Weathering mechanisms and their effects on the landsliding of ignimbrite subject to vapor-phase crystallization in the Shirakawa pyroclastic flow, northern Japan

Ignimbrite, which is consolidated by vapor-phase crystallization, is weathered in humid regions to form a special type of weathering profile that consists of a hydrated zone, an exfoliated zone, and a disintegrated zone from the depth to the ground surface, with each zone having a basal front. The ignimbrite is hydrated first and loses a significant amount of phosphorous at the hydration front, where rock-forming tridymite is dissolved and cristobalite is precipitated. The ignimbrite further loses its alkali and alkali earth components at the top of the hydrated zone by reacting with reactive water from the exfoliated zone, then the leached layers are exfoliated and become part of the exfoliated zone, and then they soften significantly. At the top of the exfoliated zone, rock is disintegrated so completely that rock texture disappears. Water from rainstorms infiltrates down to the exfoliation front, but penetrates only slightly further downward, thus saturating the weathered rock in the exfoliated and disintegrated zones and leading to a landslide with a slip surface within the exfoliated zone.