Partitioning of pyroclasts between ballistic transport and a convective plume: Kīlauea volcano, 19 March 2008

We describe the discrete ballistic and wind‐advected products of a small, but exceptionally well‐characterized, explosive eruption of wall‐rock‐derived pyroclasts from Kīlauea volcano on 19 March 2008 and, for the first time, integrate the size distribution of the two subpopulations to reconstruct the true size distribution of a population of pyroclasts as it exited from the vent. Based on thinning and fining relationships, the wind‐advected fraction had a mass of 6.1 × 105 kg and a thickness half distance of 110 m, placing it at the bottom end of the magnitude and intensity spectra of pyroclastic falls. The ballistic population was mapped, in the field and by using structure‐from‐motion techniques, to a diameter of > 10–20 cm over an area of ~0.1 km2, with an estimated mass of 1 × 105 kg. Initial ejection velocities of 50–80 m/s were estimated from inversion of isopleths. The total grain size distribution was estimated by using a mass partitioning of 98% of wind‐advected material and 2% of ballistics, resulting in median and sorting values of −1.7ϕ and 3.1ϕ. It is markedly broader than those calculated for the products of magmatic explosive eruptions, because the grain size of 19 March 2008 clast population is unrelated to a volcanic fragmentation event and instead was “inherited” from a population of talus clasts that temporary blocked the vent prior to the eruption. Despite a conspicuous near‐field presence, the ballistic subpopulation has only a minor influence on the grain size distribution because of its rapid thinning and fining away from source. Key Points Data for ballistic blocks and wind‐advected components merged as the first total grain size for an entire pyroclastic fall deposit Ballistic data (via a combination of field measurements and structure from motion) has little influence on total grain size distribution Grain size distribution of “wall‐rock” particles may be inherited and not due to explosive fragmentation