DISTAL ASH FALLOUT FROM LARGE IGNIMBRITE ERUPTIONS

Distal ashfall may be a devastating hazard of ignimbrite super-eruptions. Elutriation of ash into eruption clouds from pyroclastic flows result in ashfalls that are mainly fine ash, <100 µm diameter or even very fine (<30 µm diameter) ash, but much about the fate and transport of very fine ash in volcanic clouds is unknown. Aerodynamically fine ash is dominant in ashfall at distances approx. >300 km in many ash blankets, and the size distribution does not change much with distance. Distal ashfall seems to proceed as en masse fallout of all particles. Even small scale eruptions such as the 18 May 1980 eruption of Mount St. Helens, the 4-5 April 1982 El Chichón and the 15 June 1991 Pinatubo produced distal fall that are remarkably fine and extensive, and were deposited within a day or two. Rapid fallout suggests particles aggregated through a process driven by hydrometeor formation.

Observations of ashfall from small eruptions suggests that large ignimbrite events could lead to high volume widespread fine-grained ash, which is supported by extensive deposits found in the Miocene to Pleistocene of the Great Plains of North America, downwind of the Great Basin and Yellowstone hotspot. These ash deposits are found across many states though their correlation is incomplete and made difficult by access and preservation, but what we do know is consistent with profound continent scale environmental impacts. Mapped patterns of distal fallout are too few and markedly affected by winnowing after initial deposition, but they are clearly not simply related to the proximal deposits.

Distal ashfall is apparently governed by meteorological phenomena. Numerical models have been developed to produce accurate trajectory forecasts for volcanic clouds for a few days following eruption and for coarse ash fallout in proximal regions. The rules of fallout of coarse ash (exponential thinning away from the source and gradual fining of particle size resulting from single particle fall in the turbulent flow regime) do not apply to very fine ash falling in distal regions. A successful numerical model of distal ash sedimentation will employ meteorology. One important meteorological variable is entrainment, greatly enhanced in co-ignimbrite plumes where moist lower tropospheric air is preferentially ingested, elutriated with fine ash particles, buoyed by latent heat.