Archive for the ‘Volcanism’ Category

30-January-2016. Nevados de Chillan volcano, Chile – is it about to erupt?   8 comments

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This is the highest part of Nevados de Chillan – the aptly named Cerro Blanco, with a young (Holocene) basaltic andesite lava flow in the foreground. Cerro Blanco lies in the NE part of the volcanic complex, whereas the part of the volcanic complex that is showing signs of unrest is at the SE end, in the Las Termas area. In this blog post I’ll discuss the types of eruptions that might happen in the near future. Then I’ll provide some background on the Nevados de Chillan volcanic complex itself. I’ll finish off with some examples of lava-ice interactions at Nevados de Chillan.

Nevados de Chillan volcano in southern Chile has been showing signs of unrest since September 2015, and this is likely to lead to an eruption within the next few weeks-months. On 8 January a new vent was observed, and there are now 3 webcams focused on the volcano as well as a host of instruments measuring the developing unrest. See this link. So if an eruption does occur and the visibility is good, you’ll get a ring-side seat courtesy of the Chilean geologists and authorities.

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Webcam capture on 30 January 2016 at 20:00 GMT from Portezuelo webcam. See hyperlink above.

What makes me think that this unrest is likely to lead to an eruption? Well there are two main reasons.

Firstly, there’s clearly been a new heat source introduced into the plumbing system beneath the volcano, and this has drilled a new pathway to the surface leading to bursts of heat escaping through a new vent. This heat source is almost certainly due to magma rising up in the plumbing system. And at the moment there’s a ‘vent-clearing’ phase in place, with bursts of heat interacting with water contained within the cone (hydrothermal). There are probably magmatic gases involved as well. These energetic outbursts are clearing out material in the developing conduit, and possibly also pulverising (fragmenting) material being blown out.

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Webcam capture at 09:53 GMT on 30 January 2016 showing small and ground-hugging low plume of particulates.

Secondly, this new vent has developed on the youngest cone at this volcanic complex, which has developed through a long series of eruptions, punctuated by time gaps of a few years to decades.

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The twin cones of Nuevo and Arrau, that form part of the Las Termas, a particularly active part of the volcanic complex and one that erupts evolved and viscous dacite/rhyolite magmas.

 

In essence, this is quite a simple situation. An eruption from the new vent would simply be just the latest stage in the development of Nevados de Chillan’s youngest cone.

So what sort of eruption might we expect?

To answer this we can look at recent eruptions, as well as the series of eruptions that constructed this part of Nevados de Chillan. And this is a key point – Nevados de Chillan is not a simple cone-shaped volcano like the current Mount Fuji – instead it comprises a number of volcanic centres aligned in a roughly NW-SE direction, along with older rocks and remnants of caldera walls that are considered to have formed when large ignimbrite eruptions took place. I’ll return later to some of the older rocks – especially those that show evidence of lava-ice interactions.

But back to recent eruptions. The most recent eruption was a small eruption that took place in 2003 after a gap of 17 years (1986), lasted about a week, and produced episodic and small pulses of ash ejection that rose up to 500 m above the cone See LINK

The new vent formed on the saddle between the two newest and overlapping cones in this part of the volcanic complex – Nuevo and Arrau. (See diagram below.) A small ~64 m double crater was formed, with measurements of the two craters being 25 x 14 m and 39 x 28 m.

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Nice diagram from Naranjo et al., (2004) – see link above. Shows the 2003 vent in relation to the young cones of Nuevo and Arrau. Also shows how the young cones have been build upon the older stratovolcano of Democratico.

A thin and restricted carpet of ash was produced, much of it on snowpack. An interesting point is that if this eruption had occurred a few centuries ago, it might not have been recorded as it was so small, plus the thin ejecta blanket (being on snow) would not have been preserved for study by present-day volcanologists. The point being made is that such small eruptions in the past on ice/snow-clad volcanoes relatively remote from local populations are unlikely to have been observed, plus as the deposits have very poor preservation potential (being emplaced on snowpack), there will be no record of such eruptions. One of the little challenges for volcanologists working on snow/ice-clad volcanoes.

Such a small eruption could indeed be what Nevados de Chillan is building up to produce. But speaking personally, I’d rather see an eruption of the kind that produced the two newest and overlapping cones in this part of the volcano. These are called Nuevo and Arrau, with Nuevo being formed in the period 1945-1945, and Arrau in the period 1973-1986. The small 2003 eruption was the first since Arrau stopped erupting in 1986.

