Why people are scared of Katla

Note: 02 December 2011.
The current media interest in Katla does not stem from a recent change in activity at the volcano, but from an article published on the BBC website today.  The same thing followed a Guardian article earlier in the year.  Activity at Katla is still elevated, as it has been for six months already.  There is no new evidence today that an eruption is likely very soon.  In geological terms, imminent could mean weeks, months, or maybe years.  This post was written in early November.

Katla rumbles on.

The unrest began in the summer, when a small flood broke out from beneath the glaciers and washed away the bridge over the Múlakvísl river.  Since then, there have been a number of small earthquakes every day.  This is more activity than usual, but less than, for example, occurred at Eyjafjallajökull in the weeks before the 2010 eruption.  It is not clear where this is heading, but Iceland continues to prepare for an eruption and local towns have been running evacuation drills.

The new bridge at Múlakvísl

The remains of the bridge at Múlakvísl that was washed away in the flood of July 2011, with the replacement bridge in the background. Note the yellow earth-moving equipment high on the hillside. They aren't parked up there for the view.

While the international media focus on the potential ash cloud and ‘travel misery’, the real destruction in Iceland will be caused by meltwater floods.  Katla is covered by glaciers up to 500 m thick.  These represent a huge reservoir of water, waiting to be unlocked by the heat of an eruption.  A look at floods from the last time round, in 1918, gives an indication of what could be expected.

The jökulhlaup from the 1918 eruption

There is a word in Icelandic for such floods from beneath the ice: jökulhlaup.  Translated directly into English, it means glacier leap.  This seems appropriate, as the water bursting under, over and through the ice tears off huge chunks and carries them suddenly forward down the valley. When the water subsided after the flood of 1918, the plain was strewn with giant icebergs, up to 60-80 m high.

Eyewitnesses said the speed of the Katla 1918 jökulhlaup was “so great that a fit man could not have avoided it”.  Escaping from the eastern side of the glacier, the flood wave reached the ocean in 45 minutes.  These reports correspond to a speed of 10 metres per second (36 km per hour).  The high-water mark of the jökulhlaup is recorded on the slopes of small hills along the way that were scoured clean by the swirling torrent.  Near the glacier, the waters peaked at 25 metres deep.

Haukur Tómasson combined velocity and the depth of the flood with the shape of the channel and came up with an estimate for the peak discharge of 300,000 cubic metres per second.  The total volume of water was estimated to be around 8 cubic kilometres.  The flooded area was as much as 700 square kilometres.  There was sufficient sediment in the flow to extend the coastline by 5 kilometres.

To put these figures in context, the average discharge of the Mississippi is a relative trickle at 17,000 cubic metres per second!  The flood is equivalent to pouring out Loch Ness onto an area half the size of Greater London in less than 8 hours.

The jökulhlaup from the next eruption

To understand the hazard from future flooding, researchers in Iceland used computer models to simulate what would happen if another Katla 1918-sized flood was to occur at the volcano.  They found that floods would reach the roads to the east of the volcano in 1-1.5 hours after the eruption began; a very short window to get people to safety.  Few people live in this region, but destruction of the road would be a major blow for the tourism-based economy of the area.

Iceland is a country about the size of Ireland.  For 9 months of the year, you cannot cross the middle as the roads are blocked by snow.  Highway 1 is the tarmac ring-road that runs around the country near the coast.  If it is destroyed, it means that you can no longer drive directly from Vík in the south, to Höfn in the south east (271 km, about 3 hours).  Instead, you have to go all the way round the other way (1068 km, about 13 hours).  A severe jökulhlaup could close the road for months.

The models showed that a flood travelling westward would sweep across the farmlands of the Landeyjar district, which is home to around 600 people, within 3-10 hours.  Homes, farms and livestock would be destroyed.  Fortunately, investigation of deposits from past jökulhlaups in the region suggest that this area is flooded rarely, perhaps only once every 500-800 years.


Predicted times for Katla flooding from 1918-sized jökulhaup. Click to enlarge. Source: Gudmundsson, M. T., G. Larsen, Á. Höskuldsson, and Á. G. Gylfason (2008), Volcanic hazards in Iceland, Jökull, 58(58), 251-268.

Summary in a single picture

Why people are scared of KatlaSources: Amazon map – Natural Earth data; Iceland map – atlas.lmi.is; Iceberg image – Larsen, G. (2010), 3 Katla: Tephrochronology and Eruption History.

Categories: Uncategorized


  1. can the blast noise from Katla be heard in Widnes Cheshire UK?

  2. Pingback: Stuff we linked to on Twitter last week | Highly Allochthonous

  3. Roland says:

    Sooo how long ago did Katla last flood west? 800 years ago?

    • According to historic documents, all the jökulhlaups since AD 1179 have gone east, except for 1860, when a small part went south. That isn’t to say that a westward flood is now ‘due’, but it is important to remember that they can happen.

  4. Aidan Karley says:

    To answer the question in your title then, “because it’s a scary thing!”
    I thought that the conventional translation of jokullhlaup was as “glacier vomiting”.

    • Aidan Karley says:

      I’m just having this image of an Icelander on Sauchihall Street, shouting “hlaup! hlaup!”, instead of “hughie! hughie!”
      Well, stranger things have happened. Frequently.

  5. Pingback: Grímsvötn 2011 (Part 1): UK ash deposition from the biggest Icelandic eruption since Katla 1918 | Volcan01010

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