Jökull - 01.01.2016, Side 43

First documented surge of Kverkjökull, central Iceland
Figure 10. Discharge as derived from continuous stage records for two rivers draining from northern Vatna-
jökull in the vicinity of Kverkfjöll. Only the Jökulsá á Fjöllum receives meltwater directly from Kverkfjöll
and has statistically higher discharge and seasonal volume of runoff in the years 2010, 2011, 2012 and 2013,
in comparison to the years 2008, 2009 and 2014. – Rennsli tveggja vatnsfalla frá norðanverðum Vatnajökli í
grennd við Kverkfjöll reiknað á grundvelli samfelldra mælinga á vatnshæð. Jökulleysing í grennd við Kverkfjöll
rennur til Jökulsár á Fjöllum sem hefur tölfræðilegra marktækt meira augnabliks- og uppsafnað rennsli árin
2010, 2011, 2012 og 2013 en árin 2008, 2009 og 2014.
DISCUSSION
Glacier geometry changes
To have two airborne laser scan (ALS) surveys of
a single glacier is unusual. For two ALS surveys
to span the timeframe of a surge is extremely for-
tuitous. In general, the pattern of surface elevation
changes on Kverkjökull as revealed by the difference
between the two ALS surveys demonstrates the dis-
charge of ice from within the northern-most caldera
of Kverkfjöll and this mass transport pattern (Figure
2C) is typical of surges in land-terminating temper-
ate glaciers (Björnsson et al., 2003; Murray et al.,
2003; Murray et al., 2012). The magnitude of sur-
face elevation changes that we have detected of up
to 20 m (Figure 2C) is modest when compared to the
more spectacular elevation shifts of ∼100 m in tide-
water glacier surges (see Table 2 of King et al., 2015)
and are low in comparison with what has been mapped
of elevation changes in other surging land-terminating
glaciers in Iceland (Björnsson et al., 2003; Magnús-
son et al., 2005; Aðalgeirsdóttir et al., 2005; Magnús-
son et al., 2016), on Svalbard (e.g. Kroppbreen: 40 m,
Sund et al., 2009), in the Karakoram (Gardelle et al.,
2013), but comparable to the vertical changes of surg-
ing land-terminating glaciers in Argentina (Pitte et al.,
2016) and NW Iceland (Brynjólfsson et al., 2016).
The asymmetry of elevation changes in the termi-
nus area of Kverkjökull (Figure 2C) is remarkable. A
possible physical reason for the differing propagation
of the surge between the north and south parts of the
terminus is a control of subglacial topography. How-
ever, as we have noted above, the alignment of the
boundary between the surge front and the inactive ice
does not correspond with the alignment of ice-free to-
pographic ridges. Our ice thickness model cannot be
used to settle this question as it is a simple extrapola-
tion from a centre-line analysis and so obviously will
not reveal subglacial bedforms laterally.
The differing surge progression could also be
caused by some internal dynamics of the surge as
mentioned previously. We note that there are many
examples of surges that only activated a part of the
corresponding ice-flow basin such as at Þjórsárjökull,
Hofsjökull ice cap, in 1991 and 1994, and surge
fronts that did not reach the glacier margin as at
Western-Hagafellsjökull, Langjökull ice cap, 1997–
1998 (Björnsson et al., 2003). The north-eastern-most
portion of the terminus area appears to have become
near-stagnant, prior to the surge, as evidenced by the
lack of any significant surface morphology in the form
of crevasses and longitudinal foliation, and indeed the
rather smooth texture of that portion of the terminus
in comparison with the south-western-most portion
(Figs. 2A). Relatively intense longitudinal compres-
sion was thus caused as the surge wave encountered
slow, if not near-stagnant ice, in the north-eastern part
of the terminus area.
JÖKULL No. 66, 2016 43