Archive for the ‘Iceland’ Tag

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.

D

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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Katla’s next eruption?   8 comments

Introduction

Just when a volcano seems to have settled into a ‘pattern’ it starts misbehaving. Welcome to Katla.

The ‘pattern’ involves 16 recorded Katla eruptions over the past c.830 years, with the last 9 being the most reliable as the actual months in which eruptions took place were recorded. And from this comes the (admittedly rather bold) statement that “Katla erupts twice a century, and the last nine eruptions have all been in the May-November period “. [Not my statement by the way….]

But Katla has not had a sizeable eruption since 1918, and the current repose period of 94 years is the longest known since reliable records began. [Prior to the present one the longest is considered to be the c.80 years between the (approximately dated) eruption of c.1500 and the August 1580 eruption.]

Most geologists who study volcanoes in Iceland and who are aware of Katla’s eruptive history think it’s reasonable to expect that Katla’s next eruption is likely to be similar to her last sizeable basaltic eruption in 1918. But three additional factors may have an unknown influence on the next eruption: (1) Eyjafjallajökull’s eruptions in 2010; (2) probable small subglacial eruptions in 1955, 1999, and 2011; and (3) that an emergent Katla eruption is ‘overdue’ according to a pattern that has persisted since c.1180 AD. (Emergent is a term volcanologists used to describe when a subglacial eruption breaks through the overlying ice and produces a distinct atmospheric eruption plume.)

To add further complexity, it’s worth noting that Katla doesn’t just produce sizeable basaltic eruptions within her ice-covered caldera, so she could surprise us.

Image

Katla from the South, taken in 2012. A broad dome of ice up to c.700 m thick within which sits the Katla caldera.

Katla’s 7 types of Holocene eruptions

So let’s start by briefly describing listing the main types of Katla eruptions, with a few comments for me. And to keep it simple I’m going to restrict myself to the Holocene (i.e. the period from the end of the last glacial period till the present day, c.9,000 years), as Katla’s older geological history is largely unknown.

1. Small subglacial basaltic eruptions that have not become emergent (i.e. they haven’t pierced the overlying ice). These produce modest glacial outburst floods (jökulhlaups), and at the eruption site the overlying ice collapses to form a distinct depression (cauldron). Examples include the 1955 event, and the July 2011 event. (The word ‘event’ is used deliberately as the lack of prima facie evidence of actual eruptive products means that the heat source required to melt the ice could be geothermal rather than magmatic.)

2. Modest sub-ice to emergent basaltic eruptions. These produce modest-large jökulhlaups, and modest tephra falls. There are many examples in the historic period (i.e. last c.1100 years since the Viking settlement), including 1823, 1860, and 1612.

3. Larger basaltic eruptions that swiftly go emergent. These produce the biggest jökulhlaups, accompanied by thick falls of tephra (especially in proximal areas). Examples include the most recent sizeable eruption in 1918, and the huge eruptions of 1721 and 1755 and 1625. One interesting enigma is how these eruptions produce so much meltwater when so much thermal energy is lost to the atmosphere, especially when there is also no obvious place to store substantial volumes of meltwater at Katla. (A nice problem waiting to be solved.)

4. Modest eruptions of silicic magma. Yes, Katla also erupts silicic magma (dacite to trachydacite, and even some rhyolite). The evidence of these eruptions comes from tephra layers in the surrounding soil profiles, which have the lovely name of SILK layers. If they produced any effusive products (domes, lava flows), these either lie buried beneath the ice or have been removed by erosion. Interestingly, there have been no eruptions of this type since the massive Eldgjá fissure eruption in c.934-938 AD (described under 7 below). At least 12 SILK eruptions are known between c.6600 and 1685 years ago.

5. Large eruptions of silicic magma. Only one is known with certainty and I’m stretching beyond the Holocene to include this as it occurred c.12,000 years ago. It produced a sizeable Plinian to sub-Plinian eruption plume, and more interestingly from the hazard perspective, a number of pyroclastic flows (known as ‘pyroclastic density currents’ to the volcanology pedants). The deposits from the pyroclastic flows form the Sólheimar ignimbrite, which is exposed on the ice-free flanks of the volcano. Parts of this eruption are rhyolitic.

