Background

Askja is a lesser known major Icelandic volcano. The north volcanic zone (NVZ) where it is located is not as touristy as the east (Bardarbunga, Grimsvotn), the south (Katla) or the southwest (Fagradalsfjall) volcanic zones of Iceland, but it should not be ignored. The Krafla rift eruption here lasted (intermittently) for nine years, from 1975 to 1984, while Askja erupted (also intermittently) throughout the 1920’s.

The NVZ begins immediately north (some say south) of the infamous Holuhraun eruption of 2014. There are 5 volcanic centres in the NVZ, of which Askja is the most southerly – and the largest. Askja would be a major attraction in any other region: it has an unearthly beauty, a volcanic oasis in an uninhabited lava-desert landscape with a large caldera and two scenic lakes. Quite a few tourists still manage to get there during the summer, but only after several hours of almost off-road driving. Most come on organised trips. You can get there yourself, but will need to rent a superjeep (four-wheel drive with enough clearance to get over the rubble that pretends to be a road). Be aware that you will need to ford the unbridged rivers that cross the road, preferably when the rivers are not in spate. Many pictures of Askja show the same view of the two lakes, from the same vantage point. It is close to the point where the road gives up.

Askja has had several eruptions in the past 200 years, making it a relatively active volcano with mainly basaltic lava eruptions. This post will focus on the large -and unusual- eruption of 1875. More recently there were small eruptions in the 1920’s and in 1961. In some ways the 1875-1929 activity may represent a single episode.

Since the Krafla eruption Askja has been deflating. There is no direct connection between Askja and Krafla, although they lie along the same rift. Inflation at Askja resumed around 2010. At this time the lake became ice free, evidence of increased hydrothermal activity. There were earthquake swarms. Everything calmed down again when Bardarbunga reactivated. But since a year or so there again has been earthquake activity at Askja. This coincided with Grimsvotn entering an unexpected quiet period. Deep below the brittle crust, the volcanoes here are connected. They live off the same magma supply, drawn up through their individual conduits and stored in separate chambers. They are like different banks, dispersing the same government-issued money but with their own vaults and with different terms and conditions. If Askja is a bank, its caldera may perhaps be viewed as a ‘hole-in-the-wall’ created to dispense magma.

Inside

The volcano rises 800 meter above the surrounding area. Most of it is taken up by a large caldera, measuring 8 km across: in fact the word ‘Askja’ means caldera (or ‘box’). There are two more craters here. The larger one measures 3 by 4 km, has a large lake, and is young: it first formed in an explosion in 1875. Just a few meters north of this crater lies Viti crater, formed in a smaller explosion in the same event. It too contains a lake, and it is near the end point of the road. Brave souls can try to swim in it. But be aware: these lakes can kill. That is a story for later.

The large lake is called Öskjuvatn. It is deeper than the main caldera. The lake is 3.5 km across; the surface is 50 meters below the caldera floor and the lake is 220 meters deep. The small Askja Víti crater is a scenic, deep hole filled with a small lake. Its lake is suitable (kind off) for swimming because it is geothermally heated. Öskjuvatn tends to be frozen and should be more attractive to polar bears than to tourists for their swimming lessons. But its water is possibly cleaner. A swim in Víti crater is certainly memorable – the stench of sulphur will cling to you for days. Stay away from the boiling vent and from falling rocks, and be aware that the slippery path is easier down than back up. In rain it may be next to impossible to get up. If a trapdoor spider would run a bank, this is what its branches would look like.

The bank of Askja

The terms and conditions of the bank of Askja are hard to read. This volcano-bank offers different products at different times and at different branches. It has eruptions at the central branch (the cauldron), in the suburbs (the ring fault and flanks), and out of town (distant fissures). The main product is basalt, but the 1875 eruption produced rhyolite – a bit like the Bank of America suddenly issuing Russian rubles from its cash machines.

