Jökull - 01.01.2016, Blaðsíða 31

First documented surge of Kverkjökull, central Iceland
Figure 1. Location of Kverkfjöll in Iceland and sites mentioned in the text, detailing topography, large-scale
geological structure: curved white dashed lines approximate the caldera rim (Jóhannesson and Sæmundsson,
1998) and straight white dashed lines depict major fissure lines – structural fault lines (Carrivick, 2004). The
hill-shaded digital elevation model is that from September 2011 ALS data. – Kort sem sýnir Kverkfjöll og staði
sem nefndir eru í greininni og gefur yfirlit um jarðfræðilegar aðstæður: bogin, slitin, hvít lína gefur til kynna
brún Kverkfjallaöskjunnar (Jóhannesson og Sæmundsson, 1998), beinar, slitnar, hvítar línur sýna meginmis-
gengi á svæðinu (Carrivick, 2004). Skyggingin er byggð á leysimælingu úr flugi í september 2011.
The glacier Kverkjökull flows through an 800 m
wide gap – the ‘Kverk’ – in the northern caldera
rim. Kverkjökull is ∼10 km long, ∼18.5 km2 in area,
and extends from ∼1880 to ∼950 m a.s.l.. Thus, the
glacier is generally narrow and steep and ‘alpine’ in
contrast to the other lobate outlets of northern Vatna-
jökull. The terminus of Kverkjökull had several peri-
ods of minor advances both in the 1970s and the 1980s
(Sigurðsson, 1998) but retreated overall by 56 m from
1963 to 1971, 18 m from 1971 to 1993 and by 266 m
between 1995 and 2012 (Sigurðsson and Einarsson,
2014), leaving a series of sub-parallel, small (∼1 m
local relief) moraines that approximate annual termi-
nus positions. It is now ∼1 km behind its Little Ice
Age (LIA) position (Figure 1). The terminus supports
an ice cave which remains open all year round be-
cause the Volga river is partially fed by hydrother-
mal outflow from Gengissig, a geothermal lake sit-
uated on the western margin of the glacier accumu-
lation area. An extensive area of ice-cored moraine
lies within the proglacial area and immediately to the
north of the contemporary outwash plain. This ice-
cored moraine, a major part of which extends beyond
the LIA moraines, is conspicuous for its areal extent,
for its symmetric and sub-parallel ridges and for its
high debris content in comparison to the contempo-
rary surface of Kverkjökull.
The wider proglacial area of Kverkfjöll; ‘Kverk-
fjallarani’, holds abundant geomorphological and sed-
imentological evidence of Holocene jökulhlaups (Car-
rivick et al., 2004a,b; Carrivick and Twigg, 2005; Car-
rivick, 2007; Carrivick et al., 2009). Historically,
jökulhlaups from Kverkfjöll have occurred in 1959
(Jóhannsson, 1959), 1985, 1987, 1993, 1997, January
2002 (Sigurðsson and Jónsson, 1999; Sigurðsson et
al., 2002; Sigurðsson and Einarsson, 2005; Rushmer,
2006; Guðmundsson and Högnadóttir, 2009) and Au-
gust 2013 (Guðmundsson et al., 2013), and at least
the more recent of these events have been due to the
drainage of Gengissig (Rushmer, 2006; Guðmunds-
son et al., 2013). In both the proglacial zone and in
Kverkfjallarani there is no geomorphological or sedi-
mentological evidence that has been attributed to pre-
vious surges of Kverkjökull.
METHODS
Spot elevation measurements
A Leica GPS500 differential Global Positioning Sys-
tem (dGPS) was used to collect spot elevation mea-
surements in August 2007 and in August 2008.
Specifically, a base station receiver was set up on an
arbitrary point, and continuously recorded its 3D po-
sition at 1 s intervals for up to 8 hrs per day. These
3D positions were post-processed relative to data
from permanent Icelandic geodetic dGPS receivers at
Kárahnjúkar and Höfn and an average position, ac-
curate to ±0.5 cm was computed for the base station.
A rover receiver was used in Real Time Kinematic
(RTK) mode to collect 3D positions of our points
of interest; i.e. the 3D positions were calculated and
differentially corrected in real time with base station
data, the base and rover being linked by a radio. Rover
3D points include the 2007 and 2008 ice margin, tran-
sects of elevation of the glacier across the terminus
area, and some control points and transects along rel-
atively stable terrain in the proglacial area. They all
have a 3D accuracy of ±10 cm owing to the rover
moving (mounted on a back pack) during the survey.
Digital elevation models
A Digital Elevation Model (DEM) was produced us-
ing georeferenced Airborne Laser Scanning (ALS)
JÖKULL No. 66, 2016 31