Jökull - 01.01.2016, Blaðsíða 45

First documented surge of Kverkjökull, central Iceland
1998; 2000) and even several decades (e.g. Frappé and
Clarke, 2007), although other authors have noted ‘sev-
eral years’ or ‘3 to 5 years’ (e.g. Dowdeswell et al.,
1991; Sund et al., 2009; Sevestre and Benn, 2015).
The part of a surge that is recognised in this study
via a terminus position advance lasted >3 years at
Kverkjökull (Figure 8). Whilst this active phase is
much longer than the 2 to 3 months typical of terminus
advances during the many surges of large lobate outlet
glaciers in Iceland (Björnsson et al., 2003), the annual
records by Sigurðsson published in Jökull show that
some surging glaciers in Iceland, such as the relatively
steep outlets of Drangajökull in the 1930s and 1990s
and Búrfellsjökull in 2002–2005 have advanced for
>3 years in relation to surges. Indeed, recent studies
on the surge history of Drangajökull have showed that
Drangajökull surges last for 4–6 years and can last for
up to 10 years (Brynólfsson et al., 2016).
The surge of Kverkjökull preceded a partial
drainage (∼30 m lake level draw down) of the geother-
mal lake Gengissig (Figure 1; Montanaro et al., 2016)
in August 2013. That drainage initiated a small
glacier outburst flood, or ‘jökulhlaup’ that routed sub-
glacially for 7.5 km along the length of Kverkjökull
and exited the glacier terminus from the Volga ice
cave (Figure 1). The close coincidence of these events
means that it could be speculated as to whether thin-
ning of the ice in the vicinity of Gengissig, in the
reservoir area of the surge (Figure 2C), was sufficient
to encourage the water in Gengissig to escape sub-
glacially, although there have been many outbursts of
water from Gengissig that have not been associated
with a surge.
Surge mechanism
Given the glacier geometry changes, surface morphol-
ogy changes and the timing and duration of the surge
as described above, and our knowledge of the study
site, we can speculatively consider the mechanism(s)
of the Kverkjökull surge. A hydrological mechanism
of surging associated with ‘a thick low gradient tem-
perate glacier that can have large englacial storage’
(Lingle and Fatland, 2003) or of ‘longer, wider, lower-
gradient glaciers’ (Clarke, 1991) are not present at
Kverkjökull. Our observations do not, however, rule
out a hydrological surge mechanism of some sort as
subglacial hydrology may be expected to play a major
role in all dynamical ice flow variations in the temper-
ate environment of Kverkjökull.
The magnitude of the surge in terms of elevation
changes, terminus position changes, volume changes
and surface velocity changes are relatively small in an
Icrelandic and a global context. We therefore spec-
ulate that they reflect just a single phase of a surge.
Phases of surges have been described for Trapridge
Glacier in Alaska by Kamb and Engelhardt (1987)
and for Ryder Glacier in Greenland by Joughin et al.
(1996). Some mini-surges may be distinct events or
precede a full surge (Raymond and Malone, 1986;
Kamb et al., 1985; Kamb and Engelhardt, 1987).
Overall, Kverkjökull has been shortening and
thinning over decades in the receiving area of the
surge. The magnitude and pattern of this ice mass
loss increased the glacier ice surface gradient and this
may have been involved in the triggering of this small
surge.
CONCLUSIONS
This study has described the first documented surge
of the glacier Kverkjökull. A surge of Kverkjökull
is conspicuous in an Icelandic context and also to a
lesser degree in a global context because it is a rather
steep alpine glacier. The surge occurred after decades
of persistent and recently accelerated glacier terminus
retreat. The surge initiated after 2008 and was still in
progress in 2013, immediately preceding the drainage
of the Gengissig geothermal lake and the subsequent
jökulhlaup in 2013. The Kverkjökull surge comprised
a simple transfer of mass from a higher elevation
reservoir area to a lower receiving area and caused
vertical surface displacements that were most promi-
nent in parts of the glacier >100 m thick. Asymmetry
in the response of the glacier terminus to the surge
front may be due to the (unresolved) internal dynam-
ics of the surge. The surge of Kverkjökull remains
unexplained in terms of mechanism.
More generally, this study has shown the utility of
high-resolution airborne laser scanner (ALS) surveys
and very high-resolution satellite images for making
novel observations and quantification of 3D geometry
changes on glaciers. These data have high vertical ac-
JÖKULL No. 66, 2016 45