Askja is a shield volcano, as might be expected from the fact that it normally erupts basalt. The caldera is enormous and it takes up much of the area of the shield. It is also remarkably deep. The main way a basaltic volcano may form a caldera is by emptying the magma chamber underneath through a distant dike. This happened at Bardarbunga which collapsed at precisely the rate at which Holuhraun erupted, leaving little doubt where the magma went. The same happened at Kilauea during the Leilani eruption. It is therefore natural to expect the same happened at Askja. The caldera is 8 kilometers wide and 300 meters deep, which gives a volume of 15 km³. The sides may have been much higher before being eroded by the ice, making the caldera more than 1 km deep. Such a large volume requires large lava fields (‘hrauns’).

Some have suggested that the caldera is a fraud. It is perhaps 200,000 years old, and for 80% or more of its life has been covered by ice age glaciers. Almost all of its eruptions were subglacial: Askja is an ice-age volcano. Effusive eruptions underneath a thick glacier form hyaloclastites and indeed much of the area around Askja’s caldera is covered in these. Lava underneath thick ice cannot flow out, but forms steep ridges above the erupting fissure. The argument is that the ridges around the caldera are hyaloclastite ridges on intersecting faults, giving the impression that the enclosed area is a caldera.

Askja is now commonly viewed as a combination of both: there was also some subsidence of the central region, but the surrounding ridges are mainly hyaloclastites, perhaps on a ring fault. The bank of Askja has boosted its appearance with a big building funded with some drastic quantitative easing.

The ridges around the caldera reach around 1200 meters in elevation, and the floor of the main caldera is at 1050 meters. It used to be deeper: there is at least 50 meters and perhaps as much as 500 meters of post-ice-age lava and volcanic debris in the caldera.

The black dots show the large caldera. The lake forms an extension to the southeast, formed in the recent 1875 eruption. The centre of the new crater is on the edge of the old caldera: it used the same fault that created the caldera ridge. Shallow earthquakes (typically 2-4 km depth) now occur mainly around the southeastern edge of Öskjuvatn. This area also has hydrothermal activity. There is also at times some earthquake activity at much greater depth, up to 30 km.

Deposits

Askja is mainly, but not exclusively, a basaltic volcano with effusive eruptions both at the caldera and further afield. The largest lava fields are to the northeast, as far as 30 km for the centre of Askja. Together they contain perhaps 10 km³. There are numerous small lava flows are all around the flanks and inside the caldera. Seven such flows erupted between 1910 and 1929, fed from rifts and fissures. To the south the lava flows just reach the area of the 2014 Holuhraun lava, up to 15 km away. Askja’s rifts are lopsided, extending much further to the north than to the south. That is of course a consequence of the lie of the land: the big bulge of Vatnajokull stops the rifts going south. As they would have to flow uphill, it is a no-go area for Askja. To the north, the rifts can continue a long way, running along the side of Krafla’s rifts .

But there is more to Askja. Since the ice age there have been at least three explosive eruptions producing rhyolite. A layer of tephra up to 2 meters deep is dated to 10,000 years ago. Distant 2000-year old tephra (‘distant’ meaning that it was found in Sweden) seems to come from Askja, although it has not (yet) been found at Askja itself. The third rhyolite explosion was in 1875: it left white tephra up to 5 meters thick around the crater, and even 15 km downwind the layer was still 50 cm deep. The rhyolitic explosions produced perhaps 1 km³ (DRE) (or a bit less) in total. They were a pyrotechnic sideshow to the far more numerous basaltic eruptions .

Digging below the surface shows that Askja is more active than it may seem. This is one of the big banks of Iceland. 175 separate eruptions have been identified for the past 7000 years. This is about one eruption per 25 years. (In reality the eruptions occur in clusters, and the clusters are further apart.) In Iceland, only Grimsvotn erupts more often than that, whilst the rate is similar to that of Katla and Bardarbunga. Since the ice age, Askja has produced at least 40 km³ of lava. Not bad for a bank – Askja is a decent volcanic investment. More than half of the caldera floor is covered by lava flows younger than 1510.

The 1875 explosions

The 1875 explosion is a warning not to take this volcano for granted. In fact, the crater lake already indicates there is more to Askja than a friendly fire. Any crater lake should be viewed with suspicion. It means that something made a big hole, recent enough that it has not yet been filled with sediment. The water makes a volcano even more dangerous than it already was. Iceland has two such large lakes: Öskjuvatn and Grimsvotn. You have been warned.