6. Small-modest basaltic eruptions in the fissure swarm associated with Katla (which trends to the northeast). These are neither well studied nor well exposed, largely because they are either covered by younger and more voluminous lavas, and/or because they have been eroded.

7. Large basaltic eruptions in the fissure swarm associated with Katla. Two examples are known – the Hólmsá Fires which erupted c.6600 years ago, and the Eldgjá Fires which erupted c.934-938 AD. An interesting oddity is that unlike the other volcanoes in this part of Iceland, Katla proudly displays a prominent fissure swarm. From this fissure swarm emerged Iceland’s largest historic basalt eruption – Eldgjá. Surprised? You might be because Laki  gets all the attention, largely because it happened more recently in 1783-85, and also because its effects on the Icelandic population, and on the population of western Europe, and on the climate, are much better known. The Eldgjá eruption is estimated to have vented c.18 km3 of lava, during an eruption that lasted up to 5 years.

Image

From above the town of Vík, looking East. The black sand is derived from Katla eruptions, and is deposited by jökulhlaups (glacial outburst floods). After the 1918 eruption, the new land formed by the jökulhlaup deposits was Iceland’s southernmost point (Kötlutangi), now eroded back nearly to the ‘island’ of Hjörleifshöfði.

Katla’s 3 eruption groups

Let’s simplify and put these eruption types into three groups:

Group A ( types 1-3) are basalt eruptions within the Katla caldera.

Group B (types 4 and 5) are silicic eruptions (rhyolite-dacite-trachydacite) within the Katla caldera.

Group C (types 6 and 7) are basaltic eruptions in the fissure swarm.

From this we can formulate two simple conclusions:

  • From the ice-covered Katla volcano itself (defined by the caldera), two kinds of eruptions are known to occur – basaltic eruptions and silicic eruptions. These vary in volume and explosivity. However, basalt is volumetrically the dominant composition, and is also the most frequent composition erupted.
  • The biggest eruptions occur in the fissure swarm to the northeast, and only basaltic eruptions are known from there.
Image

Katla from the South, showing some of the older (unstudied) Pleistocene rocks forming the lower flanks, with some paler coloured silicic rocks exposed in places.

So what’s happened to the so-called ‘predictable’ pattern?

Ah, yes, the curious case of the misbehaving volcano. Well, the Viking settlement of Iceland started in 874 AD and then 60 years later the massive Eldgjá fissure eruption. And being reasonable-to-good recorders of events, especially ones that affected the populated and well-travelled coastal strip, the new settlers and their descendents wrote down some useful information on Katla eruptions – especially the ones that caused floods as these travelled over the well-travelled paths and tracks on the coastal strip on their way to the sea. From this archive comes the ‘predictable’ pattern that “Katla erupts twice a century, and the last nine eruptions have all been in the May-November period “.

This ‘twice a century’ pattern has been surprisingly consistent for the last 16 recorded Katla eruptions over the past c.830 years

However within this persistent pattern there have been some remarkable fluctuations in how much erupted and the repose periods between eruptions. I’ll mention one that really impresses me, which is that two of Katla’s largest eruptions of the past c.1100 years occurred in 1721 and 1755, just 34 years apart. I think it is rather splendid that a volcano can regularly erupt twice a century for eight centuries and only in the May-November period, whilst at the same time having zero correlation between how much is erupted and the repose time between successive eruptions. Think about it.

The ‘breaking’ of the predictable pattern is that Katla has not had a sizeable eruption since 1918. So at time of writing (2012) it’s a repose period of 94 years, which is the longest recorded.

Image

The spectacular ice cauldron produced in the surface of the ice after the probable 1955 eruption, which if it was an eruption remained entirely within the ice. Photo taken by Sigurður Thórarinsson.

A prediction?

