Kilauea's Explosive Past: Understanding Violent Explosions at Hawai'i's most Active Volcano

A sequence of explosions that occurred from Kilauea caldera around 1790 A.D. includes the most deadly pyroclastic eruptions recorded in the USA. The products, the upper part of the Keanakāko`i tephra formation, are believed to be responsible for the deaths of a Hawaiian war party on the summit of the volcano. Little is known about the parent eruptions or even how many explosions there were in or around 1790; also, no hazard maps exist at Kilauea for this type of violent yet short-lived activity, with few clear precursors. The c. 1790 deposits show both marked sectoral changes and also, on a finer length scale, rapid lateral variability over distances of less than 1 kilometer. In the southern sector, they form a series of striking alternating coarse-grained beds, cross-bedded pyroclastic density current deposits (pdc) and accretionary-lapilli-bearing fine ashes. This study focuses primarily on the coarse-grained units, which are largely absent west and east of the caldera. Through characterization of the beds in the field and analysis of grain size and dispersal data, the size and intensity of these eruptions have been defined. The coarse-grained units include both pyroclastic fall and pdc deposits. Unusually, only bedding characteristics, (and not grain size or dispersal) unequivocally distinguish between these two transport processes. This study offers a clearer, more quantitative picture of the c. 1790 events and advances the understanding of how explosive eruptions can occur at an otherwise gently effusive volcano. It also constrains the dynamics associated with these events and improves volcanic hazard mitigation at Kilauea and shield volcanoes worldwide.