyrna (THO in the map below), the other volcanoes are different volcanic systems.
The Vatnajökull volcanoes are part of the Eastern Volcanic Zone in Iceland. Volcanism here is caused by rifting and extension from the separation of the North American and Eurasian Plates. As noted in an earlier post, the Eastern Volcanic Zone accommodates 40 to 100% of the separation.
Our description of the Grímsvötn volcanic system is largely based on Magnús T. Guðmundsson and Guðrún Larsen’s description in Icelandic Volcanoes (ref. Sources below for the full accreditation).
The Grímsvötn volcanic system
The Grímsvötn volcanic system, itself, is made up of two central volcanos and fissure swarms. It is partly covered by ice.
The Central Volcanoes
The Grímsvötn central volcano is a 1722m high, 15-16km diameter caldera complex covered by the Vatnajökull ice-cap, with ice depths of 100m to 700m; she has an 8km by 10km ice-filled caldera. Grímsfall (GFUM) is the highest point on the caldera rim. There is a subglacial lake in the caldera under a 200 – 300m ice shelf with an associated geothermal area. The lake has been the source of many jökulhlaups.
The Þórðarhyrna central volcano, also subglacial, is a 1650 high with a 15 km diameter, connected to Grímsvötn by a subglacial ridge. The volcano, itself, has a small intrusive complex but does not appear to have a large magma reservoir. There is a geothermal area near Pálsfjall.
Ice cover has restricted study of the volcanoes. However, Grímsvötn has been around for long enough to develop a caldera – possibly more than 100,000 years.
Grímsvötn’s lava types are tholeiitic basalt with basaltic andesite and dacite / rhyolitic outcrops in the Þórðarhyrna central volcano. The presence of a shallow magma reservoir is inferred from the geothermal field in the caldera. The 2011 eruption of Grímsvötn produced 0.8km3 basaltic tephra.
Þórðarhyrna is less active than her neighbour; the last eruption occurred in 1903 with a VEI 4. It is possible that she had a second eruption in 1753, resulting in jökulhlaups. Again, ice cover has limited geological study. There is little seismic activity near Þórðarhyrna.
The Fissure Swarms
The fissure swarm is about 100 km long and 18 km wide. Rifting is believed to occur along the entire swarm. The northern end of the fissure swarm is covered by the Vatnajökull ice-cap; the southern 80km is ice-free. Subglacial ridges characterise the northern end of the fissure, but not the ice-free southern end where crater rows delineate the fissure, including the Laki.
Three known subglacial eruptions have occurred since 1867 at Gjálp 10km to 15km north of Grímsvötn, itself. The eruptive products include subglacial ridges and some airborne tephra. The 1996 eruption produced basaltic andesite.
Four effusive eruptions have been identified in the ice-free section of the fissure swarm southwest of Grímsvötn in the last 8,000 years; lava volumes have been between 1 km3 to 14 km3 with up to 0.7km3 of tephra. The largest fissure eruption was the Laki eruption in 1783 to 1784. No eruptions have been identified for the ice-covered section of the fissure swarm.
The Laki Fissure Eruption 1783 -1784
This eruption was well documented at the time; the Reverend Jón Steingrímsson’s 1788 account in “A complete description of the Síða Fires” gives a detailed eye-witness account.
The 1783 eruption occurred on 27km long fissure and lasted from 8 June 1783 to 7 February 1784. The early phase consisted of a series of ten or more explosive tephra events, each followed by effusive lava flows. Grímsvötn, itself, erupted in July 1783 to May 1785 causing ash fall and jökulhlaups.
The Laki eruption was pre-empted by earthquakes of increasing intensity from mid-May to 8 June 1783 when a large ash cloud and ash fall appeared, followed by lava columns over 1km high from new fissure to the north. Volcanic gases filtered out sunlight, making the Sun appear red. Accompanying rainfall was acidic, irritating people’s eyes and skin. Lava flows filled river gorges, overflowing to cover surrounding farmland. During the eruption, Mount Laki was destroyed; I am not sure how big she was and how much her destruction contributed to the vast tephra output.
