Effects of Rock Surface Temperature on Exfoliation, Rock Varnish, and Lichens on a Boulder in the Hunza Valley, Karakoram Mountains, Pakistan

Abstract

Granodiorite boulders on a moraine around the Batura Glacier in the Karakoram Mountains have exfoliation sheets and rock varnish on the south surface, and lichens and salt efflorescence on the north surface. To elucidate the formative conditions of these surface features, temperatures of a boulder at two points, one each on the north and south surfaces, were measured simultaneously with air temperature at 1-h intervals for 1 yr from July 1994 to July 1995. The annual temperature range of the south surface was 5°C higher than that of the north surface. The south surface had diurnal temperature ranges of 30°C with no seasonal change, while the north surface had diurnal ranges of 20°C in summer and 5°C in winter. Maximum heating and cooling rates of the south surface were high, being +12.6 and −14.5°C h⁻¹, while those rates in the north surface were low at +6.5 and −7.7°C h⁻¹, respectively. These results suggest that the difference in microclimatic conditions have affected the formation of rock surface features; for example, high rates of change in rock temperature on the south surface may produce exfoliation sheets.