Geology of the Hayami Volcanic Area : with Special Reference to the History of the Volcanic Activities

Abstract

The Hayami volcahic area is situated in the eastern central Kyusyu, comprising such volcanoes as Yuhu, Turumi, etc., and the famous Beppu Hot Springs., Geologically, this area is covered by various sorts of volcanic rocks in association with the Cenozoic sedimentaries, unconformably overlying on the basement rocks., The basement rocks consist of granites and mica-schists of so-called Ryoke Metamorphics, and are exposed only along the valley at the foot of the mountains., A features of the granite are shown in Table 1., The Cenozoic sedimentaries are divisible into the following two groups by a distinct unconformity : the lower is the Nozuharu group with the "Older Volcanics, " and the upper is the Oita group with the "Younger Volcanics., " Stratigraphical details are shown in Table 2 & 3., To elucidate an outline of the geologic history of the volcanic activities, discussions are attended to clarify the stratigraphical relation of the lavas and pyroclastic rocks to the sedimentary units at first., The following succession of volcanic activities were obtained: 1., The activities of the Mukainoharu volcanic-group (pyroxene-andesite, dacite, etc., ) in the Nozuharu group (Mio-Pliocene)., 2., A period of the non-volcanic activity., 3., The activities of the Ozika-Kanagoe volcanics-group (pyroxene-andesite, plagioliparite, etc., ) in the Oita group (Plio-Pleistocene)., 4., The activities of the Yuhu-Turumi volcanics-group (hornblende-andesite, pyroxene-andesite, etc., ) followed from the Ozika-Kanagoe volcanics-group., Stratigraphical setting and the petrological features of these volcanic units show that the Hayami is not simple but composite volcanic area., In other words, here the Mio-Pliocene volcanic zone "Setouti" is crossed by the Pleistocene volcanic zone "San'in" (Daisen).,