Phreatoplinian eruptions: Mechanisms derived from comparative case studies

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Fallout tephras of widespread Plinian dispersal, and their associated flow deposits, range from pumice-rich, well-sorted normal Plinian deposits through those that are unusually lithic-rich and/or fine-grained (possible evidence for water affecting the eruption) to those called phreatoplinian, which are fine-grained even proximally (possible evidence of water-flushed sedimentation) but contain highly vesicular pumice and are poor in lithics. The three tephras studied here contain deposits spanning the whole range: the Congo Tephra, the Conacaste Tephra, and the Los Chocoyos Tephra. Having determined their internal stratigraphies, distribution, lithological and geochemical composition as well as inherent magmatic parameters in the first phase of the project, this second phase will focus on more detailed studies of grain size and shape as well as pumice vesiculation textures in order to better constrain the relative roles of magmatic degassing, contact to external water in the conduit, and interaction with external water at the surface in the fragmentation and eruption processes.