Understanding the Role of Water in Explosive Eruptions of Kilauea volcano, Hawai`

Halema'uma'u crater, Kilauea volcano, Hawaii, has produced basaltic explosive activity ranging from weak explosions to sub-Plinian eruptions. The explosive eruptions that dominated the activity of the past 500 yr have been studied by carefully investigating clasts and isotopic compositions in the deposits. Changes in the eruption style are thought to result from transitions among interactions between external water and magma (phreatomagmatic/phreatic eruptions), or from shallow magma degassing (magmatic eruptions). Clast size, morphology, and dispersal depend on the eruption mechanism (phreatomagmatic, phreatic, or magmatic), and the degassing regime (open- versus closed-system) prior to eruption is recorded in the volatile contents of the pyroclasts. Generally speaking, at Kilauea volcano, phreatomagmatism is indicated by an abundance of accretionary lapilli, and poorly sorted deposits made up of smooth glass shards, blocky glass fragments with smooth and conchoidally fractured surfaces, and vitric pyroclasts with chipped edges. Mdϕ vs sigmaϕ with values between -4.25>Mdϕ>-0.5 and 1.4sigmaϕ<2.4, respectively, are also typical indicators of phreatomagmatic processes. Fine, moderately sorted deposits composed of blocky, subrounded to rounded lithic fragments, and subrounded vitric particles coated with fine angular hydrated ash chips, and 2>Mdϕ>4 and 0.5<1.75 are characteristic of phreatic processes. Magmatic eruption mechanisms are typified by the presence of droplet-shaped particles and Pele's hair in the deposits.;H and O stable isotope compositions of juvenile volcanic glasses and olivines from explosive eruption deposits do not record mixing between magma and external sources of water. Rather, correlation of ?D and total water contents of volcanic glasses indicates that open-system degassing depletes the magma in D relative to closed-system degassing. Volatile contents thus record the degassing system of Kilauea prior to eruption, and may help suggest the mechanism that might drive an explosive eruption. The results suggest that the explosive eruptions were caused by a variety of mechanisms, and are not the result of a common generating phenomenon.