A disastrous eruption of Asama volcano, the Tenmei eruption, occurred in 1783. During the eruption, “Kanbara pyroclastic flow” buried Kanbara Village located on the northern flank of the volcano, gradually changing into mudflow facies in the downstream. Furthermore, the mudflow went down along the Agatsuma and Tone Rivers. More than 1400 people were killed by this disaster. The pyroclastic flow is distributed within a parabolic area, with a narrow angle of 30deg; from a semi-circular depression with a diameter of 700 meters, which was located 4 km north of the summit. Aerial photo interpretation and field survey revealed that many gigantic blocks of essential lithics were scattered in an area of 18.1km2, and the volume of essentials was estimated to be 1.94×106 m3 for blocks with diameter upper 5m and 4.30×106m3 for including blocks with diameter lower 5m. The largest block was 49 meters in diameter, 10 meters high, and 9200 m3 in volume. Testpit survey clarified that the Kanbara pyroclastic flow deposit was 2.2 meters thick in average and hence amounted 4.70×107 m3 in total volume. Paleomagnetic measurement of the essential lithic blocks indicated that they deposited at a temperature higher than the Curie point (about 400deg;) even in the downstream of 65km from the summit. The pipe-structure observed in the several testpits was attributed to upward steam segregation, which derived from sufficient water and incandescent essential lithic blocks in the deposit.
Considering historic pictures and documents which suggest the water overflowing during the eruption, the authors interpreted that the semi-circular depression was a swamp, “Yanai marsh”, of 1.0-2.5×107 m3 in capacity.
The downflowing of Kanbara pyroclastic flow at high velocity caused an extensive erosion in the north slope of the volcano, because of much water content. The volume of the flow gradually increased since the deposit captured accidental blocks and water along the eroded area.
We propose two alternative interpretations as follows:
(1) The eruption occurred at the summit of Asama volcano and the pyroclastic flow rushed into the Yanai marsh, incorporating the water of the marsh into the flow. Consequently the Kanbara pyroclastic flow gradually changed into mudflow facies.
(2) Lateral eruption occurred at the depression (the Yanai marsh) which had been filled with water.The pyroclastic materials successively mixed with the water, which flowed out of the depression, and changed into mudflow facies.