Öskjuvatn formed in the third largest rhyolitic explosion since Iceland’s settlement. This is the oldest eruption at Askja where we are certain of both the date and the time. The eruption column was seen throughout Iceland.

The explosive eruption on 28/29 March lasted 17 hours. It started with phreatic precursor activity (i.e. steam clouds were seen: ‘phreatic’ means water). At 9pm a rising column was seen for an hour (until night fell) and tephra began to fall as far as 50 km away. This is considered a subplinian explosion. A larger, phreatoplinian explosion followed at 5:30am in the early morning, lasting about 1 hour, with a column up to 30 km high. It deposited a 2 meter thick ash layer in the caldera, and send tephra as far as Scandinavia. The ash was wet and sticky. The explosion was followed by pyroclastic flows in the region. After a 30 minute pause a new vent opened and another plinian explosion followed with again a 30-km cloud, this time dry rather than wet. Even 150 km downwind the tephra was still 2 cm deep. Local tephra shows that the explosion was accompanied by fountaining from the rifts or vents.

The eruption was entirely within the area of the modern lake. The lake did not exist at the time but we know that a small pond had formed. When people came to see some time after the explosions, there were three rifts visible which apparently had been the sites of the explosions. They are indicated by the white bars in the image above, and the labels indicate the alphabetic order of the explosions.

The explosions had formed a triangular depression, but it was much smaller than the current crater. The deep crater formed by stepwise collapse lasting almost 50 years, either by magma drainage or by rifting. This ended only with the flank eruptions of the 1920’s.

The 1874-1876 rift

This was a very busy time for the area. The first rumblings were in January 1873, with a large explosion at Grimsvotn. This is not particularly unusual as Grimsvotn erupts often but this eruption was a bit larger than most. Some tephra of this explosion reached Askja, and formed a layer just below the ejecta from 1875.

The first signs of renewed activity at Askja itself were early in 1874 when steam clouds were seen above the volcano. We don’t know what actually happened: no one lived close, and the nearest farmers were tens of kilometer away. Earthquakes were felt in north Iceland in the autumn of 1874, and these became stronger and numerous in late December of that year. (There had been two M6.5 earthquakes of the north coast of Iceland in 1872, but this is too far from Askja to have had any effect, and they also were on a different fault.) New fissures were found in the ground north of Askja. In hindsight, a dike had been forming from Askja running towards the north, and this had caused the earthquakes and the ground deformation. But this was not known at the time.

On Jan 2, 1875, the earthquake activity became so strong that damage occurred to some farm houses. The location of the earthquakes is not entirely clear, but the damage was most severe around the Sveinagjá and Veggjastyki grabens, 50 kilometers north of Askja and the reports indicate that the activity was centred near here.

On Jan 2-3, the earthquakes culminated with an eruption at Askja. There are no eye witness reports other than the eruption cloud, and the size of this event is not well recorded but it may have been a significant explosion which produced a notable crater. It appears that at the same time there was an eruption at Fjárhólahraun a few kilometers north of Askja, as two separate eruption columns were seen. The lava field of Fjárhólahraun has been attributed to this eruption, but it appears that this is much older, possibly from the 14^th century. Tephra found at Fjárhólahraun does coincide with the 1875 eruption, and there may have been fountaining activity here but without large lava flows. After these eruptions, the earthquakes diminished but did not fully cease.

The first visit to Askja (remember this was mid-winter, with deep snow, and the area was difficult to reach) was on February 16. They found new craters in the area which is now occupied by Öskjuvatn, just outside the caldera, with intense hydrothermal activity. Remember that this was before the large explosions in March, and the large new crater had not yet formed.

On February 18, 1875, activity suddenly moved far north, to the area where the earthquake activity of December and January may have been focussed. A rift eruption now occurred in the Sveinagjá graben with actual lava. A second site in this graben erupted on March 10, a third one on March 18 and a fourth one on March 25. There were eruption sites along the entire graben, although each seems to have been short-lived. A report from March 25 describes 40 active craters. This sounds more like a continuous fissure which did not develop a single dominant cone. The eruptions here produced the Nyjahraun lava field.

The next event was the major eruption at Askja of March 28/29 which has already been discussed and which affected much of eastern Iceland.

