Jökull - 01.01.2013, Blaðsíða 61

Surface deterioration of glacially abraded basaltic boulders by Fláajökull
55 years (Thompson and Jones, 1986), and given 10
years for colonization lag time, we obtained a date AD
1888 for site Ia, which correspond well with previous
dating of Da̧bski (2002, 2007).
Petrography of basalts and weathering rinds
Petrographic analyses revealed that the studied rocks
are fine-grained, hypocrystalline basalts. There are
abundant crystals of plagioclase and pyroxenes, apart
from which there are some amorphous glassy com-
pounds including very small precipitations of apatite,
ferrous oxides and sulphides. The texture of the
basalts is massive or fluidal in places with a very
low porosity. Within the weathering rinds, which are
light-gray colored (10YR 6/1 according to the Mun-
sell scale), one can observe increased porosity due
to numerous micro-fissures developed between and
within crystals (Figure 5). In weathering rinds of two
rock samples taken from a moraine deposited around
AD 1907 (site II), no chemical alteration is observed,
although some reddish grains, typical of iron oxi-
dation occur. They were previously misinterpreted
as signs of oxidation of ferrous compounds of rock
due to chemical weathering (Da̧bski, 2012b). How-
ever, they are dispersed throughout the whole thin-
section, and SEM and microprobe analyses show that
within the weathering rinds, crystals of titanomag-
netite and ilmenite remain chemically unchanged and
no hematite was found. The reddish colour can be ex-
plained by mineral inclusions within the glassy com-
pounds. Heavy fracturing (with sharp edges of micro-
cracks) is clearly visible within the weathering rind,
especially close to the outer boundary of the rock,
where cracks parallel to the rock surface can be ob-
served at depths c. 20 µm (Figure 5). Some minerals
exhibit especially heavy fracturing leading to a pro-
duction of closed cavities within the rind (Figure 6).
Micro-roughness, weathering rind thickness and
rebound values
The study revealed that there is a significant diver-
sification of micro-roughness between rock surfaces
within a single experimental site (Figure 7, Appendix
1). Micro-roughness parameters Ra and Rz from two
youngest test sites (VII+ and VI) are statistically sig-
nificantly different from parameters obtained from
older sites (Figure 7, Appendix 2). The Rzmax val-
ues follow similar pattern with the exception for sites
VI, II and Ib which are characterised by statistically
insignificant differences. Mean values of roughness
parameters increase from the youngest moraines (site
VII+) to the oldest (site Ia): Ra from 4.03±0.37 µm
(95% confidence interval for the mean) to 5.44±0.44
µm, Rz from 21.75±1.77 µm to 28.97±2.04 µm, and
Rzmax from 30.81±2.79 µm to 39.36±3.21 µm re-
spectively (Figure 7, Table 1). However, the values
increase only within young moraines deposited since
AD 1932. The micro-roughness measured on boul-
der surfaces subject to weathering for more then 80
years remains at a higher, relatively constant level.
Mean width of the roughness profile elements (Rsm)
increases insignificantly from 258.67±19.43 µm (site
VII+) to 294.78±19.02 µm (site IV), but then it falls
down again on older moraines. Spearman rank corre-
lation coefficients calculated for the micro-roughness
parameters and age-dependent test site rank, are pos-
itive but only moderate (0.37 ≤Rs ≥0.38) to weak
(Rs = 0.27), with the exception of Rsm, where there
is no correlation (Rs = 0.00).
The weathering rind thickness correlates strongly
(Rs=0.65) with the age-dependent test site rank. Mean
rind thickness ranges from 0.15±0.02 mm on boul-
ders deposited since 2000 (site VII+), to 0.60±0.05
mm on boulders from site Ia (Figure 8, Appendix 1).
However, no significant increase of the rind thickness
is observed within sites IV-Ia (Appendix 1, 2). The
time-dependent thickening of the rind is accompanied
by a general decreasing trend of Schmidt hammer R-
values from 65.17±0.66 to 61.47±0.60, the latter in-
dex remaining relatively unchanged among moraines
deposited in 1907 (site II) and earlier. The Spearman
rank correlation coefficient for R-values and test site
ranks is moderate (Rs= -0.37). Boulders of the old-
est moraine ridge (site Ib), do not exhibit any distinc-
tive roughness, weathering rind thickness or R-values
from the second oldest site (Figures 7, 8).
DISCUSSION
Lichenometry
The size-frequency gradient approach and use of
the equation elaborated for SE Iceland by Bradwell
JÖKULL No. 63, 2013 61