The Activity and Eruptives of the Myoko Volcano-IIIrd Stage

Geologic Descriptions of the Myoko Volcanoes (2)

Abstract

The Myoko volcano is situated in the northern part of the Fossa Magna, Central Japan, and constructs the Myoko volcanoes together with the Niigata-Yakeyama, Kurohime, Iizuna, and other volcanoes. The Myoko volcano in general sense, is divided into the older Myoko volcano and Myoko volcano, and the growth-history of the latter is further subdivided into the Ist, IInd, IIIrd, and IVth stages. The eruptives of the each stage, except for those of the Ist stage in which no basalt occurs, consist petrographically of basalt-andesite association.
The strata formed during the Myoko volcano-IIIrd stage are collectively called the Mitaharayama group, and consist of lava flows, pyroclastic (scoria, bomb, and others) fall deposits, pyroclastic flow deposits and volcanic mud flow deposits. They are widely and thickly distributed around the caldera (Fig. 1). The rocks of the essential eruptives are basalt, pyroxene-olivine andesite, and hornblende-andesite. They are characterized by a large amount of basalt to basic andesite and by a subordinate amount of acidic andesite (Fig. 2).
During the activity of the IIIrd stage of the Myoko volcano, the rocks changed from basalt, through pyroxene-olivine andesite, to hornblende andesite, and the mode of the eruption also changed from the eruption of the scoria fall by the Strombolian eruption, through the alternated eruption of the lava flow and pyroclastics such as bomb by the Vulcanian eruption, to that of the pyroclastic flow by the Peléan eruption (Table 1). A conical stratovolcanic cone with the elevation of 2, 800 to 3, 000m may have been formed at a certain period during this IIIrd stage (Fig. 4). The age of the IIIrd stage-activity falls in the late Quaternary, and is older than about 30, 000 years ago.