[1] The Tarawera Volcanic Complex comprises 11 rhyolite domes formed during five major eruptions between 17,000 B.C. and A.D. 1886, the first four of which were predominantly rhyolitic. The only historical event erupted about 2 km 3 of basaltic tephra fall (A.D. 1886). The youngest rhyolitic event erupted a tephra fall volume more than 2 times larger and covered a wider area northwest and southeast of the volcano ($A.D. 1315 Kaharoa eruption). We have used the Kaharoa scenario to assess the tephra fall hazard from a future rhyolitic eruption at Tarawera of a similar scale. The Plinian phase of this eruption consisted of 11 discrete episodes of VEI 4. We have developed an advection-diffusion model (TEPHRA) that allows for grain size-dependent diffusion and particle density, a stratified atmosphere, particle diffusion time within the rising plume, and settling velocities that include Reynolds number variations along the particle fall. Simulations are run in parallel on multiple processors to allow a significant implementation of the physical model and a fully probabilistic analysis of inputs and outputs. TEPHRA is an example of a class of numerical models that take advantage of new computational tools to forecast hazards as conditional probabilities far in advance of future eruptions. Three different scenarios were investigated for a comprehensive tephra fall hazard assessment: upper limit scenario, eruption range scenario, and multiple eruption scenario. Hazard curves and probability maps show that the area east and northeast of Tarawera would be the most affected by a Kaharoa-type eruption.

On May 1st 2008 Mount Chaitén (southern Chile) interrupted a long period of quiescence, generating a sequence of explosive eruptions and causing the evacuation of Chaitén town located a few kilometers south of the volcano. The activity was characterized by several explosive events each associated with plumes which reached up to about 19 km above sea level. The products were dispersed across a wide area, with the finest ash reaching the Atlantic coast of Argentina. Our field observations in the proximal-medial area (3–25 km from the vent) indicate that the May 2008 tephra deposit consists of numerous layers, most of which can be correlated with individual eruptive events. These layers vary from extremely finegrained ash to layers of lapilli and blocks, composed of both juvenile and lithic material. Here we describe the stratigraphy and physical characteristics of the May 2008 deposits, and propose a reconstruction of the timing of the May 2008 events. The deposits are mainly associated with the three main explosive phases which occurred on 1st–2nd May, 3rd–5th May and 6th May, with an estimated bulk tephra volume of 0.5–1.0 km3 (integration of both exponential and power-law fitting). For the 6th May event, represented by a layer composed mainly of lithic lapilli and blocks (>2 mm), an isopleth map was compiled from which a 19 km plume height was determined, which is in good agreement with satellite observations

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Probabilistic modeling of lava flow hazard is a two-stage process. The first step is an estimation of the possible locations of future eruptive vents followed by an estimation of probable areas of inundation by lava flows issuing from these vents. We present a methodology using this two-stage approach to estimate lava flow hazard at a nuclear power plant site near Aragats, a Quaternary volcano in Armenia.