Tag Archives: jökulhlaup

Grímsvötn, Aviation Code Raised to Orange 6th December 2021 (Since Lowered to Yellow)

Good morning!

Update 8 December 2021

The alert level has been lowered to Yellow. To date no eruption has followed the jökulhlaup or increased seismicity. As Grímsvötn may erupt with little warning, the situation remains closely monitored.

Original Post 6 December 2021

Fig 1: Líndal. A. (1 June 2011). Grímsvötn: photo 3 of 14. Retrieved from http://icelandicvolcanoes.is/?volcano=GRV

The alert level for Grímsvötn  has been raised to orange following the draining of the caldera lake over the past few days, the resulting jökulhlaup, and increased seismicity today.

Fig 2: Aviation codes for Icelandic Volcanoes.  Grímsvötn is orange.  Source: IMO

The ice-cap has subsided by c. 77 metres.  The water drained via the Gígjukvísl river; the discharge in Gígjukvísl river yesterday around noon was 2800 m3/s, reducing later in the day.

Fig 3:  Measurements of the jökulhlaup as it impacted the Gígjukvísl river.  Source: Línurit (vedur.is)

At the time of writing, there have been 25 earthquakes in the last 48 hours in the vicinity of Grímsfjall.  A magnitude 2.3 occurred at 06:15, followed shortly afterwards at 06:16 by a magnitude 3.6.  There have been a few aftershocks.  No volcanic tremor has been detected.

Fig 4: Map of earthquakes at Vatnajökull.  Grímsfjall is roughly where the green star is; the green star is the location of the 3.6M earthquake.  Source: Vatnajökull (vedur.is)

Volcanic eruptions have occurred after draining of the caldera lake in the past, attributed to the sudden decrease in pressure destabilising the system; IMO cite the 2004, 1934 and 1922 eruptions.

Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2021


Icelandic Met Office: IMO

Icelandic Volcanoes: Icelandic Volcanoes (icelandicvolcanos.is)

A Quick Update for Grímsvötn: Alert level: Yellow

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.

Fig 1: Eruption column 3 hours after the onset of the 2011 eruption of Grímsvötn. Source: Sigurjónsson, O. (2011 May 21). Grímsvötn: photo 10 of 14. Retrieved from http://icelandicvolcanoes.is/?volcano=GRV

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.

Update (02/10/2020)

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).

For more information, please visit IMO.

The Armchair Volcanologist

30 September 2020

Source and Further Reading:

Icelandic Met Office: https://en.vedur.is/   &  https://en.vedur.is/about-imo/news/the-aviation-color-code-for-grimsvotn-changed-from-green-to-yellow

Almannavarnir: https://www.almannavarnir.is/frettir/

Google Translate: https://translate.google.com/

The Katla Volcanic System, Mýrdalsjökull – the not so cuddly Katla

Good Afternoon!

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). 

Fig 2: Earthquake activity in Mýrdalsjökull plotted by the author for 01/01/2016 to 31/05/2020.  Red dots denote epicentres for earthquakes with magnitude under 3.0; black stars denote earthquake over 3.0; blue triangles are volcanic centres.  © Copyright remains with the author; all rights reserved, 2020.

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 Catalogue of Icelandic Volcanoes (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.

Fig 4: Earthquake activity in Katla plotted by the author.  Green dots denote epicentres for earthquakes with magnitude under 2.0M; orange circles denote earthquakes between 2.0M and 3.0M; yellow stars denote earthquake over 3.0M; blue triangles are volcanic centres; black triangles are local GPS stations.  © Copyright remains with the author; all rights reserved, 2020.
Fig 5: Geodensity plot of earthquake activity in Katla plotted by the author.  Black triangles are GPS stations.  Orange triangle is the volcanic centre. ENTC and AUST are on the caldera rim.  GOLA is near the Goðabunga cryptodome.  © Copyright remains with the author; all rights reserved, 2020.

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. 

Fig 6: Swarms 6 and 7 plotted by the author; dots denote earthquakes less than 3.0M, black stars denote earthquakes of 3.0M or more, black triangles are GPS stations. © Copyright remains with the author; all rights reserved, 2020.
Fig 7: Swarms 4 and 12 plotted by the author; dots denote earthquakes less than 3.0M, black stars denote earthquakes of 3.0M or more, black triangles are GPS stations. © Copyright remains with the author; all rights reserved, 2020.

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.

The Armchair Volcanologist

8 June 2020.


Fig 8:  Summary of swarms identified by the author; Swarm 0 is all earthquakes not associated with a particular swarm. © Copyright remains with the author; all rights reserved, 2020.

Sources and Further Reading

Raw earthquake data: Icelandic Meteorology Office: IMO https://en.vedur.is/earthquakes-and-volcanism/earthquakes

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

Plots are the author’s own work.

© Copyright remains with the author; all rights reserved, 2020