After this event, the main activity continued to be at Sveinagjá. Eruptions here were reported on April 4, 10, 24, July 2, August 15 and October 17. Weaker activity may have continued into 1876 but this was winter when it was difficult to visit. Askja itself also had some activity, with an eruption cloud on May 19, and unconfirmed reports of further activity and in the autumn and winter. In early 1876, the action ceased.

The magma vault

It is interesting that the main activity was not at Askja, but 50 km north in an area with rifts but no central volcano. It was a fairly normal fissure eruption, with lava erupting from separate locations along an extended rift. During this time Askja had hydrothermal activity and major explosions, but less evidence for lava. The eruptions seem related but they were quite far apart. Did the activity come from Askja? Or was Askja just a side show?

The composition of the lava and tephra show a relation between the two events. However, they are not identical. Whilst the tephra at Fjárhólahraun is identical in composition to that at Askja and clearly comes from the same source, Nyjahraun’s composition is a little different. It appears that the new magma mixed with an existing magma reservoir at Askja, but at Sveinagjá only the new magma erupted, not the mix.

The earthquake activity provides a second clue to a connection. The December and January earthquakes (which may have reached M5.5) were focussed on Sveinagjá. But it diminished after the January eruption at Askja. The earthquakes indicate a dike was growing, but it stopped growing after the pressure had been relieved at Askja. From this point the magma underneath Sveinagjá was trying to find a weakness to the surface. This took 6 weeks. After this the activity continued intermittently for a while (9 months) but eventually petered out.

We can speculate on what had happened underground. New magma had begun to collect underneath Askja, initially at a depth below the brittle crust. From there, in December a dike formed towards Sveinagjá. The earthquakes traced where it entered the brittle crust, near Sveinagjá. There was no reported activity in between Askja and Sveinagjá.

While the dike was forming, the magma also began to interact with an existing shallower magma chamber underneath Askja. The interaction rejuvenated this stale magma, by heating and mixing. This rejuvenation caused the rhyolite explosion of March, and also pushed some of the magma through an existing fault towards Fjárhólahraun.

The explosion removed the pressure below Askja, and the dike now no longer received an inflow of magma. The magma at Sveinagjá was still pushing up, and this caused several eruptions along the entire length of the graben. But without new magma they lost their oomph, and finally ceased after 9 months. This model is speculative but seems to fit the events.

Traditionally there have been two ideas for how rifting eruptions occurred. In the first idea, magma was assumed to collect underneath a central volcano, and a dike extended sideways from this chamber under a shallow or steeper upward angle. Where the dike reaches close to the surface, extension occurred and magma began to rise to the surface probing for the weak spots. The second idea began with the rifting, allowing deep magma to rise vertically into the vacated space. Here each part of the rift in a way has its own magma supply, albeit all are connected at depth.

The second version has not received much attention recently. The well studied rifting eruptions in Holuhraun and Kilauea left little doubt about where the magma came from, and this fitted the first idea. The Krafla eruption (close to the 1875 activity) also operated in this way. However, the first idea may not be perfect. If the main magma chamber underneath the central volcano is fairly shallow, then a dike may form below it, bypassing this chamber. It seems plausible that Laki formed in that way. For Askja also, a deep dike well below the main magma chamber may have intercepted some – or most – of the new magma and diverted it 50 km away.

The reason for the explosive activity is that Askja had been inactive for some time, perhaps centuries. The magma chamber had gone stale but not solid. These are the most dangerous volcanoes.

The final question is why the explosions were both wet and dry. Some of the activity was in standing water, and some was on dry land. It appears that water had ponded in the depression (caused by the rifting?) but not everywhere.

Holuhraun

Before 2014, Holuhraun was seen as part of the Askja rift system. This is relevant here because there are two lava fields in this area dating from around 1875. These have at times been considered as part of the Askja eruption. Of course, nowadays we know that this area is under different ownership, and is within reach of Bardarbunga’s rifts.

