Time-dependant surface deterioration of glacially abraded basaltic boulders by Fláajökull, SE Iceland

Abstract

he surface deterioration of glacially abraded basaltic boulders by Fláajökull, SE-Iceland, deposited since the Little Ice Age maximum, was assessed based on measurements of micro-roughness, weathering 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.