Bláhnúkur

See below for more information on the above

Walking

Bláhnúkur is a stones through from Landmannalaugar campsite. From here there is a beautiful walk up and over Bláhnúkur (route 2 on the following map). There is a clear footpath and stunning scenery for the duration of the route. My favourite way is to go straight up to the top of Bláhnúkur by the most northerly point. It's a 350 m ascent to the summit (from where there are beautiful panoramic views - see below). The footpath leads you back down to the western side. From here, there is a choice of two routes back to camp; either through the lava flow Laugahraun or between the lava flow and Bláhnúkur (I recommend both!). Both routes are about 5 km in total but allow a few hours as you may want to catch your breath and a few breath-taking photographs.

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What's it made up of?

The story of Tuffen et al., (2001)

Bláhnúkur consists of two main components: hyaloclastite and lava lobes

Hyaloclasite is a type of volcanic tephra that forms during magma-water interaction. When water interacts with magma it causes fragmentation, producing tephra. However, this can happen to various degrees. At Bláhnúkur the tephra is quite coarse and blocky in shape. This suggests that the fragmentation was by quenching but was not a particularly violent affair.

Lava lobes are mounds of rock that stick out of the hyaloclastite like warts. The fact that intact bodies of lava were preserved during the eruption of Bláhnúkur is again evidence that the eruption was not particularly violent. The lava lobes are crystalline in the middle but have a glassy (non crystalline) exterior of obsidian, which is columnar jointed. This suggests that the lava lobes were emplaced into a coolant. The two most likely options are that they were extruded into 1) ice cavities in the glacier base (Tuffen et al., 2001) or 2) water-logged hyaloclastite (Owen et al., 2012) - see also Stevenson et al., (2011). Perlite (hydrated obsidian) at the base of the lava lobes also suggests that they formed in a water-rich environment.

The hyaloclastite, columnar jointed obsidian and perlite, all suggest that the eruption of Bláhnúkur occurred in a water-rich environment. However, Bláhnúkur is a long way from the sea, has a steep topography and a high elevation. All of these suggest that a subaqueous (under water) eruption was unlikely (e.g. under the sea or under a lake). A subglacial eruption (under ice) is much more plausible. Considering that Bláhnúkur is 350 m high and these features appear consistently up to the summit, suggests that the glacier was at least 350 m thick at the time of the eruption.

Bláhnúkur erupted about 100,000 years ago, so there are no written records or photographs showing how thick the ice was. However, we know the ice must have been at least 350 m thick to cover the entire edifice (as all of the deposits show subglacial formation).

We can however, use science to tell us more than this. One of the components of magma is volcanic gas (mainly H2O and CO2). However, under the extreme pressures that are experienced by magma, these 'gasses' spend most of their time dissolved in the molten rock. It is only when the magma nears the surface that the pressure is low enough for the gasses to exsolve out of solution forming bubbles. However, if there is a glacier on top of the volcano, the weight of the ice will add to the pressure and therefore reduce the ability of the magma to degas. The thicker the ice is, the greater the pressure, the fewer bubbles will form and more 'gas' will remain dissolved in the magma. This relationship has been well studied. So we can measure the amount of H2O and CO2 left dissolved in the magma and convert it into a pressure using software such as VolatileCalc. Once we know the pressure that the magma was experiencing when it erupted we can estimate an ice thickness. (see the section 'reconstructing ice thicknesses' for more info).

We measured the dissolved H2O and CO2 content of samples collected from Bláhnúkur using FTIR (fourier transform infrared spectroscopy). We found that most sampling locations suggested as ice thickness of 400 m. This would mean that the summit was just 50 m from emerging through the glacier. However, some sampling locations were water-rich and water-poor compared to this trend. We believe that the water-rich samples (those from the Lobe Slope and Brandsgil) formed intrusively i.e. with additional loading from volcanic material (which would create greater pressure and therefore retain more H2O) and that the water-poor samples (from A-ridge) were under-saturated i.e. they did not have enough initial H2O to record the degassing trends (perhaps they were from a gas-poor part of the magma chamber). (see the section 'reconstructing the ice thickness at Bláhnúkur' for more info).

FTIR data suggests that Bláhnúkur formed under 400 m of ice. This is shown by data from Graenagil, the Feeder Dyke, the Northern Slope and the Top Ridge. By comparison the Lobe Slope and Brandsgil are water-rich and lower parts of A-ridge is water-poor. We think this shows intrusive formation and under-saturated magma respectively. From Owen, (2013).

So how thick was the ice at the time of the eruption?

The story of Owen et al., (2012)

Eruptive behaviour

The story of Owen et al., (2013)

Bláhnúkur is thought to have been a small, gentle eruption for the reasons discussed above. However, subglacial rhyolite eruptions have the potential to be very large and powerful. So why didn't Bláhnúkur erupt explosively?

We compared the composition, ice thickness and volatile content of Bláhnúkur to other more explosively formed Torfajökull edifices. We found that whilst the composition and inferred ice thicknesses were comparable to the more explosive edifices, Bláhnúkur had a considerably lower pre-eruptive volatile (gas) content. This was found by measuring the composition of melt inclusions (tiny pockets of trapped melt within phenocrysts) using SIMS (secondary ion mass spectrometry). (see the section on 'melt inclusions' for more info on the technique). We also found evidence for open system degassing at Bláhnúkur, in the form of high Cl/H2O ratios and microlite-rich glass. This suggests that although there was not much gas to start with, a lot of it was lost en-route to the surface. Again this is in contrast to the more explosive edifices where evidence suggests closed system degassing was at play. We therefore believe that it is volatiles (predominantly H2O and CO2) that are controlling eruptive behaviour at Torfajökull. The more gas in the magma, the more explosive the eruption. There was little gas left in the Bláhnúkur magma by the time it reached the surface, which is why we believe it was a gently, non-violent eruption.

a) Pre-eruptive H2O and CO2 data showing how Bláhnúkur is H2O-poor but CO2-rich compared to other Tofajökull edifices. b) a microlite-rich Bláhnúkur sample and c) a microlite-poor SE Rauðfossafjöll sample. Microlites form during degassing and slow ascent - both of these suggest open system degassing. From Owen, (2013).

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