Thordarson The most dangerous widespread hazard associated with explosive volcanism is wet ash fall during phreatoplinian eruptions, which affects areas of tens of thousands of square kilometers. Phreatoplinian volcanism is a poorly understood and neglected branch of our science, despite featuring in half of the 40 Holocene explosive eruptions in Iceland. This study targets the only historical "type example", the 1875 eruption of Askja in Iceland, to examine the roles of magmatic volatiles and external water in "driving" phreatoplinian eruptions and the contrasts between "dry" (plinian, subplinian) and "wet" (phreatoplinian) fragmentation processes. Its goals are (1) to quantify the roles of magmatic volatiles during phreatoplinian eruption by comparing patterns of nucleation, growth and coalescence of vesicles between phreatoplinian and plinian/subplinian pumices, and (2) to clarify the role of external water at fragmentation by analysis of whole deposit grainsize distribution, and clast morphologies in all eruption phases. Broader impacts: This study will promote teaching, training and learning through conjoint University of Iceland (UI)/University of Hawaii (UH) classes. Results will be broadly disseminated via the Internet in Icelandic (at UI) and English (at UH) and and in public lectures. It will also be incorporated in the volcanology courses taught at UH and UI, in summer schools taught annually in Hilo for the Center for Study of Active Volcanoes to third world volcanologists and to undergraduates from mainland colleges. It will train a young female volcanologist in a wide range of techniques available at University of Hawaii. We also hope to use this study as a "springboard" to introduce other UH graduate and undergraduate students to Icelandic volcanism and bring young Icelandic scientists to the United States. Infrastructure will be enhanced as the grant will sustain and expand the physical volcanology facilities at Hawaii that serve the needs of 3 faculty, 5 graduate students and 6 undergraduates. A major focus of this study will be to present a summary of conclusions and impacts workshop to emergency management agencies in Iceland and to present the result to a wider audience from the civil defense community at the next meeting in the Cities on Volcanoes series.