Some Volcano-Geological Significances of the Hydration Layer Observed in the Glassy Groundmass of Kozu-shima Rhyolite

Abstract

In the present study, the thicknesses of the hydration layers along the cracks in the glassy groundmass of Kdzu-shima rhyolite were measured on thin sections, and the results are shown in histograms. The histograms show a single or more peaks which give the true thickness of the hydration layer. The thickest peak is considered to be related to the original emplacement of the lava flow, and the other to the episodes after the emplacement. Based on this assumption, the relations between the peaks and the geological evidences were examined. The main results are summarized as follows : On the basis of analyses of the histograms some knowledges about the episodes were found including lava emplacement, which give the cracks accompanied by hydration layer. It was also found that the absolute ages of those episodes might be estimated from the above-mentioned analyses.