If I was a betting man my prediction is that the next sizeable eruption at Katla would be a basaltic eruption within the Katla caldera, one that is likely to be emergent but may be larger or smaller than the 1918 eruption, and that will occur within the next century. (Not much of a prediction you might say, but it is reasonable and does fit the data.)

But how likely is it that something different will happen?

Well here’s three possible scenarios based on the three groupings above (A-C), along with a few comments.

Possible eruption scenarios

Group A (basalt eruption from Katla). If it can be assumed that the 1955 wholly sub-ice eruption may have eased the pressure on the magma plumbing system (which I admit is speculation on my part), then this, along with another pressure-reducing event in July 2011, may be the reason why Katla’s long-expected sizeable 1918-style eruption is lagging behind schedule.

The July 2011 event also heralded a surprisingly and rapid change in the pattern of seismic unrest in the area, from a high level of unrest in the west (in the Goðabunga area beneath the ice and slightly outwith the caldera rim), to a high level of unrest further east focused within the Katla caldera proper. If this change in the location of seismic unrest is due to a switch in where the heat is rising through the crust (and that’s not certain) then things are heating-up within Katla herself.

Group B (silicic eruption from Katla). How silicic magma is produced and stored within Katla is not well understood. However since the massive Eldgjá eruption there hasn’t been a silicic eruption from Katla, and some scientists consider this to reflect that the magma plumbing system beneath Katla was changed. It’s a fair conclusion, and a high throughput of basalt through a volcano’s plumbing system does not provide optimal conditions for the production and storage of silicic melt. Look at Grímsvötn for example – the most frequently erupting volcano and not a sniff of rhyolite anywhere (at least not that we know of). Anyhow, I would be quite surprised if Katla’s next eruption was a silicic one, but I’d also be quite delighted as it’s my favourite magma.

Group C (basalt eruption in the fissure swarm). If this is of the scale of Eldgjá, it’s the nightmare scenario for Iceland. And it could also be a nightmare for western Europe also (remember Laki 1783). However if it follows a similar pattern (especially eruption rate, duration, meteorological conditions) as Eldgjá then it reduces from nightmare to ‘wow what a big and amazing eruption’. Laki was so damaging to Iceland and western Europe because a lot of magma and associated volatiles (especially S, F, and Cl) were vented in a short time. Eldgjá however, vented more magma with similar volatile content – but over a longer period. This is a rather critical point, because the longer the duration of an eruption the less nasty the effects of the volatiles because additional dispersal time means that their concentrations diminish, especially in distal areas like western Europe. Upwind of such an eruption (and orthogonal to the fissure direction) would be a tourist’s paradise – an opportunity to witness what some volcanologists would call a flood basalt eruption. Call it what you will, it would be truly spectacular. And to watch the development of a massive lava flow field over a few years – a volcanologist’s dream. Anyhow I’d be most surprised if Katla produced a large Eldgjá-sized eruption as its next event, but I’d be equally delighted if I was still around to witness it.

Image

Black beach formed of jökulhlaup deposits and pulverised basalt from nearby old and unstudied formations that may be connected with Pleistocene eruptions of Katla. The promontory in the background is Dyrhólaey, which is Iceland’s southernmost ‘solid’ part of the mainland, and which may also have been produced by Katla during the Pleistocene.

Concluding points

I’ll finish on two Iceland-specific points. The first is that the Icelanders know more about Katla than anybody else, as they have been living with her for c.1100 years and modern-day Icelandic scientists have been monitoring her with considerable intensity and with increasingly sophisticated equipment. And that’s why it was brilliant when Eyjafjallajökull erupted twice in 2010, as all this kit was already in place and teaching us a huge amount about Eyjafjallajökull’s plumbing system. Anyone writing a professional paper on Katla would be well advised to collaborate with the Katla experts in Iceland. This blog entry allows me to synthesise, speculate, and muse in my own individualistic fashion.

It is also worth emphasising that the Icelanders have plans on how to protect people when Katla erupts, and if you wanted evidence of how thorough and good these plans are, just reflect on what happened when Eyjafjallajökull erupted twice in 2010 and how wonderfully effective and safe the evacuation plans were.

