Weathering of acidic volcanic ash : A case study of Aira-Tn Ash

The mechanism of the weathering of acidic volcanic ash was studied on the Aira-Tn ash，w hich erupted about 24，000 -25，000 yr BPf rom Aira caldera in Southern Kyushu，J apan. The ash was spread widely from Kyushu to Tohoku district in Japan. and accumulated in lacustrine， fluvial， talus and deep-sea environments. or covered terrace surfaces. After the eruption. many of these ash -deposits have been exposed to atmospheric agents at or near the surface， and have been weathered. In consequence，s ome of these ash-deposits were altered to clayey sediments. Scanning electron microscopic analysis of grains of weathered volcanic glass in the ash shows many shallow， rounded pits like the dimples of a golf ball on the surface. On the other hand. almost fresh glass grains from the deep sea sediments in the Sea of Japan have no such pits but a few sharp. deep stab-like holes. The gradual change in the texture on the grain surfaces is observed at the outcrop of terrace deposits in Chigaki. Toyama. Jap an，w here an ash layer of 6cm thick was partly altered to clay， and many pits on the grain surfaces were generated at the southern side of the outcrop. These observations with the data of pH values of subsurface water show that the acid subsurface water dissolved the glass to generate the pits on the surface. The study of color of the ash-deposits shows that no pit was generated in the oxidation process