Subaqueous eruption and emplacement of the Shinjima pumice, Shinjima (Moeshima) island, Kagoshima bay, SW Japan

The Shinjima Pumice is a fines-depleted pumice lapilli tuff emplaced several thousands years ago at about 100-140 m below sea level. This 40-m-thick deposit comprises many poorly defined flow units, which are 1-10 m thick, diffusely stratified and showing upward-coarsening of pumice clasts with a sharp to transitional base. Parallel to wavy diffuse stratifications are commonly represented by alignment of pumice clasts, especially in the lower half of the flow units. Pumice clasts of block to coarse-lapilli size commonly have thermal-contraction cracks best developed on the surfaces, demonstrating that they were hot but cooled down to the ambient temperatures prior to their emplacement. These features are suggestive of the direct origin of the Shinjima Pumice from subaqueous eruptions. A theoretical consideration on the behavior of subaqueous eruption plumes and hot and cold pumice clasts suggests that subaqueous eruption plumes commonly collapse by turbulent mixing with the ambient water and are transformed into water-logged mass flows.