Volcano-Ice Interactions at Öraefajökull, Iceland
[*Dave McGarvie*] (Department of Earth Sciences, The Open University, UK, MK7 6AA; email d.mcgarvie@open.ac.uk); John Stevenson (Department of Earth Sciences, The Open
University, UK, MK7 6AA; e-mail johnalexanderstevenson@yahoo.co.uk).
Iceland has few stratovolcanoes, but its largest (Öraefajökull) is considered Europe’s secondlargest after Etna (Thorarinsson, 1958). Its prominence is further emphasised by a rhyolite
nunatak on the caldera rim, which at 2,119 m elevation constitutes Iceland’s highest land. The
upper c.1,000 m of the stratovolcano is mostly ice covered, whilst the summit area comprises
a 2.5 km diameter caldera filled with ice up to 500 m thick (Björnsson, 1998). Nothing is
known about the eruption(s) that formed the caldera.
Rhyolites are locally abundant at Öraefajökull, and form nunataks that surround the summit
caldera and pierce the ice-clad upper slopes, as well as substantial lava outcrops on the lower
flanks – some of which reach the foot of the edifice. All outcrops that have been examined
show evidence of interactions between erupting magmas and ice/snow.
The stratovolcano’s flanks are incised by numerous valley glaciers and one of these
(Kviárjökull) is the location of the only detailed volcanological study undertaken at
Oraefajokull (Stevenson et al., 2006). Volcano-ice interactions at Kviárjökull tell a tale of
substantial changes in ice thicknesses (of up to 500 m) in the valley glacier. At Kviárjökull a
well-preserved feeder dyke provides unequivocal evidence for flank eruption, while there is
also evidence of ridge-bounded flows similar to those reported at Mount Rainier by Lescinsky
and Sisson (2000). An unusual feature of some subglacial rhyolite eruptions at Kviárjökull
(compared to North American stratovolcanoes) is the preservation of abundant tephra,
although the significance of this has yet to be investigated.
Reconnaissance fieldwork was carried out in summer 2006 as a precursor to a new project
which will establish a timeline for the glaciovolcanic and geochemical evolution of
Öraefajökull by combining Ar-Ar dating with physical volcanology and geochemistry. This is
a methodology that has proved successful at other Icelandic volcanic systems (e.g. McGarvie
et al., 2006; McGarvie et al., in press).
The reconnaissance fieldwork has highlighted the need for a more thorough investigation of
those lava outcrops on Öraefajökull’s lower flanks which may represent a new type of
glaciovolcanic formation. These substantial lava outcrops, some of which reach the foot of the
edifice, may have originated as simple ridge-bounded flows at high elevations where ice was
thicker, but when they encountered a combination of thinner ice and steeper topography
and/or drainage channels in distal locations and at lower elevations, they became laterally
unconfined and were then able to intrude ice-edifice interfaces as sill-like sheets and lobes.
References
Bjornsson, H. (1988) Hydrology of Ice Caps in Volcanic regions. Soc. Sci. Islandica. Volume
45, 139 pp, Reykjavik.
Lescinsky, D.T. and J.H. Fink. (2000). Lava and ice interaction at stratovolcanoes: Use of
characteristic features to determine past glacial extents and future volcanic hazards. J.
Geophys. Res., 105, B10, 23711-23726.
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, 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., 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.
POSTER/ORAL
CORRESPONDING AUTHOR: DAVE McGARVIE