Neither Effusive nor Explosive: Origins of Pumice Fragments in Submarine Silicic Volcanism, Kermadec Arc, SW Pacific

Pumice textures from subaerial explosive eruptions have been relatively well documented yet little is known about their submarine equivalents. The Kermadec Arc presents a unique opportunity to investigate the processes of submarine explosive volcanism, as it contains silicic volcanoes of similar age that have erupted chemically similar magmas at differing water depths. Here we present textural analysis data for clasts collected from subaerial deposits on Raoul volcano, as well as clasts from Macauley volcano (a large submarine caldera with a 3 km{2} subaerial exposure) dredged from the seafloor and collected from the subaerial exposure. Variations in vesicle texture, size and abundance in pyroclasts from these eruptives have been quantified using stereological image analysis techniques. Initial results show that pyroclasts from five eruptions of contrasting size from the subaerial Raoul volcano show little variation in bubble number density, with similar values to other published subaerial silicic volcanoes. Submarine erupted pyroclasts from Macauley volcano, however, show vast variations in textural characteristics. These pyroclasts display a strong gradient in vesicularity not observed in the subaerial realm, with vesicularity in a single clast ranging from 50 - 80 vol. % over less than 3-4 cm. Vesicle shapes range from strongly elongate at the pyroclast margins to rounded large vesicles within the pyroclast interior suggesting that they have experienced rapid quenching with ongoing degassing at their cores. The differences in the vesicle textures of pyroclasts from Macauley volcano submarine eruptives are attributed to differences in eruptive style, and the interaction of degassing magma with a large amount of water. Although pyroclast chemistry indicates that many samples from Macauley volcano were erupted in separate events of unknown size or style, these new observations support a model of eruption that is not entirely effusive or explosive but a hybrid style of eruption not previously recognised.