 

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The Las Termas area in the SW of the Nevados de Chillan complex, showing the two young cones Nuevo and Arrau. The small 2003 vent (mentioned above) has been given the name Chudcún. The new vent that opened on 8 January is indicated. Source: LINK

These two newest cones have been built on top of (and largely obscure) an older stratovolcano (see figure above). These new cones are constructed of interesting rock types (dacite and rhyolite) that are extremely viscous and gas rich, and so eruptions tend to be explosive as gas escaping from the magma blasts apart the magma into ash and pumice. (It’s one of those counter-intuitive quirks of science that escaping gas is stronger that molten rock and can actually fragment it. This leads to the production of enormous eruption clouds composed of fragmented magma, such as the 30 km high eruption plume formed when Chaitén volcano unexpectedly erupted in 2008. But I digress….)

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Chaiten volcano erupting in 2008. A classic example of the eruption of a massive amount of viscous and gas-rich rhyolite magma is a short time (i.e. high mass eruption rate). Source LINK

However, when these viscous magmas can rise to the surface without being completely blasted apart, then they will form slow-moving lava flows that combine to form steep-sided cones just like the Nuevo and Arrau cones at Nevados de Chillan. So this is what I’d like to see – a number of lava flows effusing from the new vent and building a new cone that, given the position of the new vent, might grow to be the highest point on this part of the volcano and exceed the heights of the Nuevo and Arrau cones.

Is such an eruption likely to be dangerous? What hazards are likely? The Chileans are aware of the potential hazards from their volcanoes, and of course those that are snow/ice-capped present an additional hazard if there is a great deal of melting, as the meltwater will move downhill into drainage systems, where it can entrain particulate matter and eventually develop into a lahar. However at the moment it is late summer in Chile and consequently snowpack is close to its minimum annual mass. Good news. Sernageomin has prepared hazard maps and will work with the local authorities to ensure that populations in danger are informed and if necessary evacuated http://www.sernageomin.cl/volcanes-mapas.php. After all, this is considered the 7th most active volcano in Chile, so it’s an obvious target for hazard planning and mitigation. See LINK

So, to sum up the current activity. This latest unrest could be a damp squib and we may see nothing more than the current puffs of particulate matter being ejected a few hundred metres above the new vent, rather like the tiny 2003 eruption. Or we may (more excitingly) see magma rise to the surface and effuse out of the new vent to produce steep blocky lava flows that slowly pour down the sides of the Nuevo-Arrau cones. The above two scenarios are the most likely ones based on the past activity that built the Nuevo and Arrau cones. I’d be surprised if there was a massive explosive eruption that produced a Plinian eruption column, but volcanologists always have in the back of their minds a number of ‘worst-case’ and ‘unlikely’ scenarios – just so they have done a bit of thinking about them on a just-in-case basis.

The volcanic complex

I’ve described Nevados de Chillan as volcano complex, and not a volcano. The reason for this is that unlike, for example, Villarrica which has a single prominent cone that is the focus for most eruptive activity, Nevados de Chillan has a number of recent vents/cones (estimated to number c.13) that are aligned NW-SE for c.10 km.

 

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The Nevados de Chillan volcanic complex. Source: LINK

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Geological map of Nevados de Chillan, from Dixon et al., (1999). LINK

 

There is evidence of an ancient volcanic complex that predates the newer c.10 km long volcanic complex, in which there are remnants of caldera walls. Some work has been done on these and can be found at LINK

 

This is a pattern that occurs at many Chilean volcanoes – an early volcano complex that produces caldera-forming eruptions (with associated ignimbrite sheets), and then a younger volcanic complex develops within the caldera(s). Another quirk that occurs fairly often at Chilean volcanic complexes is that two discrete centres of volcanic activity can develop, which erupt quite differing magmas. This is the case at Nevados de Chillan, where the NW volcanic centre (Cerro Blanco) erupts mostly magma around basaltic andesite in composition, whereas the SW centre (Las Termas, which encompasses the youthful Nuevo and Arrau cones) erupts the more evolved rock types of dacite and rhyolite. I’ll be honest with you, I’ve not yet read a thoroughly convincing model that fully explains why this happens.

