Using Pyroclastic Textures to Understand Dynamic Eruption Processes in the Auckland Volcanic Field

Abstract

During the transition from phreatomagmatic to predominately magmatic eruptions, environmental factors such as the amount of external water interacting with the ascending magma change the eruption style, and hence, the associated hazards and resulting eruption deposit characteristics. Changes in juvenile magma characteristics, such as crystallinity, resulting from these transitions are recorded in the erupted pyroclast textures. For this study, volcanic deposits were collected from Motukorea (Brown’s Island) and Maungauika (North Head volcano) in the Auckland Volcanic Field, New Zealand, to gain insight into how the availability of external water, complexity of eruption processes, and changing eruption styles impact the deposit crystallinity and textures. I have specifically targeted 11 surge/fall deposits which display a variety of textures to test the impact of changes in interactions with external water, as defined by various eruption styles (phreatomagmatic, Strombolian, and Surtseyan) on clast characteristics. Juvenile clasts ranging from up to ~3 cm in size per deposit were powdered, thin sectioned, characterized texturally and analysed by X-ray diffraction (XRD) to capture the heterogeneity of each deposit’s mineralogy and crystallinity. XRD diffraction patterns were analysed using the AMORPH software to quantify the crystallinity and characteristics of the amorphous material. Analyses of both Motukorea and North Head indicate that total crystallinity values are significantly different between the Surtseyan-style deposits (~30% to ~50% crystallinity) compared to those from the phreatomagmatic and Strombolian deposits (~50% to ~100% crystallinity). Higher crystallinities and higher variability observed in Motukorea phreatomagmatic material could be explained by slower cooling of the clasts, caused by ashcoated materials, and/or faster ascent rates which could prevent juvenile materials from wholly interacting with any external water. Samples from North Head suggest a fragmentation rate that would cause any vapour film surrounding the juvenile materials to collapse, allowing exposure to water and leading to faster quenching. The cooling histories at both volcanoes suggest that a post-eruptive cooling model is too simplified. Thus, additional variables and processes, including magma ascent, degassing, magma flux, fragmentation rate, ash, and surface area must also be incorporated into the model to better reflect the complexities involved with volcanic eruptions.