Turbulent transport and deposition of the Ito pyroclastic flow: Determinations using anisotropy of magnetic susceptibility

The Ito pyroclastic flow erupted about 22,000 years ago from Aira caldera in southern Kyushu, Japan. Flow directions were determined by anisotropy of magnetic susceptibility (AMS), which measured the preferential alignment of magnetite microphenocrysts, usually ≤0.25 mm in diameter. The microphenocrysts are aligned with the long axis of the clasts during strain and fracture in the vent. The grains are then deposited parallel to the flow direction with an imbrication. There is no evidence of rolling of clasts or nonflow parallel lineations, and thus AMS can be used to determine flow directions that occurred immediately before deposition. Beyond 30 km from the center of the caldera, flow directions were predominantly down paleogradient, indicating expanded flow and that the depositional system was gravity driven and largely decoupled from the transport system. Within 30 km, measured flow directions are random, indicating that sedimentation occurred from a depositional system that was closely coupled to the turbulent transport system. Individual flow directions at all sites except one varied more than the analytical error, demonstrating that a variety of flow directions existed even within a small area of the flow. This implies that the depositional system was turbulent.