Jökull - 01.01.2013, Blaðsíða 55

Reviewed research article
Time-dependent surface deterioration of glacially abraded
basaltic boulders by Fláajökull, SE Iceland
Maciej Da̧bski and Aleksander Tittenbrun
Department of Geomorphology, Faculty of Geography and Regional Studies, University of Warsaw,
Krakowskie Przedmieście 30, 02-927 Warsaw, Poland, corresponding author: mfdbski@uw.edu.pl
Abstract — The surface deterioration of glacially abraded basaltic boulders by Fláajökull, SE Iceland, de-
posited since the Little Ice Age maximum, was assessed based on measurements of micro-roughness, weath-
ering rind thickness and Schmidt hammer R-values in order to find indices of relative age of the moraines.
Micro-roughness of boulders was analysed using a Handysurf E35-B electronic profilometer working with a
vertical resolution of 0.01 µm, which is a new method in geomorphology. A pilot microscopic analysis showed
heavy fracturing within the weathering rind developed in fine-grained basalts, which is interpreted as inherited
from mechanical weathering and responsible for deterioration of rock surfaces. A weak to moderate correlation
was observed between time-dependent moraine rank and studied indices. Surprisingly, no chemical alteration
was observed in analysed rock samples subject to weathering for more than a century. Results indicate that 80
years of weathering cause gradual increase in selected micro-roughness parameters, weathering rind thickness
and decrease in Schmidt hammer rebound value. However, these indices of surface deterioration do not change
on older moraines, which is attributed to exfoliation. The results do not provide unequivocal arguments in
the ongoing discussion about the timing of the LIA maximum of Fláajökull, but encourage further use of the
Handysurf E35-B electronic profilometer as a tool in determining initial stages of rock surface weathering.
INTRODUCTION
Timing of the Little Ice Age (LIA) glacial maximum
in Iceland is still under debate. Until recently, it
was commonly acknowledged that glaciers flowing
south from the Vatnajökull ice-cap reached their late-
Holocene maxima at the end of 19th century (Ahl-
mann and Thorarinsson, 1937; Thorarinsson, 1943;
Jaksch, 1975; Gordon and Sharp, 1983; Thompson
and Jones, 1986; Thompson, 1988; Gudmundsson,
1997; Evans et al., 1999; Sigurðsson, 2005). How-
ever, works of Kirkbridge and Dugmore (2001), Brad-
well (2001, 2004), McKinzey et al. (2004), Bradwell
et al. (2006), and Chenet et al. (2010) based mainly
on lichenometry and tephrochronology, conclude that
the LIA glacial maxima were reached in SE Iceland
at the beginning of 19th century or even at the end of
the 18th century. Discrepancies stem from different
lichenometrical approaches, uncertainties concerning
environmental factors influencing lichen growth rate
and possible erosion censoring moraine chronologies
(Da̧bski, 2002, 2007; Bradwell, 2009; Armstrong,
2011; Kirkbridge and Winkler, 2012).
Fláajökull is an outlet glacier flowing SE from the
Vatnajökull ice-cap in SE Iceland (Figure 1). Since
the end of the Little Ice Age (LIA), the glacier devel-
oped several moraine ridges, dated by Da̧bski (2002,
2007) on the basis of lichenometry as well as histor-
ical and cartographical data. Due to an ongoing dis-
cussion about the lichenometrically derived age of the
oldest moraine and the timing of the LIA Fláajökull
maximum, ranging from 1870–1898 AD (Da̧bski,
2002, 2007, 2010) to 1807–1831 AD (Chenet et al.,
2010, 2011), we looked for other age indices of the
moraines.
The following methods were employed: i) micro-
roughness of boulder surfaces determined with use
of a Handysurf E35-B electronic profilometer, a new
technique in geomorphology, ii) thickness of weath-
ering rinds, and iii) Schmidt hammer rebound values.
JÖKULL No. 63, 2013 55