Reconstructing ice thicknesses
When magma is deep underground (e.g. in the magma chamber), water, carbon dioxide and other volatiles (volcanic gases) are dissolved in the melt due to the massive pressure provided by the overlying rock.
However, as the magma rises to the surface, the overlying pressure is reduced and this allows the volatiles to come out of solution and form bubbles within the magma. The lower the pressure gets, the more bubbles there and the bigger they become. In turn, this reduces the amount of volatiles that are still dissolved in the magma
When magma rises under a glacier or ice cap, the weight from the ice provides additional loading pressure and therefore hinders degassing. The thicker the overlying ice is, the more pressure is exerted, the less the magma can degas and the more dissolved volatiles end up retained in the magma.
The relationship between water solubility and pressure is well understood. We can therefore, measure how much dissolved water is left in the erupted deposits (lava), to estimate what the pressure was when the lava cooled and therefore how thick the ice was when the eruption occured.
I use fourier transform infrared spectrometry (FTIR) to measure dissolved water content and VolatileCalc by Newman and Lowenstern (2002) to convert water concentrations into pressures.
However, nature has a habit of being complicated, so ideally one should not just rely on one water measurement to reconstruct an ice thickness. The more samples that are measured, the more reliable the result - especially if samples are collected from a good range of elevations.
If the volcano has erupted under a relatively flat topped glacier, then samples should show a decreasing trend of water content with elevation, as lava near the summit will have erupted under less ice (and therefore less pressure) than lava erupted at the base of the volcano.
We can use software such as VolatileCalc to estimate what water content we would expect at each elevation for a given ice thickness. This produces a solubility pressure curve (see right). We can then compare our measured dissolved water contents against a SPC until the most appropriate ice thickness is found which represents the whole sample-set.
If you have any comments or questions, please post them on the bottom of the page or contact me
Recommended reads:
Owen, J. (2013) Volatiles in Icelandic subglacial rhyolite, PhD thesis, Lancaster University
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.
Tuffen, H., Owen, J., and Denton, J. (2010) Magma degassing during subglacial eruptions and its use to reconstruct palaeo-ice thicknesses, Earth-Science Reviews, 99(1-2), 1-18.
Jones, J. G. (1969) Pillow lavas as depth indicators, American Journal of Science, 267(2), 181-195.
Moore, J. G. (1965) Petrology of deep sea basalt near Hawaii, American Journal of Science, 263(1), 40-52.