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The two main centres of the Nevados de Chillan volcanic complex that have been active in recent centuries. On the left (NW) is Cerro Blanco, and on the right (SE) is Las Termas where the Nuevo and Arrau cones are. This is where the January 2016 vent has appeared.

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The prominent dark lava in the aerial image above is a relatively young dacite lava flow. This image shows what this lava flow looks like on the ground, with glassy dacite (foreground) plus piles of blocky rubble that characterise the surfaces and sides of these lava lobes. This is the sort of lava that might be expected to appear if there was an effusive eruption from the new vent that opened on 8 January 2016.

 

The most recent eruption from Cerro Blanco took place from 1861-1865 and produced a lovely basaltic andesite lava flow that melted a fair bit of snow and sent meltwater down into the Santa Gertrudis valley. Not always as a steady trickle, as a portion of the meltwater was dammed and escaped as a flood.

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In the foreground is Cerro Blanco, with the 1861-1865 flank eruption of the basaltic andesite Santa Gertrudis lava with its source cone (both characteristically dark). The lava lobe on the lower left is plunging into the upper parts of the Santa Gertrudis valley.

 

As part of a wider study of lava-ice interactions at Nevados de Chillan I visited this lava flow in 2001 whilst I was supporting a PhD student based at Lancaster University (UK), and discovered a number of fractures indicative of enhanced cooling of lava, which implied water/snow/ice was involved. This was followed up the following year by the PhD student (Katy Mee) and a colleague (Hugh Tuffen) who did a more detailed study and wrote this up as a paper. See LINK

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The 1861-1865 Santa Gertrudis lavas. Taken in 2001.

The wider study of lava-ice interactions at Nevados de Chillan along with a geochemical and geochronological study was published as a separate paper, which can be accessed at   LINK or at LINK

Lava-ice interactions

One the aspects of research I am particularly interested in is what happens when lavas interact with ice and snow (or the cryosphere if you prefer a more grandiose name). What happens with sustained eruptions (point or fissure) under thick ice is arguable fairly straightforward with different volcanic rock types (lithofacies) produced in response to varying ice/water conditions, and enhanced complexity occurring as edifices collapse, vent positions shift, and intrusions permeate the edifice.

Anyhow, the point I am making is that unlike eruptions into thick ice sheets, lava-ice interactions at stratovolcanoes is a subject that has not been much studied, and yet these interactions produce a surprising diversity of landforms because of the varying thicknesses and properties of ice/snow, the type of lava erupted, how fast it is erupting/flowing, and topographic aspects such as slope angle and so on.

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A small tongue of lava displaying characteristics that indicate it has been more rapidly cooled than just by interactions with air. It has a glassy texture (indicating rapid cooling) and a set of closely-spaced fractures – the latter are produced when heat is rapidly extracted from a lava, and water/ice is very effective at doing this. Air is not.

At Nevados de Chillan as well as good examples of lava-snow/ice interactions during the 1861-1865 eruption of Santa Gertrudis lavas, there are older examples and I’ll deal with a few of these. I’ll say now that the images (taken in 2001) are a combination of early digital camera (175 kb max) and slide scans from a rusty old scanner, so don’t expect crisp and sharp images.

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A tongue of lava that has a vague similarity to an Elephant’s trunk. Structures within the lava (e.g. flow bands and folds) indicate that it started at the high point and then propagated downwards. Pale yellow material consists of quench fragmented lava. The interpretation is that a lava was ‘perched’ on the flanks of an ice stream, and then decided to exploit a meltwater tunnel in the ice, which it enlarged as it melted its way downwards. With difficulty.

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Lava lobe that encountered ice. This has a confined shape, consists of glassy and fractured lava, and in this case has a reddish (oxidised) base. The ‘Elephant Trunk’ lobe above came from another exposure of this lava, just out of shot to the right.

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Reddish base of the lava lobe shown in the previous image. This shows intense fracturing (hackly fractures for the pedants), above which a zone of platey fracturing develops. Above this occurs microcrystalline to glassy lava. The degree of fracturing is much greater than is seen in a typical subserial flow (i.e. erupted in the absence of water/ice/snow), and so it is interpreted that ice/snow was present at the time of eruption. The ‘confined’ morphology of the lava lobe along with its glassy and fractured carapace helps to corroborate this interpretation.

