Abstract for the Arthur Holmes Conference -The Physics of Explosive Volcanic Eruptions-, Santorini, Greece, 2-6 September 1996@

Transportation mechanism of the 1991-96 Unzen pyroclastic flows, Japan

Shinji Takarada (Hokkaido Branch, Geological Survey of Japan, Kita-8, Nishi-2, Kita-ku, Sapporo 060, Japan; takarada@gsj.go.jp)

A density-modified grain flow model, in which the flow is maintained by grain interaction and matrix strength, best explains the characteristics of the 1991-96 Unzen pyroclastic flows, Japan.

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 maximum runout distance and volume of a flow were 6.7 km and 1 x 10^6 m^3, respectively. Thickness of a flow unit is 20 cm-5 m. Big boulders (< 5 m) with cooling joints were observed in the deposits. Steam was sometimes observed from cracks in the boulders. Around boulders and carbonized wood fragments, segregation pipes and pods were seen, indicating they formed after deposition of the flow. Layer 2a and surge beds were observed at the bottom of the deposit. Lobes and levees were observed at the surface of the deposit. Estimated yield strengths of the flow based on lobes and levees were 1.7-9.2 x 10^3 Pa.

Calculated maximum grain size transported by turbulence was < 1.0 cm. The calculated maximum grain size suspended by fluidization was ca. 1 cm, in the case of no ingestion of air at the flow front. Therefore, the Unzen pyroclastic flows were maintained by grain interaction and matrix strength (density-modified grain flow).

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E-mail: takarada@gsj.go.jp

(Oct. 4, 1996)