MODE OF ERUPTION AND VOLCANIC HISTORY OF MUKAIYAMA VOLCANO, NII-JIMA

Abstract

Mukaiyama is a rhyolitic volcano located at the southern part of Nii jima(Figs. 1 and 2). The volcano was formed by a single continuous eruption which began in shallow water in A. D. 886. The eruption changed from submarine in the early stage to subaerial in the later stage. The volcanic history is divided into three stages on the basis of the difference in the mode of eruption. The sequence of activities from explosive eruptions in the first stage to effusive eruptions in the third stage can be attributed to the change in the degree of contact of rising magma with sea water as the volcanic body grew. In the first stage, a low-lying (less than 100 m above sea-level) and flat-topped hill was formed by repeated base-surge eruptions. Since a considerable part of the original hill has been eroded away by wave, the bedded accumulation of the base-surge deposits (Mukai yama base-surge deposits) is very-well exposed along the sea cliff around the volcano (Photo. 1). Mukaiyama base-surge deposits are characterized by various sedimentary structures such as “antidune structure” (asymmetrical dune-like structure with steep stoss-side laminations and gentle lee-side laminations) and other primary structures (Fig. 4). The deposits are divided into two major groups: the lower coarse-grained Mukaiyama-1 and the upper fine grained Mukaiyama-2, on the basis of the difference in grain size and sedimentary struc tures. The Mukaiyama-1 is composed of plane beds, lenticular beds, pinching and swell ing beds, and large-sized antidune structures. The Mukaiyama-2 is, on the other hand, characterized by abundant cross-beddings and small-sized antidune structures (Photo. 3). Some of the antidune structures in the Mukaiyama-1 have wave length up to several tens of meters, whereas the majority of the antidune structures in the Mukaiyama-2 have wave length of several meters (Fig. 6). The wave length of these antidune structures in both Mukaiyama-1 and -2 tends to decrease with distance from source, presumably, because of decay in flow energy of base surges. In the second stage, the eruptions remained explosive. In this stage, however, no base surge occurred probably as a result that the volcanic body itself (base-surge hill) hindered to some degree, the contact of magma with sea water. The .second stage activity resulted in the formation of Omine pyroclastic cone with a relative height of about 200 m and basal diameter of about 2 km, on the first stage base-surge hill. The original slope of Omine pyroclastic cone is well preserved in the north and south. Eastern slope has, in contrast, disappeared due to wave erosion, and western slope is also lacking(Figs. 1 and 2). In the third stage, the grown volcanic edifice served possibly as an effective interceptor against invasion of sea water into the crater. Thus, the eruptions were no more explosive, but viscous lava effused and formed Mukaiyama lava domes. The dome lava (biotite rhyolite) occupies the crater bottom of Omine pyroclastic cone in the east, and rests on the pyroclastic materials in the west, which were probably deposited contemporaneously with Omine cone (Fig.2). Though the present areas covered with the lava is about 1.5 km2, the original lava must have extended further west, for the western end of it is a perpendicular sea cliff. The surface feature of lava is fairly rugged (Fig. 8). The relative heights from furrow bottoms to ridges are up to several tens of meters.