The second point is that had this been a peer-reviewed paper I would have cited my sources of information throughout this account. But again it’s a blog so to keep the text flowing I didn’t. Nevertheless I have sifted and synthesised information from a number of sources and a few of these are listed below.

Finally, for the sake of one of my best friends in Iceland I rather hope that Katla erupts sooner rather than later. You see she is also called Katla, and every time there is a news report speculating about when Katla is going to erupt she gets teased by her friends with “come on Katla, stop keeping us guessing – tell us what you are going to do”. Ah the perils of naming children after volcanoes. At least she wasn’t called Eyjafjallajökull or Ok (yes, Ok is a volcano in Iceland).

Image

Katla from the East.

Sources

A useful Icelandic web source is http://earthice.hi.is/katla_bibliography

Peer-reviewed papers written by the scientists listed below (in no particular order) contain reliable and credible information on Katla, and can be accessed via Google Scholar etc. Note that this is not an exhaustive list: Guðrun Larsen; Bergrún Arna Óladóttir; Magnús Túmi Guðmundsson; Helgi Björnsson; Finnur Pálsson; Thorvaldur Thordarson; Brindís Brandsdóttir; Christian Lacasse; Olgeir Sigmarsson

Posted November 6, 2012 by davemcgarvie in Uncategorized

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Torfajökull – waiting in the shadows   2 comments

In this post I’m going to say something about a volcano that has not yet featured in the popular lists of ‘future Icelandic volcanoes that may erupt’. It is a volcano that is close to Katla and Eyjafjallajökull, if not quite in their shadows.

But I’ll start with my view of Eyjafjallajökull as this woke the world up to Iceland’s volcanoes, plus it provides a nice link to one of the points I wish to make.

Image

Eyjafjallajökull – where the flood escaped during the April 2010 eruption. Taken in August 2011.

The 2010 Eyjafjallajökull eruptions (remember there were two?) surprised us in three ways. First, the tricksy way in which the magmas uprising in March did a sudden detour to the East and erupted in the saddle between Eyjafjallajökull and Mýrdalsjökull. The seismic pattern had us all predicting a summit eruption, and then we got a beautiful and unexpected side-step. Neat. Second was the uncanny ‘perfect storm’ that made the April summit eruption so significant: an unusually efficient magma fragmentation process in the vent that produced a high proportion of fine ash capable of being transported long distances; an unusually long duration of c.45 days for such a small volume eruption; winds that took the ash direct to the UK and western Europe, and that persisted; a lamentable lack of preparedness by UK and western European governments and regulatory bodies for ash in the air; too much emphasis placed on imperfect models of atmospheric ash concentration; and insufficient ‘hard’ and real-time evidence gathered of actual ash concentrations over Europe during the eruption. All of which led to the infamous ‘no fly’ chaos. The third one is that nobody thought this volcano could produce such an eruption, as historical records showed that no eruption like this had occurred since Iceland had been settled by Nordic tribes (i.e. the preceding 1136 years, assuming a settlement date of 874 AD). Which leads neatly into the point I wish to develop….

Which is that although historical records in Iceland are pretty impressive there are omissions and inaccuracies, but the important point is that historical records cover only the past 1138 years, which is not even the blink of an eye for some volcanoes. Add to this the fact that only a few Icelandic volcanoes are sufficiently well studied that their entire Holocene volcanic histories are known with some confidence. (Quick note that the Holocene refers to the current ice-free period – i.e. interglacial – that covers the past c.9,000 years. Even more telling is that remarkably little is known about the volcanic history of any Icelandic volcano prior to the Holocene. Why is this important? Well, given that Icelandic volcanic systems are considered to have life-spans of 0.5-1.0 million years, a mere 1138 years of historical records cannot provide a representative perspective of the longer-term eruptive activity of a long-lived volcano. Even if the Holocene history of a volcano is well known that’s still just c.9,000 years we know about, which is a mere 2% of the life-span of a 0.5 million year-old volcano.

