On the slope of joint structure at Sounkyo Gorge in Hokkaido, toppling of a long columnar of welded tuff has occurred frequently and the road has hazard of rock fall. In this study, the occurrence mechanism of the toppling was considered theoretically on the basis of the observational results of the fracturing processes.
At first, it was proved from the analyses on the thermal deformation of a rock columnar that a small and a large stone fell alternately in the opened joint on the back side of the columnar in response to the seasonal change in temperature gradient and that the inclination of the columnar increased gradually with the lapse of time.
Secondly, it was clarified from the investigation on the hook-shaped crack propagations at the root of the columnar that toppling was caused when the rotational center of the inclined columnar moved from the outside to the inside just over the centroid in the processes of compressive fracture of the outside part. Additionally, the long duration of acoustic emission activity just before toppling has supported the validity of the theoretical considerations.

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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.

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