Selective weathering of granitic clasts: Discussion

Abstract

Can. J. Earth Sci., 17,956(1980) I Many granitic pebbles and boulders in the glacial outwash of northern Indiana and Illinois have been attacked by weathering leading to total disintegration to a grus-like mass. Such boulders are intermixed with sound blocks of both granitic and nongranitic character. The weathered boulders have disintegrated throughout, and, when dropped to the ground, tend to shatter into a multitude of grus-like fragments. The rusty appearance of such weathered boulders, especially near biotite flakes and biotite clusters, finds agreement with Bustin and Mathews (1979) claiming that the disintegration is a function of biotite expansion by weathering. The gradual conversion of the biotite to kaolinite is well reflected in Grant's (1969) abrasion pH. The authors do not consider the presence of the feldspars or any other mineral which may be a potential contributor to this process. In my own observation on such boulders the feldspars appear to be very little attacked by weathering as evidenced by their glassy luster, a good visual indicator of the freshness of a feldspar surface. The question also arises, how a generally dense rock mass would permit the thorough penetration by weathering up to a depth of over a foot in less than 14000 years. Did microcracking make such a rapid advance possible? Granitic rocks have shown a greater porosity, more water sorption, and less strength than their quartz-free cousins. Nur and Simmons (1970) describe microcrack porosity of quartz-containing igneous rocks: cracks tend to run both around and across the quartz grains. The P-V-T diagram of quartz, which Winkler (1973, p. 170) modified to explain the behaviour of quartz during the cooling process from the magma to ambient temperature and pressure conditions, helps to explain such cracking by differential contraction of the quartz, with 4.5%, compared with less than 2% for the other rock forming minerals. Stress relief with the advancing weathering process may have influenced the widening of such fine cracks inward. This may find an explanation here as well as for the formation of grus from granite in place, if we look at Kieslinger's (1960) concept of stress relief in granite near the terrain surface, modified by Winkler (1973, p. 60). The thorough weathering of some granitic boulders in less than 14 000 11 000 years in British Columbia appears to be strongly influenced by the amount of the minerals biotite and quartz, supported by stress relief of the pre-stressed granite boulders. This relationship may also have a bearing on the estimation of the durability of biotite-granites for the stone industry. Bustin and Mathews' paper appears to give a departure for further research and modeling of the weathering rate.