Pyroclastic flow was formed at about am 9:40 28th May, 1991, when a part of growing lava dome near the peak of Mt. Fugendake, Unzen volcano, was gravitationally collapsed and fragments of broken lava fell down through the steep slope of the eastem frank of Mt. Unzen. Although the wind near the surface blew from north, the direction of wind at the height of more than about 600 meters was from south to north, therefore the ash cloud was blown norhthward. At am 11 28th, fall-out ash with abundant accretionary lapilli was found to have covered the downwlnd area. As it was cloudy and the base of the cloud was about 1,000 meters high, most of the ash moved through the cloud. The accretionary lapilli were dry and fragile being easily disintegrated on small impact, and all lapilli got out of the shape after the light rain in the moming on 29th May.
The accretionary lapilli were distributed with in the fan-shaped area with 3km radius, the apex of which is located about 3km to the north of the crater. Accretionary lapilli ranged from 3-4mm to less than 0.5mm diameter, and the largest lapilli were found near the apex of the fan-shaped area, the maximum diameter of lapilli being gradually reduced outward.
The location of the apex of the accretionary lapilli distribution is not concordant with that of the generation of the pyroclastic flow. This discrepancy is probably attributed to that the accretionary lapilli had progressively grown during ash cloUd was moving northward, and that accretionary lapilli could not start to fall until they grew so large that their terminal fall velocity exceeded the upward current of air caused by hot pyroclastic flow and by the upward wind along the slope of the north flank of Mt. Fugendake.