Since 20 May, 1991, a dacite lava dome has been growing atop Mt. Unzen in Kyushu, Japan, and has caused many pyroclastic flows to form by gravitational collapse of the brittle lava ( Yanagi et al., 1992; Yamamoto et al., 1993). These flows included a small but devastating one that killed 43 people (including three volcanologists) on 3 June 1991. This long-lasting series of dome-collapses had led to careful studies of closely observed pyroclastic flows formed in this manner ( Sato et al., 1992; Nakada and Fujii, 1993; Fink and Kieffer, 1993), including one where sensors were placed in the path of flows to determine temperature and pressure changes within the flows ( Taniguchi and Suzuki-Kamata, 1993).