Jökull - 01.11.1998, Side 21

The uppermost part of the lava shield is richer in pla-
gioclase crystals and poorer in olivine crystals com-
pared to the lower part. The difference begins to occur
at approximately 1350 m a.s.l. This could indicate an
hiatus in volcanic activity. The lava shield forming
the summit of the mountain is petrographically de-
fined as olivine tholeiite (Guðmundsson, 1992).
The study area is located upland, about 600 m a.s.l.,
away from extreme maritime conditions of the coast.
According to Einarsson (1991) the mean summer tem-
perature is approximately 7°C (June-August, 1935-
1985) and the mean annual winter temperature is about
-6°C (January-March, 1935-1985). The mean annual
precipitation between 1966 and 1993 at Hveravellir
(600 m a.s.l.), about 50 km east of the study area, is
about 740 mm mainly concentrated in the winter
months (The Icelandic Meteorological Office, 1995).
DATING TECHNIQUES
AND DATA SOURCES
In order to date moraines, lichenometry was used
since other methods such as 14C and tephrochronology
were not possible due to lack of soil containing
tephras and organic material. All lichen dating are
based on measurements of the Rhizocarpon geo-
graphicum species, an approach pioneered by Beschel
(1950). The method used in this study has been de-
scribed by Innes (1985), and involved measurements
of the five largest lichens on each substrate. In each
case, the largest inscribed circle of the thalli (shortest
diameter) was measured in order to avoid abnormal
growth due to substrate surface characteristics (Innes,
1985, Guðmundsson, 1992). As no lichen curve was
available for the study area, a growth curve was con-
structed (Figure 2) from measurements on tombstones
and a bridge in the upper Borgarfjördur region, locat-
ed at about 64°43' N and 21°20' W at ca. 200 m a.s.l.
This calibration curve has a 19 year colonisation lag
time with a mean growth rate of 0.67 mm/a. The
Borgarfjörður reference site is located at about 400 m
below the actual measurement site around the Eiríks-
jökull ice cap and about 25 km further from the coast.
In order to minimize errors due to the climatic differ-
ence between these two sites (Figure 1), such as dif-
ferent precipitation and temperature conditions,
which greatly effect the lichen growth (Innes, 1985),
the lowest growth rate of the Borgarfjörður growth
curve was applied. This was obtained by dividing the
Borgarfjörður growth curve into four periods, 1890-
1912 (0.63 mm/a), 1912-1940 (1.01 mm/a), 1940-
1960 (0.24 mm/a) and finally 1960 to 1992 which is
considered to have a similar growth rate as during the
1940-1960 period.
Glacier positions were located on aerial photo-
graphs from 1938 (oblique), 1946, 1960, 1979, 1986
and 1987.
RESULTS
Geomorphology
The geomorphology of the Eiríksjökull table
mountain and surrounding area is greatly influenced
by recent volcanic and glacier activity. Geomorpho-
logically the present environment is highly dynamic
and is rapidly undergoing changes in terms of both
erosion and deposition. Landforms in the proglacier
area of Stallurinn are dominantly terminal and hum-
mocky moraines but include prominent megaflutes
Fig. 2. Growth calibration curve for the lichen Rhizocarpon
geographicum in the upper Borgarfjörður area, west central
Iceland. The thinner, curved lines mark the 95% significance
level for the true mean of the thallus diameter axis.
- Vaxtarhraðifléttunnar Rhizocarpon geographicum i efri hluta
Borgarfjarðar. Þunnar, bogadregnar línur sýna 95% öryggis-
mörk vaxtarlínunnar fyrir rétt meðaltal þvermálsássins.
JÖKULL, No. 46, 1998
19