Jökull - 01.01.2012, Síða 133

Reviewed research article
Removing the ice cap of Öræfajökull central volcano, SE-Iceland: Mapping
and interpretation of bedrock topography, ice volumes, subglacial troughs
and implications for hazards assessments
Eyjólfur Magnússon1,2, Finnur Pálsson2, Helgi Björnsson2 and Snævarr Guðmundsson2
1Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland
2Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, IS-101 Reykjavík, Iceland
Corresponding author: eyjolfm@hi.is
Abstract – The ice covered active Öræfajökull central volcano forms a mountain range extending south from
the central Vatnajökull ice cap, SE-Iceland. The high elevation span (7–2110 m a.s.l.) and extreme precipitation
of this coastal part of Vatnajökull results in large mass turnover and high dynamic activity. Here we present
bedrock and surface digital elevation models (DEMs) of Öræfajökull ice cap and its many outlets. The bedrock
DEM is derived from radio echo sounding profiles and point measurements carried out in 1991–2012, and
the surface from airborne LiDAR surveys in 2010–2011. At the centre of Öræfajökull is a ∼14 km2 caldera
containing 4.3 km3 of ice, reaching ice thickness of 540 m. Most of the caldera drains meltwater eastwards to
Kvíá river while the rest drains mainly westwards to Virkisá river. The caldera floor of Öræfajökull is smooth
and volcanic mounds and ridges appear almost absent. An exception is a small topographic mound, beneath
∼400 m of ice, near the water divides between Kvíá and Virkisá. The bedrock topography also suggests a
separate caldera collapse, ∼6 km2 and ∼150 m deep, within the main caldera. The subglacial topography
implies an older highly eroded caldera north of the Öræfajökull summit (Hvannadalshnúkur 2110 m a.s.l.),
similar in size to the present main caldera. The outlets of Öræfajökull, currently in some places up to 550
m thick, have excavated troughs reaching as far as 220 m below current sea level. Based on estimates of the
present sediment transport rate in the rivers draining Öræfajökull, and the volume of the troughs, it would take
over ∼4000 years to form these troughs. Hence, it is unlikely that they were all excavated during the Little
Ice Age. Marginal lakes will continue to grow and new ones form in the troughs as the outlets retreat in the
coming decades, assuming current climate conditions or climate warming. The distribution of ice volume and
area with elevation is however quite different from one outlet to another, suggesting variable glacier response
to changing climatic conditions. A persistent temperature rise of 0.5–1.0◦C may cause the lowest outlets to
disappear completely, while the outlets with accumulation areas high up at the Öræfajökull caldera will survive
even the warmest predicted climate scenarios.
INTRODUCTION
Öræfajökull is an ice covered, active central volcano
at the southern edge of Vatnajökull ice cap in S-
Iceland (Figure 1). This area has the highest recorded
precipitation in Iceland (Crochet, 2007) on the cen-
tral plateau mass balance of ∼6–8 m water equivalent
(mwe) yr−1 has been typically observed (Guðmunds-
son, 2000). The ice cap is temperate with an eleva-
tion span of 7–2110 m a.s.l. (derived from LiDAR ob-
servations in 2010–2011, as discussed later) including
the highest peak of Iceland, Hvannadalshnúkur (2110
m). Ablation is negligible in the uppermost part.
JÖKULL No. 62, 2012 131