Abstract
Volcanic tuff is a porous geo-material and easily weathered by physical, chemical and biological effects because it has mechanical characteristics of low shearing strength and high absorbency. In natural environments, there is a possibility that the weathering of volcanic tuff causes some geodisasters such as land subsidence and landslide in association with heavy rain or earthquakes. However, the weathering mechanism of volcanic tuff has not been fully understood and is still to be investigated. This would be because the phenomena are involved in not only physical and chemical factors, but also various factors such as climate, temperature, biological factors, etc. In this study, the authors focus on the volcanic tuff in Aguni Island, Okinawa, Japan, and investigate the generation and/or weathering mechanism of silicate minerals in the viewpoint of mechanical, chemical and biological factors. In particular, the generation mechanism of the silicate minerals with finger-like structures is investigated in detail, and the relationship between multi-physical mineralization and weathering process of weak inner-surface of the volcanic tuff is elucidated.
Volcanic tuff is a porous geo-material and easily weathered by physical, chemical and biological effects because it has mechanical characteristics of low shearing strength and high absorbency (Siegesmund and Dürrast, 2011). Hence, in natural environments, there is a possibility that the weathering of volcanic tuff causes some geo-disasters such as land subsidence and landslide in association with heavy rain or earthquakes. For example, the 2018 slope failure in Hiroshima, Japan was occurred by that the weathered and fractionated layers of welded tuff was flowed out to a valley (Watakabe and Matsushi, 2019). In other case, the most severe landslide in the 2008 Iwate-Miyagi earthquake was occurred along the boundary layers between pumice tuff and welded tuff (Ohzono, et al., 2012). That is, the solidification of the pumice tuff layer was not enough, so that the sliding surface was formed as a weathering layer on the pumice layer. Despite such situation, the weathering mechanism of volcanic tuff has not been fully understood, and is still to be investigated (Stuck, et al., 2008). This would be because the phenomena are involved in not only physical and chemical factors, but also various factors such as climate, temperature, biological factors, etc.
