Measures of the Degree of Chemical Weathering of Rocks

In humid regions, the silica-to-alumina ratios have been found to provide an easy way to quantify the degree of rock weathering. The reliability of this index depends on the uniformity of the bedrock, the constancy of the alumina content during weathering, the degree of correlation between silica loss and total-element loss, and on the magnitude of the difference between the silica-to-alumina mole-ratio values of fresh and thoroughly weathered rock. Examination of total silicate analyses of fresh and derivative weathered rock from sixty-four non-quartz-bearing volcanic rocks in north-east Papua and forty-eight igneous and metamorphic rocks (some quartz-bearing) from other humid areas revealed extremely good correlation, with a high degree of significance, between the silica loss and the total-element loss, and between the silica-to-alumina mole-ratio and total-element loss. The silica-to-alumina mole-ratio also correlates well with the more complex absolute and relative weathering indices calculated from total-silicate analyses. The simple index should provide a useful guide to the physical, chemical, and engineering properties of weathered rocks.