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
