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)

Tungnafellsjökull – Tectonic Earthquakes

Photograph by our own Jamie. Tungnafellsjökull seen from Sprengisandur area. Notice that the Jökull is almost gone from Tungnafellsjökull, soon to become known as Tungnafjöll only.

There has been an earthquake swarm at the northern end of the Tungnafellsjökull during the evening and throughout the night. The swarm is still ongoing. There has been a lot of speculation out there in the blogosphere about it being volcanic in nature. It is not, it is purely tectonic.

As some of you know Iceland is divided by the Mid Atlantic Rift (MAR). The MAR in turn is divided in Iceland into two separate active seismic zones, the Eastern and the Western Icelandic Seismic Zone. Lately it has been the EISZ that has been most active of the two. But the WISZ is not in any way dead or dormant. Both of them are driven by the spreading of the MAR. From the WISZ the North American Plate is spread, and from the EISZ the Eurasian Plate is spread. In between them are two micro-plates that have formed by volcanism caused by the rifting.

The map is showing the Icelandic Volcanic Zones, where the MAR runs up into Iceland, where the MAR leaves Iceland and the more important volcanic features. The Icelandic Seismic Zones are corresponding to the volcanic zone.

Along both the WISZ and EISZ are lines of volcanoes spread, it is where the spreading causes magma to pour up and fill the spaces created by the spreading.

If you look at the map you see that WISZ runs from Hengill, up to Langjökull (2 known volcanoes), via Hofsjökull (at least one volcano), onwards through Tungnafellsjökull, and then ending up at the triple-junction at Bárdarbunga.

During the last few years the area of Tungnafellsjökull has been inactive, but there is ample evidence of it having been tectonically active, something that can be found in the Sprungur (tectonic faults) found in the area. The dormancy is likely due to the area having been locked at depth, probably by old magma that has solidified the area.

Various versions of tectonic faulting. Tungnafellsjökull is suffering from strike-slip faulting.

Lately the area has been subject to an uplift not seen in Iceland since de-glaciation after the last Ice age. This is due to the melting and diminishing of the glaciers of Tungnafellsjökull (almost gone) and Vatnajökull. This uplift process has accelerated during the last decade. It is now up to 3 cm year in the area according to Sigrún Hreinsdottir (source, private email). Yes, the famed inflation of Hamarinn is not happening, it is a combination of Grimsvötn motion and isostatic rebound.

This motion might have started to release the seismic lock at Tungnafellsjökull. If that is so, there is a risk that the swarm of earthquakes is just precursor quakes for a large earthquake.

This map shows the features discussed in this text in relation to the Bárdarbunga triple-junction and the hotspots location.

What makes this interpretation the more likely one is that there is no discernible evidence of any harmonic tremoring during the earthquakes. This makes it into tectonic seismicity, not magmatic seismicity.

If there would be a large earthquake that tears the rift-lock, then magmatic movements could start in the area, but not before that. Worst case scenario here is not a volcanic eruption; it is a 6M earthquake as the slip-lock disintegrates over a large area.

Another thing that I want to point out, the earthquakes are all of low probability and some of them are as I write this due to change after revision, and some of them will be removed due to being false representations of earthquakes, so called Ghosts. And as I wrote this IMO has started to revision the earthquakes, right now there is at least one at 3M.

CARL

Icelands forgotten Volcanoes

Eldgigur seen from the Hágöngur volcano.

Sometimes a bit of honest digging will pay off in the oddest ways. Trying to utilize an unexpected free day I decided to find an alternative explanation for the uplift associated with the Hamarinn volcano. A long shot as good as any other to pursue on a rather grey day.

While on the prowl for this unknown central volcano I got a lead, and started to dig. To my surprise I found an old (1952) geological report from a Danish survey that seemed to have something to do with my suspected culprit. To my utter surprise it was about another equally unknown volcano. And as if this was not enough, it also named a few other volcanoes I had never heard about.

Image taken from the linked paper below. It shows very well the Grimsvötn fissure swarm as it goes to the SSW through Thordharhyrna volcano.

As you can see on the image the top most volcano is one of the more infamous on Iceland, Grimsvötn. And that one does not merit a lot place here, more than the obvious mentioning of its southern fissure swarm that ends up in Laki. Most of you know already that Grimsvötn was the responsible parent volcano for the Laki rifting fissure eruption, and that Thordharhyrna erupted simultaneously with Grimsvötn and Laki.

But hand on the heart, how many of you knew about Háabunga central volcano? Well, some of the more volcanoholic of the readers of this blog probably do know about it. I did at least. Also Thordharhyrna is well known, but there the fun probably ends for the readers of the blog.

SSW of Thordharhyrna resides a volcano I never had heard of before, Geirvörtur. About this one I cannot say much, at least more than that it resides on top of the Grimsvötn fissure swarm, and that probably is a remote sub feature of Grimsvötn.

SSW to Geirvörtur we have the 1200 meter high volcano of Hágöngur. And know it is starting to get really exiting. Close by to Hágöngur is the SE is the 854 meter high post-glacial volcano of Eldgigur. It resides on the Grimsvötn fissure swarm, but it is doubtful that it is magmatic subset. This is due to the rather odd nature of the lava.

Eldgigur seen from the other side.

Technically this is a very large scoriae cone, at the top resides two small and one large crater. Only the larger crater has produced a minute lava flow. The rest of the volcano is built up of 3 layers of different lavas showing as 3 concentric slag walls.

The cone is built up by very fine grained material filled in with bombs. The first type of lava is a black plagioclase containing phenocrysts of clinopyroxene. The second layer is a grey more evolved and crystallized lava that is translucent. It is a type of plagioclase-porphyric lava. The third lava is named as bytownite, be that as it may, the content of iron is high in the red lava with 15 percent by volume. Inside all 3 types of lava are found layers of clear colorless olivine (forsterite).

The size of the grain is very small, 2 millimeters and downwards. Inside of this are lava bombs prolific. Especially the red lava and black lava seems to have produced a lot of lava bombs during eruptions. The grey translucent lava and the forsterite seems to have produced significantly less lava bombs.

The lavas point to a totally atypical form of eruption for the Grimsvötn line. The eruptions seems to have been very explosive for the size of this volcano. Noteworthy is that Eldgigur shows no sign of being affected by ice, nor water, so all 3 of the eruptions has happened after the end of the latest ice age. Ice would have affected the shape of the volcano due to its loose lavas, and water would have weathered the olivine.

Iceland is famous for having 27 active volcanoes. This is the believed number of volcanoes that has erupted after the last ice age, and has the ability to erupt again. Clearly Eldgigur has had 3 eruptions after the last ice age, and due to the lack of weathering of the olivine, the last of them should have been during the last 2000 years, probably a lot later than that. So, I think I can safely say that Iceland just got a number 28 to worry about.

With weathering of olivine I mean that pulverized forsterite will decay from a member of the olivine family into a member of the carbomagnesian family due to magnesiums very reactive capabilities as it gets into contact with water and CO2. A centimeter thick layer of powdered forsterite will decay completely within 3 to 5 years if it is left in the open air. So, you can actually date it.

http://2dgf.dk/xpdf/bull-1952-12-2-222-226.pdf

CARL