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The prominent scarp in the foreground has been interpreted as a caldera fault, and it curves round (to the right) to pass in front of the highest peak (above the pool).

 

Endnote

This has been a bit of a long and rambling blog entry, but as it’s such a contrast to the limitations of writing a scientific paper I have perhaps gone a bit over-the-top. Well done if you get this far! All good wishes, Dave.

 

 

 

 

 

Posted January 30, 2016 by davemcgarvie in Chile, Eruptions, Lava, Nevados de Chillan, Volcanism

Recent volcanism at Volcán Quetrupillán, Chile   Leave a comment

This short blog entry is rich on images and short on text. Its purpose is two-fold. First – to provide a brief introduction to the recent (Holocene) volcanism of this poorly-understood volcano, and second – to provide a bit more information for candidates interested in applying for the currently-advertised PhD project at the University of Edinburgh on this very topic, on which I am a supervisor.

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The ‘beheaded’ Quetrupillan stratocone from the SW, with Laguna Azul in the foreground. The sheets and lobes on the ridge in front of the main stratocone are formed from a subglacial (Pleistocene) dacite eruption of unknown age.

So why is so little known about Quetrupillán? One key reason is that with one of Chile’s most active volcanoes (Villarrica) being such a close neighbour and with Villarrica’s past reputation for causing death and disruption, a nearby volcano that hasn’t erupted within living memory and has no obvious signs of current/recent unrest, won’t be given much if any attention. And that’s fair enough, because when you have finite resources to monitor potentially dangerous volcanoes, you need to focus those resources wisely. Even knowing what I now know about Quetrupillán, with limited resources I’d still put my monitoring equipment onto volcanoes such as Villarrica, Llaima, Calbuco, Puyehue-Cordon Caulle, and so on.

Quick big-picture context. Running through this part of Chile is a c.1200 km long approximately N-S fault zone called the Liquiñe-Ofqui fault zone LOFZ, along which many of the volcanic centres of Chile are associated. Quetrupillán lies in the middle of a NW-SE chain of three volcanoes that cuts obliquely across this fault zone, with Villarrica at the NW end and Lanín at the SE end.

I’m only mentioning this because one of the key features that distinguishes Quetrupillán from its two neighbours in the chain is that it sits astride part of the LOFZ, and on closer examination it is clear that many volcanic vents to the south and around the east and west flanks are aligned along roughly N-S fissures. This has given Quetrupillán a rather mixed morphology, with focused vent activity producing a ‘beheaded’ stratocone developed to the north, and more dispersed fissure-controlled activity producing a fascinating volcanic field to the south with abundant evidence of (Pleistocene) volcano-ice interactions as well as recent (i.e. Holocene) explosive and effusive eruptions.

I’m currently writing a paper on the volcano-ice interactions that have taken place during the Pleistocene, and I’ll write another blog entry when this is close to publication with more images than a journal will allow. Previous blog enties contain some information: Blog 1 and Blog 2

There’s also evidence of lateral transport of pyroclastic material in surrounding valleys (i.e. pyroclastic flows – or pyroclastic density currents for the pedants), as well as pyroclastic deposits formed via sedimentation from volcanic plumes. What we call ‘fall’ deposits which are important as they represent the remnants of sizeable eruption plumes in the past.

So enough of the text and onto the images.

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Western side of Quetrupillan (north to top) showing the beheaded stratocone to the north with ice-filled summit crater, along with two key geographic features – Laguna Azul and Laguna Blanca.

 

 

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Eruption 1 is a dog-leg fissure eruption (probably dacite), which in the south has been highly productive in producing lavas. In the north it has mainly resulted in initial vent clearing with the formation of craters that have cut into pre-existing Pleistocene deposits. Eruption 2 comprises part-eroded lavas that are more likely to be early than late Holocene. Eruption 3 is a thin lava flow (basaltic andesite?) that is partly covered by aeolian deposits and the rising waters of Laguna Blanca. Eruption 4 is a small lava flow than has not travelled far from its vent (hidden) to the SE.

Read the rest of this entry »

Posted November 23, 2015 by davemcgarvie in Chile, Eruptions, Lava, Volcanism

Bárðarbunga – everything you need to know (well, almost). 21 August 2014   5 comments

Yesterday a colleague decided to hold a ‘mock’ interview with me during our lunch break, which she recorded and I’ve just written up and tidied up. You may find it informative.

