Study of Distal Pyroclastic-flow Stratum from Tianchi Volcano in 1215 (±15) Eruption: Pyroclastic-flow Over Water.

In this paper, we describe three strata at the distal part of the pyroclastic-flow from the Tianchi volcano in 1215 (±15) eruption. One of the strata with crosslayers that are different from typical pyroclastic-flow strata may come from a ground-surge. The grain-size and scanning electron microscopy (SEM) analysis was performed to study the origin of the pyroclastic-flow. Characteristics of grain-size distribution show that it is similar with the ash cloud. Through the SEM analyses, we found some quench structures with less damage on the surfaces of the vitric pumices. These phenomena indicate that there has been hydration in the transportation processes at the distal of pyroclastic-flow. It has partly changed the transportation mechanism of pyroclastic-flow, which transitions form dense flow to diluted flow. This paper develops a new distal pyroclastic-flow model in the Tianchi volcano that can be divided into three stages, i.e. the quench stage, expanding stage and depositing stage. [ABSTRACT FROM AUTHOR]