Ash aggregation processes in a basaltic tuff ring (Songaksan, Jeju Island, Korea): Controls of diatreme conditions and transport processes

Abstract

Aggregation of ash generated by explosive volcanic eruptions controls the dispersal and residence time of ash in the atmosphere and, therefore, the hazard to aviation. Ash aggregation is particularly common in hydromagmatic volcanoes produced by explosive magma-water interactions. Nevertheless, few studies have been done on this process because of the lower risk of volcanic hazards posed by these volcanoes in comparison to classical stratovolcanoes. Observations of the microscopic textures of the tuff samples from a Holocene tuff ring in Jeju Island, Korea, show that the ash particles occur mainly in the form of coated particles and ash lumps, which are either rich or poor in clay minerals derived from the substrate. The clay-rich aggregates formed in the diatreme by cohesive and capillary bonding of clay-rich and water-undersaturated debris. When the diatreme was filled with clay-poor and water-saturated debris, particles were ejected mainly without ash coats and aggregation. Once emitted from the diatreme, the aggregates and uncoated/solitary particles could be further aggregated together or accreted on larger particles by liquid bonding, producing clay-poor aggregates, multiple-coated particles, and composite ash lumps. The ash aggregation occurred mainly in co-surge ash clouds and ash plumes but not in pyroclastic surges, in which the time available for ash aggregation was too short (a few tens of seconds or less). This study demonstrates that the substrate characteristics and environmental factors that can influence the material properties of the diatreme fill, such as the clay mineral and water contents, are the prime controls of ash aggregation in hydromagmatic volcanoes.