Silicic Glaciovolcanism in Iceland
[*Dave McGarvie*] (Department of Earth Sciences, The Open University, UK, MK7 6AA; email d.mcgarvie@open.ac.uk)
Iceland contains an abundance and diversity of silicic glaciovolcanic edifices. For the past
nine years a multi-faceted and multi-institutional UK-based research programme has been
unravelling the key processes responsible for silicic glaciovolcanic edifice formation in
Iceland. Five themes (below) summarise and contextualise the programme, and provide
forward directions for future work.
(1) Tuyas.
These ice-confined edifices are 300-900 m high and form during sustained eruptions into
thick ice. Individual tuyas can be up to 3.5 km3 in volume, but 0.5-1.5 km3 is more typical
(McGarvie et al., 2006). An initial phreatomagmatic phase builds a tephra pile up to 350 m
high within a well-drained vault (Tuffen et al., 2002; Stevenson, 2004). Gradual upwards
increases in highly-inflated clasts points to decreasing meltwater interaction as the tephra pile
grows (Stevenson, 2004). A final effusive phase creates a lava cap up to 250 m thick.
(2) Effusion-dominated edifices.
Typified by the small-volume (<0.1 km3) drape of Bláhnúkur (Tuffen at al., 2002) and the
larger volume (0.6 km3) edifice of Prestahnúkur (McGarvie et al., in press). Bláhnúkur is
unusual because uprising magma was dispersed into multiple subsurface conduits which then
produced distinctive conical lava lobes at the edifice-ice interface. Prestahnúkur is unusual
because there is good evidence of substantial magma-water interactions at the start of the
eruption. Despite volume differences three common features of Bláhnúkur and Prestahnúkur
are: (a) clast textures indicate that fragmentation was dominated by quenching; (b) ice
completely covered the edifices throughout the eruption; (c) space was episodically created at
the ice-edifice interfaces, allowing (for example) extrusion of substantial lava bodies.
(3) Volcano-ice interactions through time.
After establishing a sound physical volcanology footing, an additional investigative technique
was added – using Ar-Ar dating to provide timelines. Three carefully-selected volcanic
centres were studied with the aim of evaluating glaciovolcanism through time: Torfajökull
(McGarvie et al., 2006); Kerlingarfjöll (Flude, 2005); and Ljósufjöll (Flude et al., under
review). At Torfajökull there is a pleasing correlation between the eruption ages of rhyolite
tuyas and cold periods in the oxygen isotope record (McGarvie et al., 2006).
(4) Eruptions at stratovolcanoes.
Iceland largest stratovolcano (Öraefajökull) is a 2,119 m high glaciovolcanic edifice with its
upper c.1000 m ice-covered, and a c.4 km diameter summit caldera filled with ice up to 500
m thick (Björnsson, 1998). Silicic rocks occur as nunataks and as substantial flank
outpourings. The first-ever volcanological study, by Stevenson et al., (2006), discovered
substantial ice thicknesses changes (of up to 500 m) between eruptions. Recent fieldwork has
extended this work, and future plans involve blending physical volcanology and Ar-Ar dating
to unravel the evolution of this massive glaciovolcanic edifice.
(5) The Undiscovered Country.
The future of silicic glaciovolcanism looks rosy, if the number of unresolved problems
provides a measure of its health. Three examples are given. Firstly, the relative ease and
higher precision with which silicic rocks can be Ar-Ar dated currently gives them the edge
over basalts regarding useful information that can be provided on past ice sheet thicknesses
during specific glacial periods. Secondly is the much higher diversity of silicic tuya-marginal
formations (relative to mafic counterparts), in which unusual volcano-ice interactions are
preserved (e.g. Tuffen et al., in press). And thirdly, there is the challenge of understanding
silicic volcano-ice interactions at ice-capped stratovolcanoes such as Öraefajökull, where
early indications are that new models to explain volcano-ice interactions may need to be
developed.
References
Bjornsson, H. (1988) Hydrology of Ice Caps in Volcanic regions. Soc. Sci. Islandica. Volume
45, 139 pp, Reykjavik.
Flude, S. (2005) Rhyolite volcanism in Iceland: timing and timescales of eruption.
Unpublished PhD thesis, University of Manchester, UK, 257pp.
Flude, S., R. Burgess and D.W. McGarvie (under review). Eruptive History of silicic
volcanism at Ljosufjoll Volcano, Iceland.
McGarvie DW, Burgess R, Tindle AG, Tuffen H, and Stevenson JA (2006). Pleistocene
rhyolitic volcanism at the Torfajokull central volcano, Iceland: eruption ages,
glaciovolcanism, and geochemical evolution. Jokull 56: 57-75.
McGarvie DW, Stevenson JA, Burgess R, Tuffen H and Tindle AG (2007). Volcano-ice
interactions at Prestahnukur, Iceland: rhyolite eruption during the last interglacial-glacial
transition. Annals of Glaciology. In press.
Stevenson, J.A. 2005. Volcano-ice interaction at Oraefajokull and Kerlingarfjoll, Iceland.
Unpublished PhD thesis, The Open University, Milton Keynes, 330 pp.
Stevenson, J.A., D.W. McGarvie, J.L. Smellie and J.S. Gilbert. 2006. Subglacial and icecontact volcanism at the Oraefajokull stratovolcano, Iceland. Bull. Volcanol., 68, 737-752.
Tuffen, H. 2001. Subglacial rhyolite volcanism at Torfajokull, Iceland. Unpublished PhD
thesis, The Open University, 381 pp.
Tuffen, H., J.S. Gilbert and D.W. McGarvie 2001. Products of an effusive subglacial rhyolite
eruption: Blahnukur, Torfajokull, Iceland. Bull. Volcanol 63, 179-190.
Tuffen, H., D.W. McGarvie, J.S. Gilbert and H. Pinkerton 2002. Physical volcanology of a
subglacial-to-emergent rhyolitic tuya at Raudfossafjoll, Torfajokull, Iceland. Geological
Society, London, Special Publications, 202, 213-236.
Tuffen, H., D.W. McGarvie., H. Pinkerton., J.S. Gilbert., J.A. Stevenson and R. Brooker.
(2007). An explosive-intrusive subglacial rhyolite eruption at Dalakvisl, Raudufossafjöll,
Iceland. Bulletin of Volcanology (in press).
INVITED ORAL
CORRESPONDING AUTHOR: DAVE McGARVIE