It is established that Surtseyan eruptions involve extensive magma-water interaction, but the specific volumes, geometries, and dynamic consequences of such interaction have not been precisely characterized. Textural studies seeking to understand phreatomagmatism have mainly focused on fine ash—an approach that is intuitive given the abundance of fine particles in Surtseyan deposits, but that neglects additional information preserved in coarser particles. Virtually without exception, scoria bombs from Surtsey (Iceland) and similar volcanoes show composite textures, with host material having entrained smaller clasts. Entrained clasts commonly show evidence of post-entrapment groundmass crystallization, and always are surrounded by void space indicating that they were wet at the time of entrapment. The composite textures—ubiquitous in bombs but also common in lapilli—support a classical model that describes Surtseyan volcanism as being driven by mingling of magma and water-saturated slurry in periodically flooded vents. We use textures, eruption observations, and basic thermodynamics to expand the magma-slurry model and relate it directly to the vapor dynamics that characterize Surtseyan jets and plumes.

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