On the night of 23/24 July 2014 (around midnight) there was a large landslide in the SE corner of the steep inner wall of the 1875 AD caldera at the Askja volcano in central Iceland. This event is simply the latest (albeit large and spectacular) of many that have formed the current water-filled caldera of Öskjuvatn (Askja lake). It is part of the ongoing process of the formation of this youngest caldera at Askja, which is after all only 139 years old and which after its initiation in 1875 took several decades (until c.1932) to get close to the shape we see today.
This blog post contains images from before and after the landslide. I was fortunate to be doing fieldwork nearby (collecting samples from basalt subglacial mountains) when I heard that access to the ‘safe’ area above the lake had just been granted. So we went there on the first day that the area had been opened since the landslide. It was a bit special.
A. Taken in July 2011 – the site of the July 2014 landslide.
Estimates of the volume of the landslide range from 24-60 million cubic metres, and no doubt this will become refined as Icelandic scientists either gain access to the area or use digital elevation models to obtain more precise measurements.
The main hazard from the landslide was not the slide itself, as this occurred in a location well away from tourist trails. This location is visited only rarely by geologists utilising the superb exposures revealed by the caldera collapse to gain deeper insight into Askja’s past geological evolution. See Graettinger et al., 2013.
Nope, the main hazard from the landslide was the wave triggered by the sudden entry into the lake of a large mass of debris. Various people have called it a tsunami, a displacement wave, and a seiche. Tsunami will do, and estimates place it as 60-75 m high when it reached the opposite caldera wall there the vast majority of tourists gather to gaze over the lake and into the small crater of Víti (Hell) filled with turquoise coloured, warm, and sulphurous water. Fortunately nobody was in Víti at the time or they’d have had a shock (and an unwelcome cold shower) as the top of the tsunami wave spilled into Víti.
The image below shows the raft of rhyolitic pumice and ice that remains after the landslide, with the source of the pumice being loose and unconsolidated deposits from the 1875 eruption. It will be interesting to see how long this raft persists, as the strong winds of Autumn and Winter will deposit much of the material on the eastern shore.
Images from the landslide source – 2010 and 2011
In 2010 and 2011 I was co-supervising a PhD student who was mapping the older rocks that lie on the east and south of the young 1875 AD caldera right down to the lower outer flanks of the volcano. I also visited the southeast corner with an Earthwatch group in 1985 and made the surprise discovery that there was an old rhyolite dome here, which I confirmed with a chemical analysis. It was apparent that the area around the top of the rhyolite dome and to the west was unstable and that a fault system had been active given that parts of the rhyolite dome had moved downslope and been rotated to dip 5-15 degrees to the south.
To be honest, on the past occasions I was above the headwall of where the July 2014 landslide occurred (i.e. in 1985, 1987, 2010, and 2011) I was aware of the potential for a landslide in this area, but from the evidence I could see of other landslides (especially the older one immediately to the east) it looked like any future landslide may be a gentle slump rather than a headlong dash into Askja lake.
Well the eastern fringes of where the July 2014 landslide occurred formed a convenient way up to the top in this area, but this has now gone. And the landslide has covered over more (if not all) of what was a poorly exposed 1920s basalt lava. The debris dumped onto the lovely little 1920s basalt lava of Bátshraun (0robably 1921) will have covered some of the exposures I was working on – which provide evidence of lava-ice/water interactions at the time of its eruption.
Working in this area one is aware of the regular small rockfalls from the steep north-facing wall of the 1875 AD caldera, and of the larger slumps that have taken place. As mentioned above I was surprised at the rapid displacement of lake water that let to such a dramatic tsunami wave being formed, but then I’m a volcanologist and not a landslide expert.
No doubt landslide experts will evaluate the potential for additional landslides from the zones adjacent to the July 2014 headwall, as these may have been weakened and potentially be ready to go. However these zones appear fairly small in comparison to the estimated c.800-900 m length of caldera wall that collapsed on 23/24 July.
There will be further landslides at Askja simply because the 1875 caldera is still ‘settling’ and will be for some time, with the southern and eastern caldera walls being likely sources because this is the area which underwent the largest amount of subsidence as a consequence of caldera formation (i.e. a sizeable chunk of pre-existing elevated terrain disappeared from the SE corner into the developing caldera). The southern walls of the caldera are particularly steep and consequently material shed from this area has a high probability of entering the lake and displacing water.
An interesting research project would be to look specifically for evidence of past tsunamis at Askja lake, to evaluate whether the July 2014 event was an extreme/low probability event, or just the latest in a number of larger events. My hunch (based purely on the pristine surface of the Bátshraun lava flow prior to this event that is now covered in debris – see Figure F) is that these larger events are infrequent.
The spectacular large landslide of 22/23 July won’t stop me working at interesting localities along the shoreline of Askja lake in future as the risk of a repeat seems very small (though this may change if the authorities carry out a more thorough examination of the source zone and say otherwise). At present the authorities are allowing access only to the relatively safe areas well above the lake level. It will be interesting to see whether this changes over the next few weeks.
In any case, Askja is a truly spectacular place to visit even if you don’t get to go down to the lake edge. And its dynamic nature has been superbly illustrated by this recent landslide, along with its effects and aftermath.