The two lava fields are conveniently called Holuhraun-1 and Holuhraun-2. They erupted in slightly different locations: Holuhraun-1 was 15 km south and Holuhraun-2 erupted 23 km south of Askja (adjacent to the glacier). It now appears that these flows formed before the Askja 1875 eruption. Holuhraun-1 is almost a century older, and likely formed in the winter of 1797. Holuhraun-2 is much closer in time to the Askja eruption but still a bit too early: it formed around 1863. Holuhraun-2 coincided with the eruption in Veidivotn which formed Tröllahraun.

The lava of the three Holuhraun eruptions (including 2014) is identical. We can safely blame Bardarbunga. The closeness in time between 1863 and 1875 may suggest that this was a time of general rifting in the NVZ. It started from Vatnajokull, and reached as far as Sveinagjá, a length of 100 km. Askja may not have been the driving factor: it just happened to be on the rifting path and provided a convenient and existing conduit.

Öskjuvatn

Askja’s beautiful lake holds deadly secrets. Volcanologists have disappeared without trace while investigating it. That is notable in a nation where volcanic eruptions very rarely cause casualties: in one event, Askja killed as many people (2) as all other Iceland’s volcanoes have done together since.

The killings happened in 1907. Two Germans went on to the lake in boat to investigate the caldera, on July 10, 1907. They were never seen again, dead or alive.

The Germans were Max Rudolff, a painter, and Walter von Knebel, a geologist. Knebel was 27 years old at the time, and engaged. The expedition was for his research – Rudolff was there to provide the illustrations. When they did not return, his fiancee organised an expedition to find him. The chase was not sucessfull but at least it gave her a new life as she subsequently married the person who accompanied her on the journey.

Her name was Viktorine Helene Natalie von Grumbkow; she used the name Ina. Her partner in the attempt to find her fiancee was another volcanologist, Hans Reck. Ina had to proof she could do the trip (being a woman), and did so by traveling around the Reykjanes peninsula where she visited Krysuvik and Keilir – which she climbed. Having managed that journey, the two set off on a trip that went via Hekla, Laki , Myvatn and Herdubreid – the volcanology interest of Hans Reck showed – before reaching Askja.

Ina wrote a book about the failed search. Arriving at Askja, she writes

“The Askja crater has in all directions almost the same diameter of around 9 km. The diameter of the almost round lake sunk in the southeast corner of the basin and named in memory of my fiancé Knebel-lake is 4-5 km. Despite the distance, we often saw the dust from the avalanches on the south wall opposite our tents and the water splashing up on the edge of the bank when particularly large blocks had fallen into it. As already mentioned, the lake is enclosed by steep walls in the south and east. Especially on the south wall they are completely inaccessible. The over 300 m high wall here consists of a fusion of rugged almost 100 m high palagonite cliffs, the tips of which stand out sharply against the sky, with heaps of rubble that stretch from the foot of these cliffs to the lake level – the paths of the never-stopping dangerous rockfall. There is not a foot of flat land at lake level”

“On the west side, the lake is bordered by an approximately 60 m high bank. In its entire length it shows the colourful stratification of basaltic lava and coloured slag with the generally golden pumice covering in the Askja basin. The north side of the lake has the 80 m deep Rudloff crater (named after the young painter Max Rudolff who had an accident with Dr. von Knebel), at the bottom of which green-yellow sulphurous waters boil with alternatingly strong steam. The crater is very close to the lake, so only a very narrow ridge allows it to pass between it and the lake.”

After a difficult and fruitless search, the party left again, with one final look at the place where her fiancee had disappeared:

“A wonderful distant view presented itself once more from here. Towards the south the wide Askja basin, filled with black lava flows and white patches of snow, framed by multi-shaped, colourful mountains trimmed with snow. Far, far beyond, beyond the whole width of the southern Ödahraun the Vatna Jökull in light beauty. And on the whole, the transfiguring appearance of inviolable majesty, the highest solitude and original perfection. As the early morning mist melts under the rays of the rising sun, so all the glimmers of the modern world fade before the unsophisticated grandeur of this pure nature. The same word that was coined for millennia for the lilies in the field is still valid here today, in view of all this silent splendour: that Solomon in all his glory was also not clothed as this one.”

The names that she bestowed on the lake and crater have not survived modernity, but the four meter tall pyramid they left as a memorial is still there. The lake is now called Öskjuvatn, and the small separate crater is called Viti.

