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Understanding Katla, Iceland’s notorious and restless volcano: Lessons from 1918

What was the 1918 eruption like?

Big, powerful and devastating! Katla is located in South Iceland (see map) under a glacier which is up to 600 m thick; similar conditions to its neighbour Eyjafjallajökull which erupted in 2010 causing wide scale travel chaos by grounding most European and some trans-Atlantic flights (click here to read one of John Stevenson's great blogs about it). However, the eruption of Katla in 1918 was at least 10 times larger and more explosive than the 2010 Eyjafjallajökull eruption. For these reasons Katla is often referred to as Eyjafjallajökull's big sister.


Being a subglacial eruption, there were two main hazards associated with the 1918 eruption: (1) ash and (2) jökulhlaups (glacial floods).



A photograph of the 2010 Eyjafjallajökull eruption taken by Ólafur Sigurjónsson during an aerial flight on 17/05/2010, published on the Icelandic Met Office website which is an awesome, highly informative site. The 1918 Katla eruption was at least 10 times more powerful than this.

Ash production


When magma interacts with water it tends to fragment, often in a violent manner, to create ash. This happened during the 1918 eruption. The ash blasted its way through hundreds of meters of ice to produce a plume 14 km high (almost twice as high as the 2010 Eyjafjallajökull plume). When the ash fell back down to earth, it blanketed an area of 50,000 km2 i.e. half of Iceland.


The jökulhlaup


When an eruption occurs under a glacier it melts a lot of ice. This can lead to flash flooding in the surrounding area as the meltwater escapes. During the 1918 eruption, the flood waters reached a peak discharge rate of 300,000 m3 per second. This is faster than the Amazon, the world's most powerful river. An area 6 times the size of Paris was flooded which devastated several small villages on the south coast. Within the water was a great deal of ash, as well as boulders and icebergs. In fact so much sediment was transported in the 1918 flood that the Icelandic coastline was extended by several kilometres. Over the millennia, jökulhlaup deposits, such as were formed in 1918, have created vast sandur plains over south Iceland (see map).


This very difficult to climb boulder was transported over 10 km by the 1918 Katla flood

An iceberg transported by the 1918 Katla flood



There was very little warning of the Katla 1918 eruption. A large earthquake was felt at 1 pm, 2 hours later there was a plume of ash and a flood. The flood took just two hours to reach its peak discharge. 


Is another eruption likely?


Yes! Katla generally erupts twice per century. In fact since 1625 Katla has erupted at the end of the 2nd and 6th decade of every century (give or take 5 years).... until now. The last eruption of Katla was in 1918 - nearly a hundred years ago. So people have been expecting Katla to erupt since 1960. 


Then in 2010 Eyjafjallajökull went off. Every time Eyjafjallajökull has erupted within historical records an eruption at Katla has followed within a year or two. In 2010 when the eruption of Eyjafjallajökull began to quieten down, the media attention turned to Eyjafjallajökull's big sister - Katla. Now it will surely blow?


But it's now been a few years since the Eyjafjallajökull eruption and Katla has still not produced the eruption everyone expects. Perhaps the pattern has changed? 


Well she may not have blown her top but she's not exactly sleeping soundly. In 1955, 1999 and 2011 small jökulhlaups (glacial floods) emerged from Mýrdalsjökull (the glacier that acts as Katla's sleeping blanket). These could have been caused by small eruptions hidden by the ice or perhaps heightened geothermal activity. Small earthquakes are also relatively common at Katla with occasional earthquake swarms (a large number of rapidly occurring earthquake). These are often indicative of magma movement below ground and there was one associated with the 2011 jökulhlaup. This all suggests that Katla is still fairly active and perhaps getting ready for another eruption in the not so distant future.





The 2011 Katla jökulhlaup destroying part of the main road which loops around Iceland. Photograph by Gisli Olaffson

Earthquakes at Katla between 1999 and 2013. From the Icelandic Met Office website

So what's my role?


I am currently undertaking a 2 year project trying to understand Iceland's notorious and restless volcano, Katla. I have collected samples from the 1918 eruption. I will study these samples to try and answer some important questions about the 1918 eruption. e.g. at what depth was the magma stored prior to eruption? How quickly did the magma rise to the surface? Was it magma-water interaction or gasses within the magma which controlled how explosive the eruption was? By answering these questions, and assuming that the next eruption will behave in a similar way, we can perhaps help to reduce the impact of the eruption. 

If you have any comments or questions, please post them on the bottom of the page or contact me

This webpage summarises the objectives of my AXA funded Post-Doctoral Fellowoship 


Recommended reads:



Larsen, G. (2000) Holocene eruptions within the Katla volcanic system, south Iceland: Characteristics and environmental impact, Jökull, 49, 1-28.


Larsen, G. (2010) 3 Katla: Tephrochronology and Eruption History, in Developments in Quaternary Science, edited by J. K. Anders Schomacker and H. K. Kurt,  pp. 23-49, Elsevier.


Tómasson, H. (1996) The jökulhlaup from Katla in 1918, Annals of Glaciology, 22, 249-254.


Thorarinsson, S. (1960) On the predicting of volcanic eruptions in Iceland, Bulletin of Volcanology, 23(1), 45-52.


Sturkell, E., Einarsson, P., Sigmundsson, F., Hooper, A., Ófeigsson, B.G., Geirsson, H. and Ólafsson H. (2010) 2 Katla and Eyjafjallajökull Volcanoes, , in Developments in Quaternary Science, edited by J. K. Anders Schomacker and H. K. Kurt,  pp. 5-21, Elsevier.


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