Volatiles in Icelandic subglacial rhyolite
Research for my PhD was based at Torfajökull in south Iceland (see map). This is Iceland's largest rhyolitic centre and 70,000 years ago it produced Iceland's largest known eruption of subglacial rhyolite. This eruption was bigger than Mt St Helens and Pinatubo combined and would have blanketed much of Northern Europe in ash. The aim of my project was twofold:
1) To reconstruct, using the retained H2O concentration, how thick the ice was during the eruption of various subglacial rhyolitic edifices and evaluate the usefulness of this technique.
The dissolved H2O concentration was used to try and reconstruct the ice thickness for four subglacial rhyolitic edifices in Torfajökull (click here for the technique). This was achieved with varying degrees of success:
* At Bláhnúkur an ice thickness of 400 m was inferred which agrees well with independent estimates (click here for the story).
* At Dalakvísl there has been a lot of erosion which makes it very difficult to quantify an ice thickness
* At SE Rauðfossafjöll all the samples have degassed to atmospheric pressure
* At Angel Peak an ice thickness of 120 m was inferred which agrees well with independent estimates
I conclude that the dissolved volatile technique works well on small-volume edifices that erupted entirely beneath ice, in an effusive (non-violent) manner and where there has been little erosion since. As conditions stray from this ideal, it becomes increasingly more difficult to resolve an ice thickness using volatiles.
However, this technique of using dissolved H2O has offered so much more than ice sheet reconstructions. It has offered valuable inside into whether lava bodies erupted intrusively (within the eruptive deposits) or extrusively (in the cavity space under the glacier), how much erosion has taken place, what the original shape of the edifice was, where magma may have been gas-poor, how the meltwater drained and where there may have been exposure to atmospheric pressure. In short, this process has allowed us to gain a better understanding of the construction of subglacial rhyolitic edifices. Click here for the Dalakvísl case study.
2) To investigate the role of volatiles in determining the explosivity of subglacial rhyolite
The concentrations of various volatiles species (volcanic gasses such as water and choline) where measured in melt inclusions which are thought to preserve the magma chamber and therefore pre-eruptive conditions (click here for more information). We also attempted to reconstruct the degassing paths (i.e. whether degassing was closed system where the gasses stay within the magma, or open system, where the gasses are lost en route).
We did this for 5 edifices that erupted with contrasting styles. Click here for the story.
We also compared explosive and effusive deposits from Dalakvísl, which was thought to have experienced a transition in style. Click here for the story.
We found Icelandic rhyolite to be much more volatile-rich than previously thought. We also discovered that explosive subglacial rhyolitic volcanism seems to be fuelled by high pre-eruptive H2O concentrations and closed system degassing. By comparison effusive deposits have low pre-eruptive H2O concentration and open system degassing. No correlation was found with ice thickness. This suggests that at Torfajökull, at least, it is volatiles (volcanic gasses), not ice, which is controlling the explosivity of subglacial rhyolite.
However, it turns out that the 2 aims are intricately connected because....
* The ability to reconstruct ice thicknesses is dependent on the style of the eruption (success if favoured by small, subglacial, effusive edifices)
* Rapid changes in loading pressure (e.g. due to a jökulhlaup) could trigger a change in eruptive style. Click here for the story.
If you have any comments or questions, please post them on the bottom of the page or contact me
The literature behind it:
This webpage is a summary of the findings of Owen, (2013) Volatiles in Icelandic subglacial rhyolite, PhD thesis, Lancaster University; a project funded by NERC
Other recommended reads:
Owen, J., Tuffen, H. and McGarvie, D.W., (2012) Using dissolved H2O in rhyolitic glasses to estimate palaeo-ice thickness during a subglacial eruption at Bláhnúkur (Torfajökull, Iceland), Bulletin of Volcanology, 74(6), 1355-1378
Owen, J., Tuffen, H. and McGarvie, D.W., (2013a) Pre-eruptive volatile content, degassing paths and depressurisation explaining the transition in style at the subglacial rhyolitic eruption of Dalakvísl, South Iceland, Journal of Volcanology and Geothermal Research, 258, 143-162
Owen, J., Tuffen, H. and McGarvie, D.W., (2013b) Explosive subglacial rhyolitic eruptions in Iceland are fuelled by high magmatic H2O and closed system degassing, Geology, 41(2), 251-254
Owen, J., Tuffen, H. and McGarvie, D.W., (in prep.-a) Magma degassing in the effusive-explosive subglacial rhyolitic eruption of Dalakvísl, Torfajökull, Iceland: insights into palaeo-ice thickness, quenching pressures and edifice erosion.
Owen, J., Tuffen, H. and McGarvie, D.W., (in prep.-b) On what type of volcanic edifice can you use the magma degassing technique for reconstructing palaeo-ice thicknesses?
