千葉県館山市船形磨崖仏十一面観音像の劣化と水・岩石相互作用

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Daifukuji Temple rock-cliff Budda sculpture which is located halfway up on the hill near the coast of Tateyama, Chiba Prefecture was carved in 8th century on the Neogene andesitic tuff and tuff breccia alternated bed. It is sheltered by a beautiful wooden construction. The Neogene andesitic tuff and tuff breccia are composed chiefly of augite, hypersthene, hornblende, plagioclase, and volcanic glass with subordinate amounts of fine-grained magnetite, pyrite (probably biogenic), hematite and limonite. The surface of Budda sculpture is covered by a thick, up to several mm, crust or scab composed chiefly of minerals such as gypsum, thenardite, epsomite, mirabilite with minor amounts of halite, sylvite, aragonite and copiapite. Rain water penetrates into the rock of the hill where sculpture is located and the water easily comes out after interaction with rocks and evaporates from the surface of rock-cliff. The water coming out from rock mass has high contents of Ca, Na, Cl, HCO3 and SO4 with subordinate amounts of K and Mg. Gypsum-rich scab or crust was formed by the evaporation of such type of groundwater on the rock-mass surface. The growth of thus formed scabs or crusts and their falling off must be major reasons why the sculpture has been decayed so badly as shown by Fig 4. Modes of occurrence of gypsum-rich scabs and indoor experiments using bore-hole cores taken from the Neogene volcanogenic sediments mass on which the sculpture was carved indicate that the increase in the thickness of the gypsum-rich scabs is unbelievably fast, probably more than 0.15mm/year. The repeated cycle of the growth of gypsum-rich scab or crust on the surface of Neogene volcanic sedimentary rock mass and their falling off should be stopped to prevent the forth decay of this Budda sculpture. The best ways to solve this problem are considered to be (1) to minimize the down permeation of groundwater from the top surface through interstitial void and joint of Neogene volcanogenic formation by making open drainage space at the above and/or back side of the rock-cliff sculpture or by covering the top surface of the hill by unpermeable facing and (2) to keep the humidity of the room of rock-cilff sculpture as high as possible to minimize the evaporation of water extracted from the surface of the sculpture.