Introduction to subglacial rhyolite

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| Introduction | Effusive subglacial rhyolite eruptions | Subglacial-to-emergent eruptions | Models of melting patterns during eruptions |

Distribution To date, subglacial rhyolite has only been found in Iceland, where it occurs at central volcanoes (e.g. Torfajökull) and fissure zones (e.g. Prestahnúkur). Simpified geological map of Iceland, indicating the main regions of subglacial rhyolite. Image modified from Anthony Newton's website. Click on map above for a bigger version. Approximately 10 % of Iceland's volcanoes are rhyolitic, and many have interacted with ice during glacial periods. Icelandic volcanoes capable of future subglacial rhyolite eruptions include Katla, Oraefajokull and Hofsjokull. Click here to check the latest earthquakes - which may be the precursor to an eruption. Broadly speaking, subglacial rhyolite can be found either in monogenetic tuyas and subglacial ridges/domes (e.g. at Torfajokull and Kerlingafjoll) or as units within complex lithofacies at stratovolcanoes such as Oraefajokull (Stevenson et al. 2006), where a range of magma types is erupted in subglacial and subaerial environments.

Potential hazards Explosive eruptions Silicic ash shards widely distributed throughout northern Europe are attributed to subglacial rhyolite eruptions in Iceland (e.g. Lacasse et al. 1995, Zielinski et al. 1997) . It thus seems likely that subglacial rhyolite volcanoes are capable of major explosive eruptions. Field studies of subglacial rhyolite tuyas at Torfajökull and Kerlingafjoll indicate that fine-grained ash is formed during phreatomagmatic explosions within ice vaults (Tuffen et al., 2002b, Stevenson 2004). If the eruption remains explosive once the ice surface is breached, subaerial tephra may be dispersed on the glacier surface and elsewhere. However, it is unlikely that the subaerial phase of eruptions will be recorded close to the vent. Models of how melting and deformation of ice may control the mechanism of subglacial rhyolite eruptions will be coming out soon (Tuffen et al. Ann Glac in press). Click here for a pdf of this paper. Jökulhlaups Meltwater tends to drain away from the site of subglacial rhyolite eruptions (see right), hence the magnitude of jökulhlaups is likely to be much smaller than those formed during basaltic eruptions (Tuffen et al. 2001). Instead, the peak discharge is likely to be similar to the maximum melting rate. Note that little is known for sure about the hydrology of subglacial eruptions - it's an exciting but murky research area!

Differences from subglacial basalt Rhyolitic magma has different physical properties to basaltic magma: Lower temperature (850°C, vs. 1200°C) Higher viscosity (105-6 Pa s vs. 102-3 Pa s) The high temperature of basaltic magma means that it can melt sufficient ice (up to 14 times its own volume) to accommodate the injected magma (Hoskuldsson & Sparks 1997). This favours the accumulation of meltwater at the vent area, the formation of bedded hyaloclastite sequences (Smellie 1999) and generation of high-magnitude jökulhlaups. Cooler rhyolitic magma is unable to melt sufficient ice, leading to an increase in pressure which favours drainage of meltwater. Fragmental lithofacies mostly lack evidence for deposition within standing water (Tuffen et al. 2001, Tuffen et al. 2002 a,b) and more detailed studies at Kerlingafjoll (Stevenson 2004) show that subglacially-erupted tephra was dominantly transported by moist pyroclastic density currents. The higher viscosity of rhyolitic magma leads to higher aspect ratio lava bodies and a greater tendency for magmatic fragmentation. Perhaps even more importantly, the eruption rates of basaltic and rhyolitic magma may differ. A paper discussing these differences and including some modelling of melting of ice is in press in Journal of Geophysical Research (click here for a pdf).

Field observations of subglacial rhyolite volcanoes

• Furnes H, Fridleifsson IB, Atkins FB (1980) Subglacial volcanics - on the formation of acid hyaloclastites. J Volcanol Geotherm Res 8:95-110
  

• Smellie JL (1999) Subglacial eruptions. In: Sigurdsson H. (Ed.) Encyclopaedia of volcanoes. Academic Press, pp 403-418.
  

• Tuffen H, Gilbert JS, McGarvie DW (2001a) Products of an effusive subglacial rhyolite eruption: Bláhnúkur, Torfajökull, Iceland. Bulletin of Volcanology 63, 179-190.
  

• Tuffen H, Pinkerton H, Gilbert JS, McGarvie DW (in press a) Ice-melting patterns during small-volume subglacial volcanic eruptions: evidence from Bláhnúkur, Iceland. Sedimentary Geology Special Issue: Modern and Ancient ice-marginal landsystems (A.J. Russell & F.S.Tweed, eds.).
  

• Tuffen H, McGarvie DW, Gilbert JS, Pinkerton H (in press b) Physical volcanology of a subglacial-to-emergent rhyolitic tuya at Rauðufossafjöll, Torfajökull, Iceland. Journal of the Geological Society of London special edition: Volcano-ice interaction on Earth and Mars (J.L. Smellie, ed.).
  

Thermodynamics of eruptions

• Tuffen H (in preparation) On the melting, deformation and filling of subglacial cavities during eruptions.
• Hoskuldsson A, Sparks RSJ (1997) Thermodynamics and fluid dynamics of effusive subglacial eruptions. Bull Volcanol 59:219-230
  

Silicic tephra in Northern Europe

• Dugmore AJ, Larsen G, Newton AJ (1995) 7 Tephra isochrones in Scotland. Holocene 5:257-266
  

• Lacasse C, Sigurdsson H, Johannesson H, Paterne M, Carey S (1995) Source of Ash-Zone-1 in the North-Atlantic. Bull Volcanol 57:18-32
  

• Zielinski GA, Mayewski PA, Meeker LD, Gronvold K, Germani MS, Whitlow S, Twickler MS, Taylor K (1997) Volcanic aerosol records and tephrochronology of the Summit, Greenland, ice cores. J Geophys Res 102:26625-26640
  

• Hafliðason H, Eiríksson J, VanKreveld S (2000) The tephrochronology of Iceland and the North Atlantic region during the Middle and Late Quaternary: a review. J Quat Sci 15:3-22