Physical volcanology and emplacement history of the Ben Lomond rhyolite lava flow, Taupo Volcanic Centre, New Zealand

Abstract Currently, there are two contrasting models concerning the state in which rhyolite lava reaches the surface: (1) the permeable foam model, where the lava reaches the surface as an expanded foam and collapses into dense obsidian during lava flow; and (2) the traditional model, where decreased load pressure leads to vesiculation of the upper surface of the dome. In order to test these models, textural parameters were documented on outcrops of the Ben Lomond rhyolite dome, which has two flow lobes comprising an eroded dome (c. 100 ka old) cut by the Whangamata Fault. A composite stratigraphy of the upper 60 m of the flow, assembled from three sections (roadcut, airstrip and fault scarp), comprises, irom the flow top: finely vesicular pumice, black aphyric obsidian, and a spherulitic transition zone above a central crystalline rhyolite core. An explosion breccia occurs as a pod at a depth of c. 35 m and cross‐cuts the upper two units us an inverted cone‐shaped deposit—an infilled explosion pit. Eight physical parameters (density and porosity, void aspect ratios, spherulite size and proportion, microlite size and abundance, and volatile contents) were measured throughout the exposed thickness of the flow. Results show that primary vesicles—those that deform pre‐existing flow banding—are most numerous within the finely vesicular pumice layer but are suppressed c. 10–15 m below the surface. Vesicularity and volatile contents are anomalously high in explosion breccia fabrics. Furthermore, microlites are ubiquitous in the flow but absent in the explosion breccia. Incomplete degassing of the ascending magma took place, resulting in volatiles being distributed within the flow thickness during lava emplacement. The lava flow carapace revesiculated during extrusion (primary vesiculation), whereas secondary vesiculation (spherulites and lithophysae) was associated with groundmass crystallisation of the flow centre, causing localised volatile enrichments responsible ior the formation of explosion breccias. An emplacement model, formulated for the Ben Lomond dome, shows that vesiculation can occur within different units throughout emplacement and after flow cessation. Furthermore, textural data indicate that the permeable foam model is not a viable model for the Ben Lomond dome.