PHYSICAL VOLCANOLOGY AND EMPLACEMENT HISTORY OF THE BEN-LOMOND RHYOLITE LAVA FLOW, TAUPO VOLCANIC CENTER, NEW-ZEALAND

Currently, there are two contrasting models concerning the state in wh ich rhyolite lava reaches the surface: (1) the permeable foam model, w here the lava reaches the surface as an expanded foam and collapses in to dense obsidian during lava flow; and (2) the traditional model, whe re decreased load pressure leads to vesiculation of the upper surface of the dome. In order to test these models, textural parameters were d ocumented on outcrops of the Pen Lomond rhyolite dome, which has two f low lobes comprising an eroded dome (c. 100 ka old) cut by the Whangam ata Fault. A composite stratigraphy of the upper 60 m of the flow, ass embled from three sections (roadcut, airstrip and fault scarp), compri ses, from the flow top: finely vesicular pumice, black aphyric obsidia n, and a spherulitic transition zone above a central crystalline rhyol ite core. An explosion breccia occurs as a pod at a depth of c. 35 m a nd cross-cuts the upper two units as an inverted cone-shaped deposit-a n infilled explosion pit. Eight physical parameters (density and poros ity, void aspect ratios, spherulite size and proportion, microlite siz e and abundance, and volatile contents) were measured throughout the e xposed thickness of the flow. Results show that primary vesicles-those that deform pre-existing flow banding-are most numerous within the fi nely vesicular pumice layer but are suppressed c. 10-15 m below the su rface. Vesicularity and volatile contents are anomalously high in expl osion breccia fabrics. Furthermore, microlites are ubiquitous in the f low but absent in the explosion breccia. Incomplete degassing of the a scending magma took place, resulting in volatiles being distributed wi thin the flow thickness during lava emplacement. The lava flow carapac e revesiculated during extrusion (primary vesiculation), whereas secon dary vesiculation (spherulites and lithophysae) was associated with gr oundmass crystallisation of the flow centre, causing localised volatil e enrichments responsible for the formation of explosion breccias. An emplacement model, formulated for the Ben Lomond dome, shows that vesi culation can occur within different units throughout emplacement and a fter flow cessation. Furthermore, textural data indicate that the perm eable foam model is not a viable model for the Ben Lomond dome.