Torfajökull – waiting in the shadows

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.

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.

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.

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.

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

____________________________________________________

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