Image

c.1480 AD rhyolite lava at Torfajökull (the darker lava). Brighter and paler colours are older and hydrthermally altered rhyolite.

So what about Torfajökull?

It has impressive credentials, the most prominent being that it is Iceland’s largest active rhyolite volcano. And rhyolite is the type of magma that is so viscous (sticky) that it is most easily blasted into small fragments (ash) during explosive eruptions. This is one of the very few Icelandic volcanoes for which we have reasonably accurate ages for any of its pre-Holocene eruptions, and this led to an exciting discovery which is revealed at the end of the next paragraph.

Image

Basaltic maar near Torfajökull produced during the c.1480 AD eruption.

Let’s start with the most recent eruption, which took place c.1480 AD when two small rhyolite lava flows effused (accompanied by only minor explosivity) on a linear fissure that to its NE erupted a substantial amount of basalt. There were a further c.9 other eruptions like this earlier in the Holocene, but before you think there’s a decent pattern here, step back into the glacial period just before the Holocene (the Pleistocene) and prepare for a surprise. Around 70,000 years ago when the area was covered by at least 500 m of ice a ‘ring fracture’ opened up around the margins of the volcano and out poured c.16 cubic kilometres of rhyolite. This was hypothesized as Iceland’s largest known rhyolite eruption back in 1984 (by me), but until the ages had been determined we didn’t know when it took place.

Image

One of the c.70,000 year old rhyolites produced during the ‘ring fracture’ eruption at Torfajökull. This is Kirkjufell.

There is evidence that the rhyolite erupting 70,000 years ago pierced the overlying ice during explosive activity and spread ash far and wide, as a 5.5 cm thick ash layer in the Norwegian Sea has been attributed to this eruption. This is an enormous thickness by the way, as the Eyjafjallajökull 2010 ash layer at the same location won’t even be 5 mm thick.

At the moment at Torfajökull you can bask in hot pools beneath one of the 1480 AD rhyolite lava flows, blissfully unaware that minor amounts of magma are on the move beneath you (indicated by occasional earthquake swarms of a specific type). And that beneath the western part of the volcano a small magma chamber has been ‘imaged’ using patterns of seismicity. The potential ‘mush’ zone – an important birthplace of rhyolite magma – is likely to be much larger than this, but is notoriously difficult to detect and ‘image’. But it will be there.

Conclusion? The next eruption at Torfajökull is likely to be similar to the other Holocene eruptions, so expect a small rhyolite fissure eruption triggered by a basaltic fissure eruption to the NE of Torfajökull. However if the rhyolite that erupts happens to be loaded with volatiles (gas) then a decent amount of ash will be produced as the gas expands like crazy during its rush to the surface and in doing so fragments the uprising magma into ash-sized fragments. And there is some research I’m involved in that is showing that Icelandic rhyolites contain more gas than previously thought, thus enhancing the potential for greater explosivity and greater ash production, the combination of which could put enough ash into the atmosphere to merit diverting flights. But that’s another story….

Undereath this geothermal field lies the ‘imaged’ small magma chamber.

The links below will take you to papers dealing with:

Dating the c.70,000 year old and other eruptions at Torfajökull

Volatiles in one Torfajökull eruption

Explosive rhyolite eruptions beneath Icelandic glaciers

Rhyolite volcano-ice interactions in Iceland

Ice melting and potential effects on eruptions

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https://docs.google.com/open?id=0B9nifcNoPqF3TEs5QUNPYnFQYkU

https://docs.google.com/open?id=0B9nifcNoPqF3ZVJZRnZsYmZIMDg

https://docs.google.com/open?id=0B9nifcNoPqF3Q1JKVDRoazczX0E

https://docs.google.com/open?id=0B9nifcNoPqF3SURyeWZOeGd3MEU

https://docs.google.com/open?id=0B9nifcNoPqF3R01EaXRFSURFNDQ

Posted May 18, 2012 by davemcgarvie in Uncategorized

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