ON THE DEPOSITION OF THE PUMICE FLOW AND ACCOMPANYING TOPOGRAPHIC CHANGES IN THE NORTHEASTERN PART OF HAKONE VOLCANO

Abstract

The author studied the pumice flow field at the northeastern foot of Hakone volcano, about 80km. southwest of Tokyo, in order to make clear both the relation between the mechanism of flowing of the pumice flow and its basal topography, and also accompanying topographic changes. The major conclusions of this investigation are as follows: 1. The pumice flow deposits of this field are destributed in the lower area between the Hakone volcano and the Ashigara Mountainland made up essentially of older Pliocene sediments, forming Sekimoto Hill-land (Fig. 5 and 6). The Sekimoto Hill-land is composed Topographically of many ridges with flat topped surfaces, whose summit level shows the fan-shaped topography (Fig. 3). 2. The pumice flow deposits are separated into two stratigraphic members, the upper one more than 60m. thick and the lower one more than 5m. thick. But the lower one is recognized at a few . small out-crops (Fig. 7), whereas Sekimoto Hill-land is composed mainly of the upper one, which consists of pumice fragments (decite), lithic fragments (pyroxene andesite) and the fine materials of both fragments, and is usually consolidated but not welded as the lower one. 3. Stratigraphic and topographic evidences might indicate that the basal topography of the upper pumice flow deposit was as follows (Fig. 14-2): In the time just before the deposition of the upper pumice flow, the Kari River cut deeply into the old somma of the Hakone volcano, running in parallel with the Uchi River originated in the Ashigara Mountainland and the old somma, and flew into the area now occupied by the Sekimoto Hill-land, which was a lower small relief land, probably a flood plain. 4. The grain size and the number (in unit area) of both the coarse pumice and lithic fragments contained in the upper pumice flow deposit vary areally, but their variation series are very interesting as mentioned below. The author measured, as the indicators to find the above-mentioned variation series, both the maximum sizes of each fragment and the mumbers of coarse fragments with more than 5cm. in diameter per 1m2, at 22 stations. Each value was then plotted on the map and the equivalent lines were drawn. The results are as follows: in the case of lithic fragment (Fig. 10 and 12), many coarse fragments are concentrated along the line which runs across the center of the field of the upper pumice flow depotit almost from west to east, and the grain size, together with the number contained in the unit area, decreases in proportion to increasing distances away from this line to north and south. And along this line the grain size decreases toward east. On the contrary, in the case of pumice fragment (Fig. 9 and 11), distribution patterns of the grain size and the contained number do not show the clear regularity as seen in the case of lithic fragment.5. These variation series might be resulted from the mechanism of flowing and deposition of pumice flow. When the flowing of pumice flow occurs, the cross-sectional differences of the flowing energy take place by the effect of the basal topography, resulting in the occurence of the center line of flowing with larger energy and probably higher speed like talweg in stream. As the result of the effects of both the depositional agency by gravity (as already pointed out by H. KUNO and others, 1961) and the occurence of the center line of flowing, the coarse heavy lithic fragments tend to sink down near the crater, and the grain size and the volume of the carried lithic fragments decrease with increase in distance away from the crater. At this time, although along the center line of flowing even the coarser lithic fragments are carried owing to the larger energy, at a distance away from the center line of flowing only the line Ethic fragments are carried owing to the decrease of the flowing energy.