Cavernous Weathering in the Taylor Dry Valley, Victoria Land, Antarctica
TAYLOR VALLEY (77° 33′ S., 163° 25′ E.) is part of a deglaciated valley complex called the McMurdo Oasis, which is located about 80 km to the west of Ross Island across McMurdo Sound. Since the IGY many New Zealand and American geological parties have worked in this area1–3, which is easily accessible by helicopter from Scott Base and McMurdo Station at the southern end of Ross Island. Now that the topographical and geological reconnaissance stage has been completed and adequate maps cover the 2,500 km2 of the McMurdo Oasis, more attention is being devoted to the study of various anomalies and problems.
Weathering of a Quartz Diorite at Marble Point, McMurdo Sound, Antarctica
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.
Petrography of some erratics from Cape Royds, Ross Island, Antarctica
Cape Royds, located in about latitude 77"33' S and longitude 166"07' E, is a promontory on the west side of Mount Erebus, Ross Island, Antarctica. Results of other studies of Cape Royds erratics have been published. Benson's (1916) contribution relates to the dolerites collected chiefly from the moraines. Jensen (1916) records descriptions of anorthoclase trachyte, phonolitic trachytes, biotite-hornblende trachyte, acid and basic types of kenytes, shonkinitic kenyte porphyry, and porphyritic feldspar-olivine basalt. Smith (1954) refers briefly to the erratics oI the Cape Royds district. Thomson (1916) in his microscopical studies of inclusions of the volcanic rocks of the Ross Archipelago mentions the occurrence of certain erratics, including a basic type of alkaline trachyte or orthophyre, sanidinite, and biotite microsanidinite. Walkom (1916), in discussing the petrography of pyroxene granulites, groups these rocks into acid, scapolite-bearing, and basic. Woolnough (1916) describes pegmatites, aplite, sodalite syenites, qtiartz diorite, granophyric granite porphyry, granophyre, feldspar porphyry, minettes, vogesite, porphyrite, sericitized diabase porphyry, sdlvsbergites, sapphire-bearing trachyte, corundum-bearing trachyte, spherulitic trachyte, dense porphyritic basalt, actinolite gneiss, tremolite gneiss, actinolite schists, f ine tremolite schist, spotted schist, phyll ite, q\artz schist, and micaceous sandstone.