This research deals with seasonal change of salt weathering and XRD analysis of salts for the stone caves, called “yagura, ” built in Kamakura, central Japan, during the Kamakuraera, about A. D. 1200 to 1300. The detected salts are gypsum, epsomite, thenardite and calcite. Among these salts, the soluble epsomite and thenardite occur from winter to early spring, while insoluble gypsum and calcite are observed through the year. In order to consider this observation result, we measured temperature and humidity in stone caves and also performed solution experiment using rocks of the same lithologies with the yagura stones. Consequently, the seasonal change in salt precipitation is shown to be correlated well with that in humidity change, and considerable quantity of salt-forming ions are shown to dissolve out from the rock samples into water.
Salts are considered to precipitate on rock surfaces through evaporation of solutions made by water-rock interaction, especially during dry season of winter. Three types of collapse are recognized for rock surfaces of yagura's; (1) salts pulverize rock surfaces into powder (“powder-type”), (2) salts form crusts on rock surfaces, and crusts fall down together with thin films of host rocks (“crust-type”), and (3) salts grow inside cracks and fracture them into blocks (“blocktype”). Salt weathering is well known as weathering process in arid/semi-arid areas. This research shows that salt weathering is also important in humid areas like Japan in case dry and wet seasons occur. Gypsum is reported from many ruins of other areas, while the seasonal change of soluble epsomite and thenardite causes the “powder-type” collapses and considerably affects yagura's.