Bárdarbunga calling

Image Wikimedia Commons

Á SPRENGISANDI

Ríðum, ríðum, rekum yfir sandinn,

rennur sól á bak við Arnarfell.

Hér á reiki’ er margur óhreinn andinn

úr því fer að skyggja á jökulsvell.

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This is one of the most famous Icelandic folk songs, talking about people riding over the highland desert, on the Sprengisandur track, hurrying on horseback over the barren land between the majestic glacier volcanoes, thinking about ghosts haunting the wind-blown valleys between Hofsjökull and Vatnajökull.

This is Hofsjökull glacier volcano. But they will also be traveling past Bárdarbunga volcano. Without being as well-known and popular with the media  as her colleagues Katla and Hekla, Bardarbunga is a volcano which deserves remembering.

Bárdarbunga, a big volcanic edifice and central volcano located under Vatnajökull in Iceland, has been showing unrest for some time now. There was esp. the glacier run (jökulhlaup) from June 2011 into the reservoir Hágöngulón in the highlands of Iceland. But also lately, growing unrest showed on the tremor charts of IMO:

Under the five big ice caps which together cover about 10 % Iceland’s surface, there are located 9 or 10 central volcanoes incl. parts of their fissure systems. Under Vatnajökull alone, there are 2 twin systems of basaltic and rhyolitic central volcanoes, i.e. Grímsvötn-Gjálp and Bárdarbunga – Hamarinn/Loki-Fögrufjöll,  Kverkfjöll in the north, all of these part of the EVZ (Eastern Volcanic Zone of Iceland), and two separate active central volcanoes, Esjufjöll and Öraefajökull, forming a separate belt in the east. (Thordarson, 2007)

Bárdarbunga central volcano, proudly presenting a 80 km2 caldera with a depth of 700 m, and reaching heights of  2100 m at its summit, is placed in the northwestern part of Vatnajökull ice cap, north of Grímsvötn.

Of these volcanic systems, Grímsvötn is by far the most productive with 480 eruptions in 6.500 years, whereas Bárdarbunga prehistoric eruption frequency (which means in Iceland till 1.000 BP) reaches 330 eruptions during the same period of time and an average of 5 per century. On the other hand, tephra layers from Bárdarbunga are dispersed more widely, which indicates „more voluminous and/or longer lasting eruptions than these of Grímsvötn“. Additionally, there were 135 tephra layers which could be identified with Grímsvötn, 87 originating from Bárdarbunga and only 17 from Kverkfjöll since Icelandic settlement started in the 9^th century. This gives Bárdarbunga the place of the 2^nd or third most active volcanic system in Iceland in historic and prehistoric times – rivaling Hekla. (Larsen,  2011). But probably there were much more eruptions, mostly very small ones which couldn’t break the glacier surface. See also GVP:  http://www.volcano.si.edu/world/volcano.cfm?vnum=1703-03=&volpage=erupt

But Bárdarbunga also has the longest or perhaps – if Askja is competitive enough – the second longest fissure system of Iceland (around 190 km), extending to the south into the Landmannalaugar region (Veiðivötn, Vatnaöldur) and also some 50 km in northeastern direction from her outlet glacier Dyngjujökull. It produced not only one of the most important flood eruption events in historic time (Veiðivötn in 1480), but also an enormous lava field, the so-called Þjórsárhraun (2130 km3) 8.500 years ago.

There has now been some earthquakes, one of them magn. 3.3. Like many other calderas, she shows unrest from time to time, which may or may not mean more. This is anyways a volcano which deserves some attention.

See also Carl’s post about Hamarinn: http://volcanocafe.wordpress.com/2012/01/25/hamarinn-volcano-burns-night/

Literature:

• Volcanism in Iceland, by Th. Thordarson, G. Larsen (2007)
• Basaltic Tephra Layers reveal the Eruption History of the Icelandic subglacial Volcanoes, Grimvötn, Bardarbunga and Kverkfjöll during the last 7000 Years, by B. A. Oladottir, G. Larsen, etal. (2011)

PS: Disclaimer – I am just a layman interested in geology.

Inge B.

(Images Wikimedia commons)

Herðubreið – Renewed activity at Askja

Photograph by Zanthia. On the picture one can see Mount Herðubreið. Herðubreið (Broad Sholder) is a tabletop mountain, or in another word, a Thuya.

Yesterday an earthquake swarm started with a 3.4M earthquake at Herðubreið. So far the swarm has had 15 earthquakes above 2M, among those 3 where at Askja proper. This swarm as well as the previous ones at Herðubreið have been to the west of the volcano. And those earthquake swarms have been deemed to be lateral intrusions from Askja.

Image by Icelandic Met Office (IMO). Askja to the left of the green star, the black “circle” above is Frémrinamúr. Kverkfjöll is due south and not showing here.

Image by Icelandic Met Office (IMO).

Only problem here is the 3 earthquakes that happened within Dyngjufjöll (Askja). Having 3 earthquakes above 2M at the same time as a medium sized earthquake swarm takes place rather beggers coincidence. I think when the hubbub of this is over the area will be removed from Kverkfjölls fissure swarm. One should also remember that Kverkfjöll is the smallest volcano on the riftline.

Image by Icelandic Met Office (IMO). There seems to be magmatic components to the earthquake swarm when looking at a higher resolution.

On the other hand, this is as far as known not anywhere near any part of Askjas fissure system. We should remember that. Personally I thought up untill now that Herðubreið itself belonged to the Frémrinamúr volcanic fissure swarm. Apparantly I was as wrong about that as the ones who thought it belonged to Kverkfjöll.

Image by Icelandic Met Office (IMO). The earthquake swarm shows well also at the Dyngjufjöll SIL-station.

So, now we are back to a long dormant volcano that had it’s last eruption before deglaciation. And that put it as having erupted at 6000BC latest (time when the glacier withdrew). How do we know that? Thuyas only form under glaciers that are big enough to contain the erupted lava thusly forming the tell-tale tabletop look of a thuya. So, we are talking about a long dormant volcano here.

Image by University of Iceland and Professor Sigrún Hréinsdottir. Inflation showing at Askja. The inflation at Herðubreið started 2 years before.

If we look at the 12 +2M earthquakes we find that 9 of those are between 2.2 and 7.9 kilometres deep. 2 of them are 1.1km deep, and that is a dummy value when an actuall depth has not been set, then we have the original 3.4M quake that has a suspiciously undeep figure. The current given depth is almost certainly around 5 to 7km and will be revised sooner or later. What does this then tell us? That the figures point towards a magmatic intrusion into an old chamber. Remember, this is my interpretation.

So, back to Herðubreið. What is Herðubreið? In my eyes Herðubreið is starting to look like a volcano on it’s own. One of the reasons is that it started to inflate just to the east before Askja started to inflate. It in fact started inflating and having earthquake swarms to the east before Askja stoped deflating. So, I am actually contemplating that Herðubreið and Askja had a common origin and has been rifted apart by the EISZ part of the MAR over the course of millenia. What I am trying to say is that they might actually share a deep root found in the current EISZ. We could think of them as two non-twins sharing the same womb and umbilical cord.

Untill we have new data from the area this is a bit speculative, but I do not think it is that much way off.

CARL