Clarke Population density around active volcanoes has increased rapidly over the last century, and in fact, roughly 500 million people today live in regions of the world directly threatened by volcanic hazards. Pyroclastic flows, or hot, high-speed avalanches of pumice and gas, are a major cause of destruction and loss of life in explosive eruptions. Their effects can be extremely destructive, as indicated by the deaths of 27,000 inhabitants of St. Pierre (Martinique) in 1902, the devastating eruption of Lamington (Papua New Guinea) in 1951, and the eruptions of Merapi (Indonesia) in the last century. Similar impacts can be anticipated in the future at other volcanically active locations. Two of the most important eruptions of the last 20+ years, with respect to monitoring, hazards mitigation and post-eruptive research, have been the eruptions of Mount St. Helens (MSH) during the 1980's and the ongoing eruption of the Soufriere Hills volcano, Montserrat (SHV). Each of these volcanoes, at different times during a single eruptive period, exhibited two different scales and styles of explosion - one style aborting after only a few minutes, generating what are termed Vulcanian explosions, and the other style continuing rather than aborting, thus producing larger-scale, sustained Plinian eruptions. The controls on such explosions are poorly understood, are imperative to hazard mitigation and are therefore the focus of the proposed research. Our previous studies successfully compared observational and field data from Vulcanian explosions at SHV with conduit and explosion models, validating our computational methods. The proposed research will build on this work in order to identify the salient controls on explosive style in two ways. First, because our previous studies required several simplifying assumptions, we intend to further develop the conduit flow model allowing for increased complexity of the magmatic system. Second, we will analyze explosions of different scales and styles: the 1996 sustained, sub-Plinian explosion at SHV and a series of short-lived eruptions at MSH, in July and August 1980. Our improved model combined with field-based studies of real explosions will lead to a better understanding of the potential impact of such volcanic eruptions.
