Archive for September 2014

So what if there was an eruption at the Bárðarbunga caldera…?   2 comments

Health warning! I am heading out soon to explore a relatively unknown volcano with a new PhD student but I wanted to put some thoughts down before heading off. It’s a stream of consciousness account. So what you’re going to get here is a lot of text and some links plus some images. There will be a rambling preamble providing underpinning information before I get to what Bárðarbunga might actually do.

I will try and add links and images later. Thought it best to get the text posted first in case I don’t manage to do any more work on this post.

 

Introduction

The c.15 m subsidence measured at the Bárðarbunga caldera is the largest that has been measured reliably in modern times at any Icelandic caldera IMAGE. And it has been accompanied by a massive release of seismic energy via a series of large earthquakes triggered by the downward movement (along faults) of the inner blocks against the stationary outer rim of the caldera.

And quite rightly there is concern that this large downward movement and major disturbance of the structural integrity of the caldera may lead to an eruption. The problem is that because such an eruption has never been witnessed in the modern era, we really don’t know what will happen. We can take some clues from activity at other volcanoes such as Krafla which has a major volcano-tectonic event in 1975-84 involving dyke intrusion, but at Krafla the caldera did not subside as it is now doing at Bárðarbunga LINK. Askja has a well preserved caldera that formed during a major eruption in 1875 IMAGE, but the amount of subsidence prior to the major eruption is not known with certainty. However the subsidence afterwards was measured reliably and so we know that formation of the present Askja caldera took nearly 40 years. And some subsidence is still happening. (LINK) The amount of subsidence at Askja was considerable, and at 220 m deep the lake formed in the caldera is Iceland’s second largest. If you want to read more about the Askja 1875 caldera formation check out LINK

 

The Bárðarbunga caldera and other Icelandic calderas

But let’s get back to Bárðarbunga. It has a c.10 km diameter caldera filled with c.700 m of ice. The caldera rims are also covered in ice, so we don’t even get some clues about the compositions of the volcanic rocks erupted here. And this is fairly important, because these rocks could provide clues on how the caldera formed. For example, there’s also a massive ice-filled caldera at Hofsjökull IMAGE, where all of the nunataks (rocky outcrops sticking above the ice surface) on the caldera rim are of rhyolite. From work done by colleagues and myself we know that the effusive phases of rhyolite eruptions into ice form tall towers and ridges because the erupting lava finds it mechanically easier to grow upwards through thinning ice than to try and melt its way sideways through endless ice IMAGES. In essence the ice confines the erupting rhyolite. We also know from studies of well-exposed calderas in other parts of the world that the ‘ring faults’ that are integral to calderas can be leaky, and that it’s not uncommon for lava effusions to escape from these and form domes, which are often of an evolved composition such as rhyolite. Put this process into a subglacial context and hey presto you can create really impressive and tall caldera walls by erupting only a modest amount of lava.

There are other examples in Iceland that corroborate this such as the basalt-dominated caldera walls of the Askja volcano, and the elliptical chains of subglacial rhyolite domes at the Torfajökull volcano. And I’ll finish this list with mention of a most impressive example – the tallest volcano in Iceland – Öraefajökull. IMAGE At Öraefajökull there is a c.8 km diameter caldera filled with up to 500 m of ice, and fortunately there are nunataks on the caldera rim to give us clues. These have not been studied properly, but accounts I have read suggest that most are rhyolite and a few are basalt. The highest point in Iceland – a land dominated by basalt – is ironically the rhyolite dome of Hvannadalshnúkur. The largest explosive rhyolite eruption since Iceland was settled took place from this volcano in 1362 AD IMAGE and the devastation caused to the rich farmland to led to the unusual event of a volcano changing its name, from Hnappafellsjökull to Öraefajökull. Öraefi means ‘wasteland’.

