DEFORMATION OF IWAKI VOLCANO, NORTHERNMOST HONSHU, JAPAN

Abstract

The final purpose of this study is to clarify the relation between the deformation of Iwaki volcano and that of its adjacent areas from a viewpoint of “tectono-volcanology”. In this paper, however, the author discussed mainly on the deformation of the volcano itself. The geomorphological and volcanological significances of the deformation of Iwaki volcano and the relation to the deformation of adjacent areas will be discussed in the following paper which will be appeared in this Review. Iwaki volcano (1625m) rests on the tilting hills which are mainly made up of the gently inclined monoclinic sedimentary rocks older than upper Pliocene (Figs. 1, 2 and 3). This volcano is a composite volcano composed of a main strato-volcano, three parasitic lava domes, and some craterlets (Figs. 4 and 5). Some tens of eruptions of rather small scale took place on the flank of the main cone in historic times. The latest one was solf atara activity in 1970. It is noteworthy for geomorphology of the Iwaki volcano that conspicuous asymmetry between the western half of the volcanic cone and the eastern is found in some features such as the gradient of restored original slope of the main cone, the distribution of “planezes” or volcanic initial surfaces of the main cone, the development of alluvial fans at the volcanic foot, and so on (Figs. 4, 6 and 11). There are, however, no evidences showing that the above-mentioned asymmetry depends on the geologic structures of the volcano (Fig. 6). Lateral volcanoes and craterlets are also distributed asymmetrically (Fig. 5). At the foot of the volcano except the western foot are found some deformed geomorphic surfaces. They are the Rdaino surface which is a part of older volcanic initial surface in the earlier stage of Young Iwaki volcano, the Nenoyama surface which is a part of volcanic initial surface of the main cone, the Hiramitai surface which is a part of Oldermost alluvial fan at the volcanic foot, and the Oldermost, Old and Middle alluvial fans at the volcanic foot, in descending order of their formations (Figs. 12, 13, 15, 16 and 17). Among these geomorphic surfaces, the older three were reversely inclined toward the main cone from which the materials forming these surfaces were derived, and also the younger three were decreased in their gradients in similar trend to that of the older ones. The present main cone is surrounded except at the western foot by the circular hills, named Old Iwaki volcanic hills, which are higher than the foot of the main cone (Fig. 18) and are composed of lavas and pyroclastics of Old Iwaki volcano (Fig. 6). Old Iwaki volcano is presumed to be a strato-volcano and now to be buried in the inside of the present main cone of Young Iwaki volcano. The inner border line of the Old Iwaki volcanic hills is circular in shape and is about 5.5 to 6.5 km in radius from the topographic center of the present main cone. At the southern foot of the present main cone, the innermost part of Old Iwaki volcanic hills is deformed by normal faults dipping toward the center of the volcano (Figs. 12 and 19). These faults run in the direction which is not parallel to the general trend of fault system in adjacent areas of the volcano (Fig. 2), but concentric to the center of present main cone. Deformation processes of Iwaki volcano inferred from the facts mentioned above are as follows: The central part (about 5km in radius) of Old Iwaki volcano subsided forming the circular faults before the beginning of the activities of Young Iwaki volcano. The main cone of Young Iwaki volcano had been also subsiding or down-warping slowly so as to incline eastward during the period from the formation of most parts of volcanic initial surfaces of the main cone to at least just before the formation of Young alluvial fan at the volcanic foot.