Q. So Dave, stop the 50:50 stuff when asked ‘will it erupt’. What do you really think?

A. It’s still 50:50! Whether it will erupt or not depends on a number of factors, some of which cannot be monitored. So that people can better understand why predictions are so difficult let me list some:

The magma is sitting at depth in a vertical fissure and slowly moving NE. It’s a dyke intrusion.

A key question is whether new magma is joining the magma in the dyke. If not (or it’s just a small amount), then there is unlikely to be an eruption. It will stall and cool.

However should a fracture suddenly appear above the dyke, then the magma is going to move upwards, and then it’s more likely to erupt.

Because as it moves up, it will reach a level where any dissolved gases (mostly water) will stop being dissolved, expand dramatically and accelerate upwards, and ‘push’ the magma to the surface. This is actually how eruptions are powered – bubbles.

Another scenario is if magma keeps being pumped into the dyke. The dyke has a number of choices: use the extra energy to keep moving NE; expand by moving to the SW, or grow up and/or down.

Get the picture?

Q. Thanks Dave, and stop calling me Bubbles. Right, we all love an apocalyptic story, so what’s the worst case scenario?

A. Ah, well, there’s more than one with this particular volcano – sorry. But these are nowhere on the horizon at the moment. Here are three.

  1. This presently benign little dyke intrusion is the forerunner to the uprise of large packets of melt from below (from the mantle) and it suddenly turns into a Laki-type flood basalt eruption. There’s still controversy over how these massive eruptions are fed in Iceland, but they always occur in fissures, and they have to involve the mantle because we have no definitive evidence that 10s of cubic kilometres of melt are stored under each central volcano just waiting to erupt. A little puzzle to solve is why these flood basalts (if they are fed directly from the mantle) have ‘shallow’ pressure signatures, but this might just mean they spend enough time at shallow dept in transit to ‘equilibrate’ to lower pressures.
  2. This event triggers activity within the heart of Bárðarbunga, beneath the summit, where there’s almost certainly some melt and or mush (melt+crystals) stored. This could be all basalt, or there could be some more ‘sticky’ magma around, such as rhyolite. Evidence from ash layers in Iceland indicates that explosive basalt eruptions from Bárðarbunga do happen, and that they are powerful. The good news is – and myself and John Stevenson have said this many times – is that we have less to worry about if this happens because we’ve already had one – Grímsvötn 2011. So we know that fewer flights will be cancelled simply because the old “ash in the sky you don’t fly” rules no longer exist. Everyone is much better prepared for a big and powerful explosive eruption. I’ve seen a few geologists say things like “Icelandic magmas do not contain enough gas to drive powerful explosive eruptions”. This is utter rubbish, incorrect, and misleading. These are invariably geologists who lack a true understanding of Icelandic volcanism because they have done little or no research there.
  3. Probably the worst-case scenario for Iceland is that this leads to a massive volcano-tectonic event in the fissure system to the SW of Bárðarbunga, as this is where a number of large flood basalt eruptions have occurred. The hydroelectric power plants on the rivers near to this fissure system would be in trouble, and we know that in the past large ash piles have dammed the rivers. The abundant water in this area results in spectacular (but fairly local) explosions and a high production of fragments as the abundant river water cools the erupting magma.

Q.Final question. You mentioned over coffee that you’d been very active on Twitter trying to get what you called the ‘right information’ out there. But isn’t there a danger that others will pinch your work and re-cast it as their own?

A. That comes with Twitter territory. I’d much rather try and provide an informed and scientifically-based set of views and ideas that can be pillaged and re-used (usually without credit) than leave it to those who don’t understand Icelandic volcano-tectonics to mislead (not always deliberately I hasten to add). I appreciate it when folks give me credit, but I don’t expect it. If you are being paid from public money to do your science, then put your knowledge to good use for the benefit of the public. Getting credit for it is a bonus, not a right.

Q. OK – late for the next meeting Dave. Maybe continue with a pint or two later?

A. Only if it’s a real ale acceptable to my palate.

Posted August 21, 2014 by davemcgarvie in Uncategorized, Volcanism

Askja Volcano – 26 July visit to the 22/23 July 2014 Landslide   1 comment

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. Takn in July 2011 - the site of the July 2014 landslide.

