Weathering of a Quartz Diorite at Marble Point, McMurdo Sound, Antarctica

Abstract

Methods established by Goldich (1938) in his classic study of rock weathering have been applied with some modifications to analysis of a weathered rock series collected in the Antarctic. Exposures of a finegrained quartz diorite exposed at Marble Point, McMurdo Sound, were selected for study because they appeared to have undergone extensive chemical weathering beyond the effects to be expected in a region of such extreme cold and aridity. The light-gray, well-knit rock is altered in its most weathered form to a rich brown sand with angular particles as small as 0.003 mm. Complete chemical analyses of the rock in six of its weathering stages were obtained, and the trends established by these data were correlated with the mineralogie changes. Contrary to the impressions gained in the field, the bulk chemical and mineralogie composition of this rock has remained constant throughout the observed weathering sequence. There is no substantial change in alumina, combined silica, soda, lime, or potash which are normally sensitive indicators of chemical action. No clay forms in the weathering process. The only appreciable change involving an original constituent of the rock is the progressive oxidation of ferrous iron in pyrrhotite and biotite and the resulting formation of limonite which is responsible for the pronounced difference in color of the physically weathered debris. Seventy per cent of the original ferrous oxide is lost, and 45 per cent of total iron oxide is leached from the rock. Small amounts of halite deposited by evaporation of sea spray and native sulfur, presumably derived from volcanic fumes of Mt. Erebus, are found in the more-weathered materials at Marble Point. Their abundance relates to weathering only in that the disintegrated rock is physically more favorable to accumulation of these exotic substances. Oxidation of the native sulfur has contributed acid to the surface debris and thereby promoted rapid decomposition of pyrrhotite in the fresh rock and the pronounced iron staining in the weathered outcrops. The maximum volumetric changes caused by oxidation and hydration of the original minerals is negligible, and it is concluded that the mechanism primarily responsible for the observed breakdown of the rock is the combined action of frost-wedging and the crystallization of salt in pores and crevices in the weathering rock. Continuous records of the ambient air temperatures taken at Marble Point during the summers of 1957-58 and 1958-59 suggest that about thirty freeze-thaw cycles affect the site each year. However, on unclouded days rock-surface temperatures are considerably higher than air temperatures, and the number of freeze-thaw cycles in the rocks probably does not correspond to that indicated by air-temperature measurements. This study began as an investigation of what appeared to be a case of strong chemical weathering in the Antarctic and has ended in providing quantitative proof of the predominance of physical processes, a predominance commonly accepted on qualitative grounds.