Jökull - 01.01.2012, Page 143

Öræfajökull central volcano, SE-Iceland
Table 1. The dimensions of lakes that would form if ice in the ablation area of Öræfajökull were removed and
no sediment replacement occurred. Names of lakes refer to the name of the outlet glaciers (Figure 1) except the
lake Fjallsárlón in front of Fjallsjökull, which already has a name. The dimensions of a lake filling the caldera
of Öræfajökull if the ice cap were removed are given separately at the bottom of the table. – Flatarmál, rúmmál
og dýpi lóna sem myndast ef skriðjöklar Öræfajökuls hverfa án þess að setfylling komi í staðinn. Neðst eru
samsvarandi tölur fyrir stöðuvatn sem myndast ef ísinn í Öræfajökulsöskjunni hverfur.
Name Area (km2) Volume (km3) Max. depth (m) Average depth (m)
Morsárjökull lagoon 1.9 0.11 145 55
Skaftafellsjökull lagoon 10.8 0.76 230 70
Svínafellsjökull lagoon 5.1 0.61 320 120
Virkisjökull lagoon 0.8 0.02 70 25
Kvíarjökull lagoon 3.7 0.20 0 55
Hrútarjökull lagoon 0.6 0.01 25 15
Fjallsárlón 9.8 0.65 210 65
Total 33 2.4
Öræfajökull caldera lake 8.6 1.3 280 155
the glacier rivers draining from the outlet glaciers cov-
ering these troughs is ∼1.5 x 109 kg yr−1 (Tómasson,
1990). At these rates the rivers would need ∼4000
years to excavate these troughs in a pre-existing sed-
iment plain. An order of magnitude increase in the
sediment transport rate and an average sediment con-
centration in the rivers, on the order of tens of kg per
m3 of river discharge, would be required to narrow
the period of excavation down to the Little Ice Age.
The outlet glaciers of Öræfajökull do not surge nor
have significant jökulhlaups been reported from the
outlets overlying the largest troughs beneath Skafta-
fellsjökull, Svínafellsjökull and Fjallsjökull (jökul-
hlaups occurring during historic eruptions did not af-
fect these outlet glaciers). Such events are therefore
not likely to have aided significantly to the excavation,
in contrast to what may have been the case for other
troughs underneath outlets of S-Vatnajökull including
Breiðamerkurjökull (Björnsson, 1996), Hoffellsjök-
ull (Björnsson and Pálsson, 2004) and Skeiðarárjökull
(Magnússon, 2008). It is therefore likely that some of
the troughs underneath the Öræfajökull outlet glaciers
have existed since the settlement of Iceland (∼900
AD) and even much longer, either in form of lakes,
subglacial troughs or both. It may also be questioned
if troughs like the one underneath Svínafellsjökull
have ever been filled completely with sediments since
the last Ice Age maximum, given the small drainage
area of Svínafellsjökull above the present trough com-
pared to the volume of the trough.
ICE CATCHMENTS AND WATER
DRAINAGE BASINS
Results
The derived ice thickness and ice divides of the main
ice catchment basins of Öræfajökull are shown in Fig-
ure 8a. The thickest glacier ice within our study area
is in the ablation area of Skaftafellsjökull and Svína-
fellsjökull where it exceeds 550 m. The ice divides are
generally forced by the rugged topography. In the few
exceptions where drawing the ice divides is not trivial
we use a trace algorithm to obtain ice flow lines from
the LiDAR DEM. Small scale surface changes have
limited effects on the ice flow direction. The LiDAR
DEM was therefore degraded to 100 m x 100 m grid
size, prior to the tracing and filtered with a moving
circular window. The weight of the circular filter de-
creases linearly towards zero with distance from the
centre as suggested by Kamb and Echelmayer (1986).
The filter width was equal to the 5-fold ice thickness
at each location.
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