Joyo Ossaka

On the factors influencing the weathering of tephras in various parts of Japan.

The relations between the factors influencing the weathering of tephras and evolved clay minerals were studied for specimens of pumices and scorias in various parts of Japan. The factors of weather like rainfall, temperature and humidity largely influenced the species of clay minerals formed by weathering. The correlation that the formation of halloysite became easier with increasing amount of water supplied for that system was recognized. The concentration of SiO2 in the interstitial water of tephras changed to higher as their burial depth became deeper and this relation was considered to contribute strongly to the formation of halloysite. The change of chemical compositions of tephras by weathering always showed the leaching of SiO2 and relative condensation of Al2O3 compared with those of the host rocks independent of the chemical compositions of the host rocks. In pumices and scorias allophane and/or imogolite were transformed into halloysite with the long lapse of time and this time was considered to be largely influenced by the factor of weather. The following clay mineral change was found as the burial depth of tephras became large: (volcanic glass)→(allophane, imogolite)→(halloysite)→(kaolinite)→(smectite).