In an attempt to explore the mechanism for rock weathering, temporal changes in various kinds of rock properties due to weathering were investigated selecting Kozu-shima, a small island in the Pacif-ic, as the study site (Fig. 1). In this island four lava domes formed successively with eruption of rhyo-lite, at 1, 100, 2, 600, 20, 000 and 40, 000 y BP (Table 1). These four rhyolites with different ages are de-noted here as the 1.1-ka, 2.6-ka, 20-ka and 40-ka rocks, respectively. It was found in preliminary analy-ses that all four are biotite rhyolite, with (1) a mineral composition of about 85-90% glassy ground-mass and about 10-15% phenocrysts such as quartz, plagioclase, and biotite (Photo 2); (2) a chemi-cal composition of 74-77% SiO₂ and 12-13% Al₂O₃ content (Table 2); and (3) a porosity of about 30% (Table 3). These findings suggest that the rocks had similar original properties when the lava domes formed.<br> The following field evidence is recognized. (1) The rocks show little change in lithofacies with in-creasing depth from the ground surface on each outcrop of quarries having a height of 10-20m: no common weathering profile has developed (Photo 1). (2) Schmidt hammer rebound values do not vary with depth on each outcrop (Fig. 2). These observations lead to the first conclusion, that the porous rhyolite has characteristics of so-called “deep weathering”. This high susceptibility to weathering seems to be due to rapid extension of water-rock interaction to the deeper zone, which is facilitated by the presence of (1) a large number of cooling joints with the vertical and horizontal surfaces in a rock mass and (2) a high porosity in a rock block, say, about 30 %, both yielding high permeability in rhyolite.<br> Deep and uniform weathering in each lava dome indicates that the present-day rock properties can be assumed to represent the degree of weathering in the time elapsed from each eruption. Rock block samples taken from the outcrop in each dome are analyzed in detail in the laboratory. Minera-logical characteristics of the four rocks were investigated: (1) X-ray diffraction analyses show that clay minerals such as kaolinite and mica clay minerals are formed in older rocks such as the 20-ka and 40-ka rocks (Fig. 3); and (2) SENT photographs show that flow structure in the glassy groundmass is clear in younger rocks such as the 1.1-ka and 2.6-ka rocks, but unclear in the older rocks (Photos 5 and 6) . Chemical composition obtained by X-ray fluorescence analysis and thermo-gravimetry indicates that the amounts of SiO₂, Na₂O, and K₂O decrease slightly in the younger rocks and decrease rapidly in the older rocks, while those of H2_O and FeO^* (FeO+Fe₂O₃) increase slightly in the younger rocks and rapidly in the older rocks (Fig. 5). The SiO₂/Al₂O₃ ratio gradually de-creases in the older rocks (Fig. 6). High leaching of SiO₂ suggests that the solution percolating through the rock mass and block has a high pH. This is supported by the fact that Kozu-shima has been located in the humid and warm environment through the late Quaternary.