Abstract
Accretionary lapilli show two types of occurrence in pyroclastic flow deposits-those fill steam pipes (INSIDE PIPE) and those occurring in the pyroclastic flow deposits not containing pipes (OUTSIDE PIPE). The host pyroclastic flow deposits with pipes contain much less lapilli than those without having pipes. The measurements of diameter, porosity and bulk density of lapilli, as well as the observation of the internal structure revealed no difference between accretionary lapilli of INSIDE PIPE and OUTSIDE PIPE. Thus lapilli of both occurrences were formed by the same process, i.e. by volcanic hailstone. Whereas pyroclastic materials including pumice and lithic fragments are found in all the directions from the Aira caldera, accretionary lapilli are found only in the southeast region of the caldera. This difference results from different modes of transportation; the pumice and lithic fragments were controlled by the flows, whereas accretionary lapilli were strongly affected by the wind, as they form in high-altitude.
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