The in-land tsunami

At the time, the caused of the disappearance remained unknown. A popular suggestion was that they had used a poorly build boat which had sunk in the lake. But it now appears that the two Germans may have vanished because of the frequent rockfalls that Ina described.

On July 21, 2014, a landslide occurred on the southeastern cliff above the lake. The cliff collapsed over a length of 800 meters, between 150 and 370 meters above the water. Half of the debris, or 0.01 km³, slid into the lake, mostly as a single block. It caused devastation. In hindsight, movement on the cliff had started 7 years earlier, and the creep had already reached 25 meters before it gave way. But no one had noticed the change and the collapse came as a complete surprise. The landslide was among the largest in Iceland of the preceding century.

The original cause was probably the long-term subsidence of the crater along the ring fault. The final collapse came after the ground had been weakened by snow melt (asn effect already described by Ina) and by heavy rain.

Luckily, the collapse happened at midnight and no tourists were present near the lake. They would have seen a frightening event, and probably not have survived. The block slid into the lake at a speed of 140 km/h, and traveled 2 kilometers along the bottom, disintegrating along the way. The block pushed up the water, and this wave started traveling out at high speed. Even lakes as small as Öskjuvatn can have tsunamis! And lake tsunamis are even more dangerous than their open water counterparts. The water bulge can’t spread out – instead the water becomes focussed towards a small area on the far side. The wave here can reach enormous heights. The run-up at Öskjuvatn along the shore reached up to 80 meters! The wave traveled at 50 m/s, and reached the opposite shore in 1 or 2 minutes. By the time people exploring the lake would have recognised this phenomenon was not put on for the tourists but deadly dangerous, there would not have been enough time for them to climb to safety.

The debris was sufficient to raise the level of the lake by 0.5 meter. But that was a minor side show compared to the lake tsunami.

Ina reported numerous small rockfalls into the lake. This was at a time when the caldera was still forming through slow subsidence along the faults at the edges. The disappearance in 1907 was at the same time of the year as the 2014 collapse. This is the time when snow melt can destabilize the ground, something also noticed by Ina. The suggestion has therefore been made that the two Germans were on the lake during a collapse, and were swamped by the wave.

Final words

Askja is a highlight of Iceland, worth the effort it takes to visit, at least when the roads are passable in summer. But it is also unpredictable and dangerous. White Island has shown us the dangers of volcano tourism. The lessons also apply to this giant of Iceland’s sleeping beauties.

And activity is on the rise. Hydrothermal and earthquake activity increased in 2010, and for a while the lake even became ice free. After a calm period, earthquakes resumed last year and since a month there is rapid inflation. Magma is on the move. The inflation is centred not at the new crater of Öskjuvatn, but at the centre of the old caldera. This is a significant influx. Will there be an eruption? Will it be in the caldera, along the surrounding ring faults, to further away? Will the bank of Askja collapse? It is far too early to call wolf. But perhaps it is a good thing that the tourist season here is ending.

references

Gylfadóttir, S. S., et al. (2017), The 2014 Lake Askja rockslide-induced tsunami: Optimization of numerical tsunami model using observed data, J. Geophys. Res. Oceans, 122, 4110–4122, doi:10.1002/2016JC012496.

Hartley, M. E., and Thordarson, T. (2013), The 1874–1876 volcano-tectonic episode at Askja, North Iceland: Lateral flow revisited, Geochem. Geophys. Geosyst.  14, 2286– 2309, doi:10.1002/ggge.20151.

Carey, R.J., et al. (2010), Tephra dispersal and eruption dynamics of wet and dry phases of the 1875 eruption of Askja Volcano, Iceland. Bull. Volcanol. 72, 259–278

Hartely, M.E., et al. (2016), Postglacial eruptive history of the Askja region, North Iceland.Bulletin of Volcanology 78, Article 28

Brandsdottir, B., (1992), Historical accounts of earthquakes associated with eruptive activity in the Askja volcanic system. Jokull, 42, 1

Ina von Grumbkow, 1909. Ísafold: Reisebilder aus Island. http://www.isafold.de/klassiker/default.htm

Albert, September 2014