Jökull - 01.12.1999, Page 73

The morphogenesis of kettles in the Höfðabrekkujökull
forefield, Mýrdalssandur, Iceland
Antoni Olszewski and Piotr Weckwerth
Institute of Geography, Nicholas Copernicus University, Fredry 6/8, 87-100 Torun, Poland
Abstract — Sixteen melt-out kettles located in the southem part ofthe near forefield ofthe Höfðahrekkujök-
ull outlet glacier were analysed. The kettles most likely fonned during the 1918 jökulhlaup as they are a geo-
morphological proof of sudden and catastrophic, but territorially limited, frontal deglaciation, linked with
the sudden outfiow of melt-waters and the resulting deposition oficebergs. However, we can not exclude that
the icebergs originated in the cores ofend moraines which were swept away by surging water.
INTRODUCTION
Höfðabrekkujökull (also called Kötlujökull) is the
main outlet glacier of the south-east margin of the
Mýrdalsjökull ice cap (Björnsson, 1979; Kriiger,
1994). The Höfðabrekkujökull forefield (Mýrdals-
sandur) is a sandur (alluvial) plain created by huge
floods (jökulhlaups) associated with subglacial vol-
canism beneath Mýrdalsjökull (Figure 1). The south-
em part of the forefield can be classified into five dif-
ferent levels in relation to the Sandvatn river (Figure
1B); the highest (I), high (II-III), middle (IV-VII), low
(VIII-IX), and the lowest (X), the current river basin.
Level VI has a length of about 1.2 km and a max-
imum width of about 250 m. It rises 5.5 m above
the adjoining plain. An integral part of level VI is
its lower fragment Vla containing kettles (Figures 2
and 3). The kettles cover a relatively small area of 90
m in length and 70 m in width, intersected by a small
outlet channel. The kettles are located directly west of
the so-called “main moraines” (Heim, 1983), which
have most likely been destroyed in this place during
subsequent meltwater floods.
Geomorphological mapping made it possible to
distinguish three morphogenetically different outwash
plains: flat (level VI), pitted (a small marginal part of
level VI), and collapsed and pitted (level Vla). Three
kettles occur on level VI and fifteen, including the
three largest, on the lower outwash plain, Vla. Ket-
tles 6 and 13 were too damaged to be analysed. The
slope between levels VI and Vla has a locally sinu-
ous course which is very distinct but barely 0.5 m in
height. This sinuosity is caused by chaotic distribu-
tion of the ice blocks, testified to by the inclination of
the deposits exposed in it.
The difference between the pitted outwash and
collapsed and pitted outwash lies in the surface struc-
ture. The pitted outwash contains kettle holes of vari-
ous sizes being created by ice melting in an uniformly
sloping plain (Goldthwait and Smith, 1968) whereas
the collapsed and pitted outwash formed by melting of
icebergs which were completely or nearly completely
covered by deposits (collapsed) or where the blocks
protruded above the surface (pitted). Goldthwait and
Smith (1968) emphasise that the degradation of em-
bedded ice leads to the development of “ undulating
gravel deposits with accordant flattopped residuals ”.
Such surfaces exist in the study area in the form of
small islands and narrow peninsulas.
The location of the 16 kettles (Figures 1-3) poses
a question on their morphogenesis. The closeness of
Höfðabrekkujökull’s snout and the position of the ket-
tle field in the prolongation of the end moraine from
the beginning of the 20th century, suggest the need to
consider the origin of glacier ice in this place. The
kettles could have been formed by melting glacier ice
which had remained in situ within the early 20th cen-
tury moraine. Situations where glacier ice is buried
JÖKULL, No. 47 71