Jökull - 01.11.1998, Blaðsíða 25

1987. No evidence of re-advance was observed from
the LIA maximum until 1987, although a slower re-
cession occurred during 1960-1987.
No lichens grow on the moraines in front of Stal-
lurinn outlet glacier. It is therefore impossible to state
exactly the date of the outermost extent of this outlet.
Precisely 20 moraines cross the line rnarked A-B on
Figure 3 reflecting its retreat history. However, by
comparing aerial photographs from 1960 to 1986, at
least a part of this history can by drawn. It is apparent,
despite the age of the outermost moraine, that the
glacier retreated 2 km from its outermost extension to
the 1960 position (Figures 3 and 6). In the early 1960s
the glacier began an advance, which lasted at least
until 1987, when the study period ended. The glacier
advanced approximately 50 m during the period
around 1960 to 1987 (Figure 6).
Area and thickness of the Eiríksjökuli ice cap
The area of the Eiríksjökull ice cap has decreaced
considerably during the last 110 years. Maps from
Thoroddsen (1901), measured 1880 to 1898, and from
the Army Map Service (1951), measured from aerial
photographs taken in 1945 and 1946, were used to
map the ice cap fluctuations. In addition, measure-
ments from Thórarinsson (1943), Williams (1986)
and Bjömsson (1978) were also used. To establish the
difference between the maximum and minimum area
of the glacier in historic times, the area was measured
from a map surveyed in 1986,1987 (Icelandic Geode-
tic Survey, 1988) and by geomorphic evidence (outer-
most moraines) obtained from aerial photographs
(Table II). The measurements show that the Eiríks-
jökull ice cap was 49.4±0.4km2 during the LIA (1880-
Table II. Area of the Eiríksjökull ice cap during the last 110
years.
- Flatarmál Eiríksjökuls síðustu 110 árin.
Year Area (1
1880-1884 49.4*
1938 23.3
1945 22.0
1980 23.8
1986-1987 23.8
According to geomorphic evidence on aerial photographs.
- Samkvæmt landslagsummerkjum á loftmyndum.
1884), but only 22 km2 in the early sixties, and
23.8±0.5km2 in 1987. This gives a maximum reduc-
tion of 55% during 60 or 70 years following the max-
imum LIA glaciation and an overall reduction of 52%
100-110 years after the maximum advance of the
glacier. Most of the ice lost was located on Stallurinn
(Figure 6).
Ice thickness (/?) can be estimated assuming that the
basal shear stress is equal to yield stress for perfect
plasticity (Paterson, 1981). If we assume a perfectly-
plastic ice sheet of a total width of 2L we can write
/r = (2x0/pg)(L-x)
where x is the distance from the center of the icecap, x
Sand-supported
ca. 4 m Ép medium pebbles i Sandborin möl
to medium pebbles Siltborin möl
s o
oð I ,nr<re. rnhhles nr
boulders Hnullungar
planes Skerfletir \
The bottom of this unit is not known
Fig. 5. A schematic cross section of the Klofajökull stratigra-
phy, strongly indicating its depositional origin as an old rock
glacier. The upper unit is the debris cover, well known in ac-
tive rock glaciers, but the lower unit indicates the ice mass,
which in this case has melted away, leaving a bulk silt for-
mation. A very sharp contact, of only a few millimetres, dis-
cerns the units.
- Dœmigert þversnið Klofajökulsurðarinnar sem gefur
sterklega til kynna grjótjökulsuppruna. Efra lagið er urðin
sem hylur virka hluta jökulsins en neðra lagið sýnir hvar
ísinn lá undir efnishulunni. Hann hefur síðan hráðnað og
skilið eftir sig mjög mikla siltmyndun. Mörk þessara tveggja
myndanna eru mjög skörp, aðeins nokkrir millimetrar.
JÖKULL, No. 46, 1998
23