The eruption is rated a VEI4, having produced 0.7km3 of tephra which covered more than 8,000 km2, and 14 km3 of lava. Volcanic gases, including fluorine, killed more than half of the livestock and the “Haze Famine” killed 20% of the Icelandic population. Further afield, 100 million tonnes of sulphur dioxide, having reached the jet stream, spread acidic sulphate aerosols round the Northern Hemisphere, damaged vegetation and crops in Europe and Alaska, caused severe winters and annual cooling of around 1.3°C that lasted for two to three years.
According to GVP, the Grímsvötn volcanic system has had 86 Holocene eruptions ranging from VEI 0 to VEI 6. The VEI 6 occurred around 10200 BP and is the thought to be the source of the Saksunarvatn Tephra, a basaltic tephra which covered an area of 2 million km2 around the North Atlantic. The Saksunarvatn Tephra, like the Vedde Ash from Hekla, is a geological time marker, although radiocarbon dating of the Saksunarvatn Tephra shows that it may have come from seven eruptive events over a 500 year period from 10400 BP to 9900 BP
Grímsvötn’s most recent eruptions from 1996 to 2011 range from VEI 3 to VEI 4. They were preceded by a small increase in seismicity and small earthquake swarms, except for the 1996 Gjálp eruption. The 1996 eruption was preceded by a 5.4 earthquake on Barðabunga’s northern caldera rim, swarms over a two day period at Barðarbunga’s north and northwest caldera rims and at Grimsvotn’s southern caldera rim, followed by a swarm from the north Bardarbunga caldera rim that migrated to Gjálp.
So, what does our earthquake data set tell us about the likelihood of an eruption at Grímsvötn? The answer is a disappointing “not a lot”. We can see that Grímsvötn has a fairly steady stream of earthquakes but no obvious swarms. However, given the proximity of Grímsvötn to other volcanoes, we may have attributed some of Grímsvötn’s activity to another volcano in error. Plots are shown below, including one for Vatnajökull which shows the problem.
The earthquake plots of the Vatnajökull region show the SW-NE trending fissure swarms and also a SE-NW trending line of earthquakes. The head of the mantle plume is considered to be under the Vatnajökull ice-cap; perhaps we are seeing its influence on the plate junction? We can also see the proximity of Grímsvötn to Bárðarbunga.
The Grímsvötn system, with 3,326 earthquakes, is not the most seismically active volcano; activity is overshadowed by seismic activity at Bárðarbunga (5,464 earthquakes), Askja and Herðubreið (a combined 15,645 earthquakes) and Öræfajökull (4,770 earthquakes). The 2014 eruption of Holuhraun was both preceded and accompanied by intense seismic activity at Bárðarbunga, notably near the edges of the caldera, and deflation at Bárðarbunga. Since the eruption, Bárðarbunga has started to re-inflate. Our data set starts a year or more after the end of that eruption.
Looking more closely at Grímsvötn we see that earthquake activity is focused on the south east of the caldera and at an E-W trending fissure to the north east of the volcano. The E-W fissure is parallel to similar lines of activity further north at Bárðarbunga’s caldera. We also picked up some activity at Þórðarhyrna.
The earthquakes are telling only part of the story. Grímsvötn has had a steady stream of earthquake activity during the period, but without the SO2 measurements from scientists, we would not be certain that magma, itself, was near the surface.
For updates on Grímsvötn, please visit IMO’s website (details below).
The Armchair Volcanologist
22 June 2020
Sources and Further Reading
“Grímsvötn”, Magnús T. Guðmundsson and Guðrún Larsen (Institute of Earth Sciences – Nordvulk, University of Iceland) 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=GRV
“Þórðarhyrna”, Magnús T. Guðmundsson and Guðrún Larsen (Institute of Earth Sciences – Nordvulk, University of Iceland) 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=THO
Fig 2: Map: After Guðmundsson and Miller (1997), Guðmundsson et al (2013a), Jóhannesson and Sæmundsson (1998a), Jóhannesson et al (1990). 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=GRV
Having visited Katla, let’s go further north to the Eastern Volcanic Zone and take a look at Hekla.
Hekla is an active snow-covered elongated stratovolcano, lying at the southern end of the Eastern Volcanic Zone in Iceland in a rift transform junction. Following the 1104 AD eruption, Hekla was called “The Gateway to Hell” – a name that stuck until the 19th century.
