Takarada S., Kawanabe Y., Hoshizumi H., Suto S., Geological Survey of Japan, Kita-8, Nishi-2, Kita-ku, Sapporo 060, Japan (takarada@gsj.go.jp)
More than 9400 Merapi-type pyroclastic flows (0.2 km^3) were produced due to lava dome collapse from May 24, 1991 to May 1, 1996. The pyroclastic flows had overhanging, tongue-like heads, producing a cauliflower-shaped ash cloud behind them. The flows swept down a topographic low. The measured average velocities were up to 42 m/s. The maximum runout distance and volume of a flow were 6.7 km and 1 x 10^6 m^3, respectively.
Depositional features of the deposits were surveyed along the Mizunashi River (E of the Volcano), Akamatsudani Valley (SE) and Senbongi area (NE). Thickness of a flow unit ranges from 20 cm - 5 m. Some flow units showed reverse grading of larger blocks. The blocks were subangular to rounded with an average Mdf of -1.3 and sf of 2.7. Big boulders (1 m - 5 m) with cooling joints were observed in the deposits. Steam was sometimes observed from cracks in the big boulders. Around big boulders and carbonized wood fragments, a fines-depleted part (Mdf=-2.3, sf=2.7), segregation pipes and pods were seen, indicating they formed after deposition of the flow. Layer 2a (Mdf=1.5, sf=1.7, <18 cm thick), and surge beds (Mdf=3.3, sf=0.8, <20 cm) were observed at the bottom of the deposit. Lobes (<10 m wide, <2 m high) and levees (<3.5 m high) were observed at the surface of the deposit. Levees were sometimes clast supported. Estimated yield strengths of the flow based on lobes and levees were 10^3 -10^4 Pa.
Reverse grading, rounded boulders and layer 2a indicate grain interaction within the pyroclastic flow. Well-defined lobes and levees indicate enough yield strength to support blocks during deposition. Therefore, a density-modified grain flow model (Lowe, 1976) and a high-density turbidity current model (Lowe, 1982) are plausible two end-members to explain the depositional features.