 

Bárðarbunga caldera and a major eruption

So back to Bárðarbunga (at last, I hear you groan). What it will share with other large Icelandic central volcanoes possessing calderas is a set of sub-circular faults on which upwards and downwards movements takes place to accommodate subsidence and inflation of the underlying plexus of magma bodies (or a large single chamber). Think of a cafetiere of coffee with a leak at the bottom – push it down (subsidence) and the lid goes down and the coffee moves out. Now reverse the process by replenishing the cafetiere via pumping coffee via the leak and the lid will rise up. The cafetiere is the magma chamber, and the lid is the caldera roof. Coffee = magma.

A crucial point is the composition of the magma sitting near the top of the Bárðarbunga magma system. It could be rhyolite,  but as the tephra layers representing explosive eruptions from Bárðarbunga are all basaltic let’s assume a large explosive basaltic eruption occurs. Well now we are on reassuringly familiar ground, because back in 2011 we had Iceland’s most powerful basaltic explosive eruption in over a century. Yes, good old Grímsvötn. IMAGE From this we learned a great deal and even though twice as much ash was injected into the atmosphere that Eyjafjallajökull did the year before, the disruption during 2011 was a fraction of that during 2010. If you want to know more and understand more, I can strongly recommend John Stevenson’s blog entries, as he is a top expert on how explosive Icelandic volcanoes may affect northern Europe. LINK So, a worst-case scenario of a large basaltic explosive eruption from Bárðarbunga is something we are fairly well prepared for given the lessons learned in 2010 and 2011. Yes there will probably be disruption to commercial air travel, but for the simple reason that all planes were grounded by law in 2010 and this law has changed means it will never again be as bad as 2010.

 

What if it’s not a major eruption?

If an eruption at Bárðarbunga happens and it’s not a major 2011 Grímsvötn-type eruption, then what’s likely to happen is one or more eruptions along the ring fractures as magma leaks from below, and depending on the rate and amount of magma erupting, it may or may not reach the ice surface. What is fairly certain though is that any magma that does not escape the ice confines of the glacier will form a tall tower or ridge that will contribute to the caldera structure within the ice.

So there you have it in a nutshell – one worst-case scenario that is less threatening because of we had a similar one in 2011 from Grímsvötn. And other scenarios that are even less threatening to international air travel.

 

Flooding in Iceland

However any subglacial eruption will produce a lot of meltwater as basaltic magma can (under ideal conditions) melt up to 14 times its own volume, though perhaps 5-10 times is more realistic. This will cause flooding in Iceland which may do damage to the road and power infrastructure. Iceland usually bears the brunt and cost of its volcanic eruptions – 2010 was a rare exception.

 

Final thoughts – keep an open mind

I have focused only on an eruption taking place at the Bárðarbunga caldera itself. There are other possibilities such as a major eruption in the fissure system to the south-west. But as there is absolutely no current indication of this it’s best ignored. The caldera is in turmoil, and that’s why it’s best focusing on this for now.

 

Final thoughts 2 – the oddness of subglacial calderas?

I have also revealed via a few tweets and in the above that I consider an important mechanism in the evolution of subglacial calderas in Iceland to be due to a combination of ‘normal’ flexing of the caldera floor in response to subsidence plus upwards growth of caldera rim zones from the icy confinement of effusive eruptives. Other scientists may disagree. That’s not to say that Icelandic calderas cannot also form via major explosive eruptions as happens elsewhere in the world. I am merely pointing out that there’s something special about subglacial calderas that can make them seem much more impressive in their vertical extent without them being formed solely during a single major explosive eruption. After all, there is as yet no evidence that has convinced me that the formation of any major Icelandic caldera is linked to a single cataclysmic eruption….

 

(Aside. Rhyolite is a rock type that is much more viscous that the basalt being erupted at Holuhraun, and usually contains more gas so it has a higher potential to erupt explosively and produce higher proportions of fine ash relative to basalt.)

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Posted September 7, 2014 by davemcgarvie in Uncategorized