Jökull - 01.01.2016, Page 18

Eyjólfur Magnússon et al.
B: The layer may be a product of polythermal condi-
tions with the glacier snout frozen to the further up-
glacier it is not. This may result in strong longitu-
dinal compression enabling ice folding and squeez-
ing of debris from the bed to the surface (Hambrey
et al., 1997). Evidences of polythermal environ-
ment have not been reported for Icelandic glaciers at
present. Winter snow cores drilled at 12 locations on
Drangajökull during the fieldtrip in March 2014, re-
vealed average core temperature varying from -1◦C
to -3◦C, manifesting the current mild winter condi-
tions on Drangajökull. DEM differencing for the win-
ter 2014–2015 (Belart et al., in open review, 2016)
revealed 6–7 m of winter snow at the snout in front
of Langahraun. Various aerial photographs and satel-
lite images show snow often covering the snout in late
summer and autumn. The insulation of high winter
accumulation at the glacier snout makes polythermal
glacier less likely during present conditions. The de-
bris layer may, however, have been formed in substan-
tially colder climate during the Little Ice Age (LIA),
particularly if combined with less winter accumula-
tion at the snout. Evidences of polythermal environ-
ment during LIA has been reported for Tungnafells-
jökull ice cap in central Iceland (Evans, 2010).
C: The ice mass beneath the internal debris layer
was separated from the ice cap at some time in the
late Holocene (Figure 13). A close analogue to
such a situation is the area west of Drangajökull at
present, where several ice patches persist separated
from Drangajökull (e.g. an ice patch in the upper left
corner of Figure 12a). These ice patches, typically
facing towards west and south, are probably retained
by snowdrift (Figure 11). At some later time the
glacier advanced due to cooling or wetter climate.
When the debris-rich advancing glacier front reached
the ice patch, instead of pushing the ice patch forward,
the advancing glacier slid over the stagnant ice leav-
ing a debris layer between the two ice masses. The
debris at the glacier front most likely facilitated the
sliding over the ice patch, similar to what is found at
Mýrdalsjökull ice cap, South Iceland (Krüger, 1993).
Additional debris was conveyed to the surface where
it continued to accumulate. This explanation for the
formation of Langahraun, may also explain thick de-
bris covers near the snout of other temperate glaciers
with relatively little ice motion as well as the forma-
tion of hummocky moraines associated with glaciers
in similar environments.
Figure 13. A schematic figure with longitudinal pro-
files of a glacier at different times explaining how the
debris patches on the surface of SW-Drangajökull,
called Langahraun may have formed. Black arrows
indicate ice motion near the bed and where the glacier
slides over the old ice patch. Red dots indicate a
debris layer, functioning as conveyer of debris from
the bed to the surface. – Skýringamynd sem sýnir
hvernig Langahraun gæti hafa myndast. Myndirnar
sýna langsnið upp jökulinn á ólíkum tímum. Einhvern
tíma seint á Nútíma (tímabilið eftir ísaldarlok) var
jökullinn aðskilinn í meginjökul og jökulfönn. Vegna
kólnandi veðráttu gekk meginjökullinn fram uns hann
náði að jökulfönninni, skreið upp á hana og ýtti urð
á undan sér yfir jökulfönnina. Eftir það er skrið-
flötur milli gömlu jökulfannarinnar og meginjökulsins
sem dregur með sér efni eftir þessum skriðfleti (rauð
punktalína) frá botni upp á yfirborð þar sem urðin
heldur áfram að safnast upp.
18 JÖKULL No. 66, 2016