Jökull - 01.01.2013, Blaðsíða 63

Surface deterioration of glacially abraded basaltic boulders by Fláajökull
(2004) gave dates AD 1966 or AD 1975, for the old-
est moraine ridge (site Ia). These dates are much
too young according to numerous sources of infor-
mation (Ahlmann and Thorarinsson, 1937; Thorarins-
son, 1943; Jaksch, 1975; Snorrason, 1984; Evans et
al. 1999; Da̧bski, 2002, 2007). Based on the work
of Da̧bski (2007), who tested the size-frequency ap-
proach on Fláajökull moraines and confirmed its ro-
bustness in dating post-LIA glacier landforms, we
conclude that failure of this approach can be attributed
to the small population of measured thalli.
We are aware that lichenometrical dating of
moraines is not precise, undergoes continuous devel-
opment and must be interpreted with caution (Da̧bski,
2002; Bradwell, 2009; Chenet et al., 2010, 2011).
Nevertheless, the date obtained with the use of the
mean of 5 largest thalli and a changing growth rate
(AD 1888), supports the notion that the oldest Fláa-
jökull moraine ridge, deposited by its most prominent
eastern lobe, occurred at the end of the 19th century.
A question remains unsolved whether it is represents
the LIA maximum of the glacier or, possibly, older
deposits have been eroded and thus are not available
for dating (Kirkbridge and Winkler, 2012).
Petrography
Abundant micro-fissures with sharp edges present
within the weathering rinds are probably inherited
from frost weathering (French and Gugliemin, 2000).
The lack of signs of chemical alteration within the
weathering rinds is counter to expectations, because
oxidation of iron compounds and dissolution of glass
within basaltic rinds is commonly found elsewhere
(Yoshida et al., 2011) and can develop within decades
following deposition (Etienne, 2002). In the Fláa-
jökull marginal zone, there were only few boulders
with reddish varnish, presumably resulting from ox-
idation, and fulfilling the size requirements for sam-
pling. Therefore we decided to conduct research on
basalts having light-coloured rinds (10YR 6/1). Eti-
enne (2002) identified a very similar colour of the
rind (10YR 5/6 and 7/3) resulting from dissolution
of glass. We do not exclude the possibility of this
process occurring on Fláajökull moraines; however,
we observed glassy compounds within the rinds with-
out signs of dissolution. Moreover, the sharp edges
of micro-cracks testify to mechanical breakdown of
rock, probably frost shattering rather than dissolution.
The colour therefore should be attributed to looser
structure of the weathering rinds and the presence of
air in the micro-pores.
The direction of fracturing parallel to the rock sur-
face allows us to infer the process of flaking (exfolia-
tion) which is in accordance with Etienne (2002) who
elaborated a model of exfoliation on boulder surfaces
subject to weathering in front of Sólheimajökull. He
observed micro-cracks parallel to the rock surface at
1–3 mm depth, which is much deeper than in our case,
and also observed flakes 2–3 mm thick on boulders
on moraines 150–200 years old. Micro-organisms oc-
cupy the cracks, thereby adding to their widening and
preparing the rock for frost shattering (Etienne, 2002).
In a sample taken from the Fláajökull moraine de-
posited about AD 1907 (site II), micro-cracks parallel
to the rock surface are found within the 0.6 mm thick
weathering rind at depths of 12–50 µm (Figure 5).
We interpret the micro-cracks as well as heavily shat-
tered crystals found below the rock surface (Figure
6), as signs of frost weathering. Micro-roughness of
glacially abraded basaltic surfaces (Figure 5) results
from erosion as well as from post-depositional weath-
ering processes. An increase in micro-roughness to-
wards the oldest moraine can be attributed to the de-
velopment of the weathering micro-relief.
Indices of relative age
There are no doubts that micro-fracturing is responsi-
ble for weaker rebound of the Schmidt hammer (lower
R-values) on older moraines, as Schmidt hammer
readings are extremely sensitive to discontinuities in a
rock (Goudie, 2006). The coefficient of determination
R2 = 0.55 (Figure 9) shows that the micro-fissured
rind thickness probably controls R-values. Results
from test site IV seem anomalous, because surpris-
ingly thick weathering rinds are accompanied by rel-
atively high R-values. If we omit site IV, assuming
possible mistake in the data collection (inconsistency
in petrography of sampled boulders), the coefficient
of determination is much higher R2 = 0.85.
A very high diversification of micro-roughness
parameters within one test site, can be explained by
textural inhomogenities of basalts, differential glacial
JÖKULL No. 63, 2013 63