Formation processes of tafoni on pyroclastic rock surfaces with hydrothermal alteration on the Isotake coast, Shimane, Japan

Cliffs along the Isotake coast, Shimane, Japan, have two types of distinctive surfaces with different colours, with tafoni and without tafoni, even though they consist of the same pyroclastic rock. Rockfalls, which result in severe accidents, occur only on the surface without tafoni. Geochemical analyses of the cliff materials and the Schmidt hammer rebound test were conducted to elucidate the process of salt weathering in the formation of tafoni. The strontium stable isotope ratio (87Sr/86Sr) of the calcite (i.e., 0.706298) found on the backwall of tafoni indicated that the calcium in this calcite is derived from cliff materials, although the cliff receives seawater through splashing. Carbon, hydrogen, nitrogen and sulfur elemental analyses and X-ray diffraction pattern measurements revealed that clinoptilolite formed by hydrothermal alteration was more abundant in the cliff surfaces with tafoni than in those without tafoni, indicating a high abundance of calcium available for incorporation into calcite. The hardness of the cliff materials increases as the clinoptilolite content increases in any surface type. Consequently, tafoni preferentially form on surfaces with abundant clinoptilolite produced by hydrothermal alteration, although the surfaces have also been strengthened by hydrothermal alteration. Our results imply that the presence of tafoni indicates an overall lower risk of rockfalls.