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The rock of Taya Caves is a Pleistocene tuffaceous marine siltstone, grain-supported and well sorted, composed mainly of quartz, plagioclase, and lithoclasts rich in illite and smectite clay minerals, also distributed in the matrix. Two slightly different siltstone varieties are observable, which can be distinguished by their color and the presence (or not) of calcareous bioclasts.

Yoshimi Hundred Caves are excavated into a Miocene tuff with dacitic to andesitic composition, which shows a certain lithological variability, ranging from a coarse-grained pumiceous type – with porphyritic texture, plagioclase phenocrysts, abundant lithoclasts, and hypocrystalline groundmass – to a fine grained, mostly glassy type.

Finally, Oya stone is a Miocene ignimbrite having a rhyolitic to dacitic composition, with fiamme and porphyritic texture, phenocrysts of plagioclase and quartz, and hypocrystalline groundmass. The grain size is highly variable, and the typical clay clusters may reach a size of several centimeters.

The rocks of all the studied sites share, other than the pyroclastic-related sedimentary origin, also the silicate composition (plagioclase, quartz, and clay minerals) (Fig. 2). Moreover, they are all soft rocks with very high porosity. We determined an average porosity of around 45 %, and a pore-size distribution characterized by the prevalence of capillary pores (> 0.1 µm), those most involved in liquid water absorption.

Figure 2: Thin-section photomicrographs in plane- and cross-polarized light of the studied rocks.