Jökull - 01.01.2016, Síða 37

First documented surge of Kverkjökull, central Iceland
Figure 5. Slope maps (A) for 2007 and 2011 both depicting increased crevassing and serac formation in 2011
compared to 2007. The leading edge of the most chaotic ice surface in 2011 is outlined with a white dashed line
in A and forms a coherent lobe shape. The combination of changing slope and changing ice thickness produced
a change in driving stress (B). In B, note the spatial pattern and zones of low (L), medium (M) and high (H)
relative changes in driving stress as annotated and delimited by dashed white lines. – Hallakort (A) sýna um-
fangsmeiri sprungusvæði og ísturna árið 2011 en 2007. Jaðar uppbrotna svæðisins árið 2011 er afmarkaður
með hvítri, slitinni línu í A sem sýnir að framhlaupið myndar tungu niður miðjan jökulinn. Breytingar í halla
og ísþykkt leiða hlutfallslega til lítillar (L), nokkurrar (M) og mikillar (H) breytingar í botnspennu á svæðum
sem afmörkuð eru með hvítum, slitnum línum (B).
to the alignment of ridges in the ice-free topography.
It is possible that the surge only affected the south-
westerly part of the terminus region for some dynam-
ical reason and that the increased crevassing and am-
plified hummocky ice-surface undulations are formed
at the boundary between ice affected and unaffected
by the surge. There are many examples in Iceland of
surges affecting only a part of the corresponding ice-
flow basin as further discussed below.
Therefore, we contend that the asymmetric pat-
tern of surface elevation changes in the terminus
area of Kverkjökull between 2007 and 2011 is best
explained by a hypothesis of a different speed of
surge front propagation between the north-eastern and
south-western portions of the terminus. The surface
expression of this more rapid front propagation in
the south-western part of the terminus area is more
widespread and more intensely hummocky surface
texture of the 2011 DEM, as depicted in slope maps
(Figure 5A) and elevation range maps (Figure 6), and
field photographs (Figure 7), as well as increased
depth of crevasses and height of seracs. Quantita-
tively, crevasses and seracs had local relief in adjacent
grid cells (i.e. over 10 m horizontal distance) of <8 m
in 2007 but up to 18 m in 2011, and local relief of
>5 m is found over 80% of the example transects in
2011 compared with <10% in 2007 (Figure 6). In the
field, those accessible had the form of stacked thrust
blocks revealed by exposed thrust planes between rel-
atively ‘clean’ and ‘dirty’ ice (Figure 7). The com-
bination of changing slope gradient (Figure 5A) and
changing ice thickness produced a change in the driv-
ing stress with a relatively complicated pattern, which
can be interpreted to have zones of relatively high and
low changes in driving stress (Figure 5B). Notably, a
‘corridor’ of relatively little change in driving stress,
JÖKULL No. 66, 2016 37