The Hekla volcanic system comprises a 1490m high central volcano and a 60 km fissure swarm. The Heklugjá fissure, 5.5 km long cutting across the central volcano, is the site of many eruptions and gives Hekla its elongated shape. The Vatnafjöll fissure system, 40 km long and 9 km wide is considered part of the Hekla volcanic system. Hekla may have a small magma reservoir 4 km below the surface. She has permanent snow cover but no large glacier.
Hekla’s lavas differ from the rift zone volcanoes; her lavas are andesite, basaltic andesite, basalt / picro basalt, rhyolite and dacite. She erupts tephra and silicic to intermediate lavas from the central volcano. Eruptions tend to be a short plinian / subplinian phase followed by lava flows. Larger explosive silicic eruptions have produced enough tephra for the deposits to act as time markers in dating other eruptive activity in Iceland. She is a large fluorine producer which is hazardous to livestock. The hazards listed are tephra fallout, fluorine gas, pyroclastic flows and lava flows; the absence of a large glacier means that jökulhlaups are not a major hazard for Hekla.
The fissure system produces basaltic lavas and a small amount of tephra; its hazards are listed as lava flows and volcanic gas pollution.
According to GVP, Hekla has had 65 Holocene eruptions ranging from VEI 1 to 5. Larsen and Thordarson state that there have been 100 eruptions in the past 9000 years, 23 of which occurred in the last millennium with VEIs ranging from 0 to 5. The central volcano produces eruptions of VEI 2 to 6, VEI 3 to 4 being the most frequent; the longer the repose time between eruptions, the larger the ensuing eruption. The fissure system produces less explosive eruptions (VEI 1 to 2). Hekla’s largest known eruption, a VEI 6, occurred between 3000 to 4300 years ago. Traces of ash from Hekla have been found in Scandinavia, Germany, Ireland and the UK.
Hekla’s eruptive style has changed over time: from effusive basalt 9000 to 7000 years ago; to large explosive silicic eruptions between 7000 to 3000 years ago; and, then smaller more frequent mixed silicic and basaltic eruptions from 3000 years ago to the present day.
The most recent eruption was in 2000, with a VEI 2 to 3, 0.01km3 of airborne tephra 0.01km3 and 0.12km3 of lava; it thought that magma rose through a conduit from a depth of more than 10km to 1km below before heading towards a fissure on the Hekla ridge.
Hekla has an unusually low level of seismic activity. Her largest earthquakes are in the order of 2M when dormant and 3M during an eruption. She does not give much warning of an eruption: known precursors are earthquakes 25 to 90 minutes beforehand. Monitoring has increased since the last eruption so there should be more information about any precursors to future eruptions.
Recent Seismic Activity
From the data set of earthquakes downloaded from IMO’s site for period 1 January 2016 to 31 May 2020, we extracted those for the Hekla – Vatnafjöll area: 63.7578°N, 19.4687°W to 64.0952°N, 19.9399°W. We found 1,018 earthquakes, compared to Katla’s 6,505 for the same period. The largest quake was 2.62 and the deepest 25.11km.
In our latitude v longitude scatter plot we can see that most earthquake activity is scattered along the fissure systems, with an E-W “cluster” to the south of GPS station HEK3, north of Hekla. The latitude v depth plot shows the activity near Vatnafjöll is occurring in the lithosphere, whereas there appears to be a conduit under Hekla. A close up of the cluster confirms this impression.
Our geodensity plot of the earthquakes shows a hot spot south of HEK3, north of Hekla.
We looked for swarms in the data set to see if these account for the hot spot but did not find any large ones; the hot spot appears to be an accumulation of activity over the period. Hekla seems to have a slow magma feed.
“Hekla”, Guðrún Larsen (Institute of Earth Sciences – Nordvulk, University of Iceland) and Thor Thordarson (Faculty of Earth Sciences, University of Iceland). 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=HEK
Fig 2: Map: After Jóhannesson and Einarsson (1992), Jóhannesson and Saemundsson (1998a), Larsen et al (2013a), 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=HEK
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.
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.
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.
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.
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.
“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