Katla is a large volcano in southeast Iceland, that sits beneath the large glacier known as Myrdalsjökull. For those that remember the eruption that grounded air traffic over Europe (which came from the Eyjafjallajökull volcano), Katla is extremely close in proximity, and tends to erupt on a larger scale than Eyjafjallajökull.
Over the last half year or so (as of 1/23/2017) Katla has been experiencing some levels of unrest, with an uptick in earthquakes and an increase in other signals that are often associated with increased activity. But before we get into the oddities, I want to give a brief overview of Katla.
There has been a lot of speculation over whether Katla is approaching an eruption in the last few years. While volcanoes do not follow schedules, Katla has been highly active, and is one of the most regular volcanoes over the past 10,000 years.
How Often Does Katla Erupt?
Just how regular is Katla? Based on data provided by the Global Volcanism Program (1), we can derive some interesting tidbits:
- Katla has erupted 126 times since 6230 B.C. (although not all of these are fully verified or studied).
- Of all the 126 eruptions that have occurred at Katla, there has never been a period of dormancy that lasted over 260 years in this time frame.
- The average time between eruptions at Katla is 64.7 years
- Only 24.6% of the eruptions that occurred took longer than 100 years before the eruption occurred.
So while we wouldn’t say Katla is overdue for an eruption, we are currently in a significantly longer waiting period than the average time between eruptions.
Current Earthquakes and Activity at Katla
Katla has been having a lot of unrest in the last 6 months. For a short period of time in the late summer / early fall of 2016, some people thought Katla was about to erupt during an intense earthquake swarm that created quite a few quakes larger than 4.0 in short succession.
That swarm died down, but similar swarms have occurred ever since. Prior to 2016, it was somewhat rare to see any earthquakes above 3.0 in magnitude, but more recently, these have become somewhat normal. Additionally, locals have reported smelling sulfur, volcanic gas emissions have grown in local rivers, and there have been minor glacial floods occurring, all of which point to amplified volcanic activity.
Despite the heightened activity, other signs seem to be relatively benign. No extended earthquake swarms have occurred, no harmonic tremor has popped up, and no significant movement in the GPS stations that monitor Katla has been observed.
The earthquakes themselves are also quite a curiosity. During this period of heightened earthquake activity, almost all the swarms have occurred at an extremely shallow depth of .1km as listed by the Icelandic Met Office (2). Katla’s magma chamber already is estimated to be quite shallow, with a bottom only 3km below the surface (3), but that does not explain why these quakes are all occurring at only .1km in depth.
Putting some Pieces Together
Normally, tectonic earthquakes occur deeper than .1km in the crust. Volcanic (or volcano-tectonic) earthquakes tend to occur deeper as well, located in the conduits and faults around the magma chamber and magma feeding system.
With so many shallow earthquakes, you would traditionally assume there is a relation to hydrothermal activity (volcanic activity related to heating in the aquifers near the volcano). Hydrothermal activity has a tendency to be very shallow, and can often come out of nowhere (The tragic Mount Ontake Eruption that killed many hikers in 2014 was a hydrothermal event). Hydrothermal activity can be caused by volcanic activity, as magma rising through the crust can cause explosions and changes in that system, although sometimes changes in the hydrothermal system can occur without any additional volcanic activity. The hydrothermal argument is bolstered when you account for the increased glacial floods as well as the increased gas content in the rivers.
Flaws in the Hydrothermal Argument
If I had to bet, I would still say that the current activity is being caused by hydrothermal activity, but there are some oddities here that don’t make tons of sense to me.
- Earthquake has occurred all around the caldera: The earthquakes have not seemed to follow any pattern in terms of location. There has not really been one singular locus of activity, and magnitude 3+ earthquakes have occurred across different regions of the caldera. This tells me that if there is some sort of non-volcanic hydrothermal changes going on here, those changes must be occurring across a very large portion of the 10km wide caldera+ , which would be very surprising to me. See comparisons from two dates below.
Photo Courtesy of Iceland Met Office: From January 23, 2017
- The activity has not died down: With hydrothermal changes, typically once the system stabilizes, you don’t see any new activity until something causes another shift in the groundwater system beneath the volcano. Since we have been seeing similar quake swarms since August of 2016, it seems strange that this is still occurring. Some argued initially that this could be related to seasonality of Katla, which seems to be more active in the Fall and late summer, but considering that activity is ongoing in the dead of Winter, that no longer seems to be a valid argument.
What to Make of the Activity at Katla?
The whole thing here, is that I really have no confident clue what is going on. I would assume there is something hydrothermally related occurring at the volcano, but it’s truly impossible to discern what is causing this activity. The lack of noticeable GPS movements, tremor, or other signatures makes it tough to understand what is causing the recurrent ultra-shallow swarms and m3+ earthquakes. If there were magma moving into the caldera lid, causing alteration of ground water systems, then there should be additional earthquake signatures at slightly deeper depths than what we’ve been seeing. Also, this would possibly show up as tremor or on the GPS map.
Based simply on past history of Katla, there is a strong likelihood that we will see an eruption within the next 10-20 years, but it’s extremely difficult to say with any confidence whether the current activity could be related to activity that leads towards an eruption. The current activity certainly points towards an increased likelihood of an eruption in the geologically near future, but it’s difficult to say if this is something benign and unrelated to unrest, or if it is a long-term sign of an eruption.
Unfortunately, part of the fun of volcanoes is in trying to figure out how they work and what may happen, almost in the same way that a forensic specialist pieces together the clues of a crime scene. As we can see here however, not all evidence leads to solid conclusions, and sometimes we just scratch our heads at what is happening.