PRODUCTS OF SHORT-LIVED PYROCLASTIC EVENTS DURING THE 2008-2009 HALEMA'UMA'U ERUPTION, Kīlauea: IMPLICATIONS FOR ERUPTION PROCESS AND HAZARDS

The ongoing eruption of Halema‘uma‘u crater at Kīlauea, which began on 19 March 2008, has been an unprecedented opportunity to observe a small volume pyroclastic eruption at close quarters. It has been characterized by low mass discharge rates (≤ 4 x 10⁴ kg s^-1), with a relatively small total volume of ejecta (< 0.01 km³). For much of the eruption magma has been ponded in the funnel-shaped vent system, accompanied by open-system degassing, and the activity has taken two forms: rare, short-lived impulsive discrete explosions and long intervals of pulsating, nearly continuous degassing with minor vent-wall collapses and ash (± spatter) emission.

The 8 largest discrete events ejected blocks up to 90 cm in diameter to distances of 400 m, with fallout of ash to greater than 15 km downwind. The two largest deposits are predominantly lapilli-sized and contain two elements: near-circular aprons of ballistic clasts, with lithic blocks weighing 140 kg thrown 150 m from the vent, and wind-attenuated convective fall deposits which extend kilometers downwind but barely reach single-clast thickness even on the dispersal axes. Both were studied within 24 hours of eruption, permitting the convective falls to be measured to isomass values of 1 g m^-2. Cross-wind, they show subtle and diffuse margins where the abundance of clasts progressively declines to less than 1 clast per m^-2, when the new clasts can no longer be distinguished from the substrate. Thinning half-distances (b_t) for the deposits are 20-30 m, consistent with the exceptionally low discharge rates. The juvenile ejecta show systematic changes in vesicularity. The early erupted magma was largely non-vesicular (and outgassed) but more powerful explosions in August-September 2008 ejected a significant proportion of microvesicular foam-like clasts.

The 2008 explosive events demonstrate that even very small eruptions in frequently visited areas create significant issues, which are compounded by uncertainties in factors such as weather patterns and vent-wall stability. Such eruptions necessitate contrasting approaches to short-lived, intensive sudden onset hazards, i.e. the discrete explosions (closures) and long duration, diffuse slow onset hazards like vog (land-use planning, informed individual decision-making via public information processes).