Weathering mechanism of pelitic schist and its relationships with landslides: analysis of undisturbed drilled cores from the Sambagawa Metamorphic belt in Shikoku.

Abstract

Weathering mechanisms of pelitic schist are clarified by analyzing the vertical variations from fresh rock zone to weathered rock zone in densities, porosities, chemistry, mineralogy and petrological features of undisturbed drilled cores from a landslide body. An oxidation front is made within a weathering profile by the reaction between rock and oxidizing water percolating from the ground surface. At the oxidation front, chlorite is transformed into Al vermiculite, goethite is formed, and pyrite and graphite are depleted by oxidation. Pyrite is also oxidized by Fe just below the oxidation front. The sulfuric acid yielded by the oxidation of pyrite, percolates further downward and interacts with rock, which consequently decreases its strength in conjunction with fractures induced probably by unloading. The deteriorated rock mass could be easily sheared and be transformed into a shear zone, which is a sliding zone in other words. The sliding zone is less permeable and hence interrupts the downward filtration of groundwater through it. Abstract