THE LONG SHADOW OF VOLCANIC ERUPTIONS: THE ORIGIN AND IMPACT OF VOLCANIC ASH

The April 2010 eruption of Eyjafjallajokull volcano, Iceland, highlighted the hazards posed to air traffic by even small explosive volcanic eruptions. It also highlighted gaps in our knowledge of ash generation, particularly the particle size range expected for different eruption conditions. Challenges to addressing this question include both documenting the total grain size distribution (TGSD) of past eruptions and relating grain size distributions to processes that operate during magma ascent and eruption. Recent detailed studies of several well constrained pyroclastic fall deposits have partially addressed the first challenge, and show that the TGSD of magmatic fall deposits is closely linked to eruption style: energetic silicic Plinian and subplinan eruptions commonly produce fall deposits where > 50% of the erupted mass is ≤ 100µm in size, while mafic Strombolian and Hawaiian eruptive deposits are dominated by clast sizes of > 10-100 mm. These differences can be explained by different fragmentation mechanisms: in low crystallinity silicic systems, fragmentation is brittle and controlled by local stresses caused by rapid expansion of individual bubbles or bubble clusters; in mafic systems, fragmentation is ductile and controlled by instabilities in the liquid phase – bubble expansion serves mainly to accelerate the liquid. More poorly understood are controls on fragmentation in eruptions that are intermediate in both intensity and composition. Pyroclasts from these eruptions often have moderately to highly microcrystalline groundmass textures and correspondingly variable bulk vesicularities; notably, mean clast size is typically at least an order of magnitude larger than individual bubble and crystal sizes and pyroclast shapes range from angular (brittle) to rounded (ductile). External shape, in turn, controls the abrasiveness of the ash particles. Understanding the relationships among conditions of magma ascent, internal clast textures, external clast shape, and fragmentation processes represents an important step for improving our understanding of hazards posed by volcanic ash.