Iizuna volcano (1871m) is formed at the southern end of the Myoko volcanoes which are situated in the northern part of the Fossa Magna. It is a Quaternary stratovolcano built on an anticlinal axis of basal Neogene strata, and has a horseshoe caldera with 2.5×2.0km in diameter at its summit. The volcanic cone is considerably dissected in contrast to the Myoko and Kurohime volcanoes. At the western to northern midslope of the cone, the five domes of Kenashiyama, Nakanomine, Takadekki, Tengudake, and 1340 m-ridge are arranged in NE-SW direction (Fig. 1). The growth-history of the Iizuna volcano may be divided broadly into the Ist and IInd stages, and the latter may be subdivided into the stratovolcano, caldera, and lava dome stages (Table 1). The eruptives from this volcano are full of variety such as lavas, pyroclastic (scoria, pumice, lapilli, and others) falls, pyroclastic flows, and volcanic mud flows. The rocks consist of basalt, pyroxene andesite, and hornblende andesite (Fig. 2). Augite-hypersthene andesite is predominant in volume. During the activity of the IInd stage, the mode of eruption and rock nature generally tend to change as follows, respectively: mode of eruption; eruption of scoria fall→alternated eruption of lava flows and pyroclastics→eruption of pyroclastic flows and falls, and viscous lave flows: rock nature; basalt→pyroxene andesite→horhblende andesite. The activity of this volcano began at the middle Pleistocene, and had already closed at least 32, 000 years ago.
The amount of plant opals and the constitutions of pollens in humic horizons of present and buried volcanic ash soils distributed in Tohoku, northern Kanto, eastern Tokai and Kyushu districts were investigated. The results obtained are summarized as follows; 1) It is assumed that the constitutions of plant opals in the humic horizons of volcanic ash soils have an effect on a relationship between the amounts of plant opals (X) and organic carbon contents (Y). In a group having ratios, Panicoid (Pa.)+Festucoid (Fe.) vs. Sasaoid (S.), not less than 0.5, the relationship between X and Y is expressed in the following formula: Y=1.58X+3.92 In another group having the ratios less than 0.5, the following formula is obtained: Y=0.63X+3.14 This may be explained by the fact that Sasaoid grasses have higher amount of plant opal production per dry matter than other ones. 2) The constitutions of pollens in the humic horizons of volcanic ash soils are characterized by high content of nonarboreal pollens (NAP), which allows an assumption that herbaceous plants were dominant in the vegetation concerned in formation of the humic horizons of volcanic ash soils. 3) A maximum mean annual production of plant opal is shown by the humic horizons of Kirishima volcanic ash (Ks-d) soils which had been formed during a period from 4, 600y. B.P. to 3, 700y. B.P. (14C dates), possibly a part of the climatic optimum in the post glacial age. This period nearly agrees with that derived from buried humic horizons of volcanic ash soils in Hokkaido (SASE and KONDO, 1974).