GEOMORPHOLOGICAL DEVELOPMENTS OF THE EASTERN FOOT OF THE ZAÔ VOLCANOES AND ITS ENVIRONS

Abstract

In the Japanese Islands, volcanic activities and geotectonic movements which have been very active since the later Tertiary, have exerted profound influences on the present landforms. Accordingly, stratigraphic and morphogenetic examinations of volcanic landforms and of geotectonic developments are extremely important. From this point of view, the writer investigated geomorphological developments of the eastern foot of the Zaô volcanoes and its environs. The Zao volcanic group is composed of four volcanoes: Aoso, Ryû-zan, South Zaô and North Zaô. The results of this investigation are summarized as follows: 1) After deposition of the Numata Formation, Aoso volcano was being active throughout the first half of the Quaternary period when the initial form of this volcano was developed. It appears that the activity of the volcano started under a lacustrine environment. Therefore, most of the Aoso volcanic deposits show submerged facies. 2) When this volcano rose above the sea level, its activity stopped. A red weathered crust was created during the following warmer period over the initial surface of the volcano. Thereafter, the center of the volcanic activities moved toward the west of Aoso volcano, and South Zaô became active. 3) The activities of the Zaô volcanoes started again with South Zaô. Volcanic mudflows from South Zaô are three in number; namely, Karasawa, Kawarago, and Nanokahara. The Kawarago mudflow formed flowmounds and the Nanokahara mudflow formed a fan-like topography. 4) After the down-flow of the Nanokahara mudflow, the center of volcanic activities moved to North Zaô. 5) There are five topographic areas which are not of volcanic origin; namely, hill-lands, the Hara terraces, the Nagano terraces, the Nagafukuro terraces, and recent terraces, including fans and flood plains. The Nanokahara mudflow is related with the recent terraces. 6) In the Murata-Ôgawara area, terraces are not discernable, and this, as well as the hilly landforms, indicates the existence of geotectonic movements, the western part of this area shows an elevating tendency, whereas the eastern part has tended to subside. The boundary of these two movements closely coincides with the Komurasaki-Shiroishi geotectonic line.