A. Taken in July 2011 – the site of the July 2014 landslide.

B. Taken in July 2011. Yellow line shows the major fracture system that was exploted during the July 2014 landslide.

B. Taken in July 2011. Yellow line shows the major fracture system that was exploited during the July 2014 landslide.

C. Taken in July 2011. Purple shaded area shows roughly the part of the inner wall that collapsed during the July 2014 landslide.

C. Taken in July 2011. Purple shaded area shows roughly the part of the inner wall that collapsed during the July 2014 landslide.

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D. Taken on 26 July 2014, 3 days after the 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.

Hazards

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 small water-filled crater of Viti, which lies just north of the rim of the 1875 AD (youngest) caldera at Askja. It was one of the vents of the 1875 eruption - the rest are buried beneath the lake water.

Figure E. The small water-filled crater of Viti, which lies just north of the rim of the 1875 AD (youngest) caldera at Askja. It was one of the vents of the 1875 eruption – the rest are buried beneath the lake water. ‘Spillover’ marks the low point where water from the tsunami wave poured into Viti.

 

Aftermath

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.

Raft of rhyolitic pumice and ice occupying the northeast corner of Askja lake. Debris-covered areas clearly indicate inundation bythe tsunami wave. Access to these areas to check extent of inundation was not possibe as the area is closed.

Figure F. Raft of rhyolitic pumice and ice occupying the northeast corner of Askja lake. Debris-covered areas clearly indicate inundation by the tsunami wave. Access to these areas to check extent of inundation was not possible as the area is closed.

 

Images from the landslide source – 2010 and 2011

August 2010, at the eastern end of the headwall of the July 2014 landslide. Outcrops show downward movement relative to the ridge crest, and multiple parallel troughs indicating fault development.

Figure G. August 2010, at the eastern end of the headwall of the July 2014 landslide. looking to the west. Outcrops show downward movement relative to the ridge crest, and multiple parallel troughs indicating fault development. Some block rotation resulting in dip to the south (left) was apparent on closer inspection.

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.

From the eastern end of the 2014 landslide headwall, looking across the lake to the Viti crater.

Figure H. From the eastern end of the 2014 landslide headwall, looking across the lake to the Viti crater.

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.

Older landslide immediately to east of July 2014 landslide. This older one contains large intact blocks of rotated rhyolite lava from the dome above. Look carefully and you can see these be seen on images A-D above.

Figure I. Older landslide immediately to east of July 2014 landslide. This older one contains large intact blocks of rotated rhyolite lava from the dome above. Look carefully and you can see these be seen on images A-D above. In the foreground is one of the basalt vents from the 1920s, when a number of basalt (and mixed-magma) eruptions occurred around the 1875 caldera margins. This vent erupted a number of silicic lithics (non-juvenile clasts), some of which have chemical affinities to the old rhyolite dome nearby, whilst some suggest that other rhyolite sources lie buried.

 

Consequences?

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.

Consequence. A straightforward route up to the rim at this point has now gone. It went up the eastern edge of what came down in July 2014.

Figure J. Consequence. A straightforward route up to the rim at this point has now gone. It went up the eastern edge of what came down in July 2014.

Photo taken August 2010 shows a basalt lava flow from the 1920s which is now largely/wholly covered by debris from the July 2014 landslide.

Figure K. Consequence. Photo taken August 2010 shows a basalt lava flow from the 1920s which is now largely/wholly covered by debris from the July 2014 landslide.

Flow front of the 1920s Bátshraun basalt lava, showing typical a'a upper surface (to left) with glassy and block-jointed lava at lake level indicating more rapid cooling of the loer part of the lava flow.

Figure L. Consequence. Flow front of the 1920s Bátshraun basalt lava, showing typical a’a upper surface (to left) with glassy and block-jointed lava at lake level indicating more rapid cooling of the lower part of the lava flow. See Figure F for location (debris-covered lava).

Figure M. Detail of blocky and glassy texture of Bátshraun basalt lava, showing pseudopillow fractures (long and curving with small joints perpendicular to main fracture). On right is actual pseudopillow fracture surface.

Consequence. Figure M. Detail of blocky and glassy texture of Bátshraun basalt lava, showing pseudopillow fractures (long and curving with small joints perpendicular to main fracture). On right is actual pseudopillow fracture surface.

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.

The Future?

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.

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Posted August 2, 2014 by davemcgarvie in Volcanism

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