Jökull - 01.01.2016, Síða 16

Eyjólfur Magnússon et al.
Figure 11. The location of the surface crest
(blue line) and the bedrock ridge (red line).
The polar plot in the lower left is a precip-
itation rose showing the relative frequency
(%, numbers on concentric circles) of the
wind direction (direction of bars) associ-
ated with precipitation rates above 1 mm /
3 hours (bar colours) during winter (Oct.-
April, 1957–2011) from a downscaled at-
mospheric hindcast (Rögnvaldsson et al.,
2011) at the surface crest of the ice cap. –
Lega háhryggjar Drangajökuls frá Jökul-
bungu að suðurenda jökulsins á yfirborði
(blá lína) og botni (rauð lína) jökulsins.
Vindrósin sýnir hlutfallslega tíðni (%, fjar-
lægð frá miðju) vindáttar (stefna stólpa)
og vindstyrks (litakvarði) er hermd úrkoma
er yfir 1 mm/3 klst að vetrinum (október-
apríl), nærri háhrygg jökulsins á árunum
1957–2011 í RÁV-gagnasafninu (Ólafur
Rögnvaldsson og fl., 2011).
try. The maximum thickness for the delineated ice
catchments IV (Reykjarfjarðarjökull, Þaralátursjökull
and Ljótarjökull) and V (SE-Drangajökull) are actu-
ally west of the bedrock ridge. If the area west of the
ridge is not included, the corresponding max thick-
ness for catchments IV and V would be 138 m and
199 m, respectively instead of 180 m and 252 m.
Debris patches on SW-Drangajökull
Patches of rock debris near the SW margin of Dranga-
jökull are so prominent that they have been given a
name by the local people: Langahraun (Figure 12).
From optical imagery it is hard to distinguish whether
they are nunataks or debris layers covering glacier ice.
During the RES-survey in late March 2014 thick win-
ter snow covered Langahraun. It was traversed several
times during the survey without even being noticed
apart from rare small debris mounds sticking out of
the snow. RES-profiles typically reveal 40–60 m thick
ice beneath Langahraun confirming that it is a debris
layer on top of glacier ice.
The RES-profiles also show a faint reflection in-
dicative of a thin debris layer that can be traced from
the ice surface at the upper edge of Langahraun down
to the glacier bed over a distance of ∼300 m (Fig-
ure 12b). The weak reflection from that layer indi-
cates that the debris is much thinner than the debris
cover on the glacier surface. We conclude that Langa-
hraun is at least partly formed by delivery of debris
removed from the bed to the surface via this internal
debris layer. In order for the layer to function as a con-
veyer a discontinuity in the ice motion is required. To
explain the discontinuity in the glacier motion causing
the glacier above the layer to slide on the ice below we
propose three plausible explanations:
A: The discontinuity was formed by high shear strain
enabling fracturing through the whole ice block. Frac-
turing >50 m thick ice, requires substantially higher
glacier motion than at present on this relatively gently
sloping, thin glacier. Such fracturing may have hap-
pened during a surge. Other indications of surges for
this part of Drangajökull have not been reported.
16 JÖKULL No. 66, 2016