Hodder, PeterBriggs, Roger M.Stevenson, Richard John2026-06-162026-06-161989https://hdl.handle.net/10289/18357Rhyolite lavas have not been observed to erupt in New Zealand. No detailed measurement of the physical volcanological properties of lavas has been undertaken to date. However, the detailed measurement of fabric parameters from lava flow unit profiles, provides an insight into emplacement history, rheological behaviour, and ultimately volcanic hazards of rhyolite lavas. Two contrasting Cenozoic rhyolites were studied: Ben Lomond flow - a calc-alkaline rhyolite flow 3.5 km length of 110 ka age; and a pantelleritic flow sequence of Quaternary age from Mayor Island. The general stratigraphy of a rhyolite flow comprises: finely vesicular pumice; upper obsidian layer; central crystalline rhyolite; lower obsidian layer; and a basal breccia. Fabric measurements include density, proportions of primary and secondary voids, void and autoclast aspect ratios, and microlite length. Porosity data provide information on the distribution of volatiles in an emplacing lava, and spherulites on post-emplacement volatile migration. Primary voids are suppressed below 10 m depth. The largest spherulites most commonly occur above the central rhyolite core, suggesting late flow migration of volatiles from a crystallising flow centre. Supportive evidence is: vesiculation of explosion breccia at c.30 m depth; a coincident peak in water content; and subtle trace element variations. Microlites of uniform length formed prior to emplacement. Subsurface evidence (microlite shape) supports an increasing undercooling, and degassing of an originally hydrous magma, collapsing from an inflated foam before emplacing as a viscous flow. Effervescence occurred within the carapace during flow. Isobaric crystallisation within the flow centre, triggered explosion breccia formation just before and after the flow front stopped advancing. Sixteen years elapsed as the flow centre cooled below the glass transition temperature. Late-flow migration of soluble trace elements is consistent with subtle variations in Na₂O, Cl, H₂O and the development of secondary fabrics. Groundwater metasomatism is manifest in pumiceous fabrics. Viscosity, calculated from major element chemistry, temperature, and water content, is 10⁷ poise for the magma (chamber); 10¹² poise (conduit); and 10¹¹ poise for the central flow. Three peralkaline flows from Mayor Island were studied: Panui flow (a lava pond within a tuff ring); Pre-8 ka flow; and the 8 ka flow sheet. No explosion breccia and no significant spherulitisation or lithophysae occur. Carapace sections show ramp structures, anticlines compressed into spines, and synclinal troughs. In contrast to Ben Lomond, more bubbles occur in the upper obsidian layer. A high flow temperature ( ~ 950°C), variations in microlite length throughout the flow profiles and viscous deformation of powdered and block samples (furnace experiments) point to a fluidal rheology. The main differences between pantelleritic and calc-alkaline emplacement models is a spatter-fed emplacement mode, microlite growth and high temperature isobaric crystallisation during flow. Significant variations in Na₂O, La, Ce and Cl support high temperature migration of volatiles associated with the crystallisation of rhyolite. Sodic fluids were more concentrated in Cl and F in fluid lavas containing 2 - 10 times less water than Ben Lomond. Estimated viscosity of Mayor Island lavas is between 10⁷ - 10⁸ poise. Temperature has a significant contribution to the fluidity of the lava allowing a spatter-fed emplacement and rehomogenisation of the spatter as a distally tapering flow sheet. Void aspect ratio and porosity are parameters that best approximate the theoretical viscosity profile, whereas mean microlite length approximates the temperature curve only for the Mayor Island lavas. The calc-alkaline emplacement model was tested on two poorly exposed rhyolitic centres: Pauanui (Miocene age); and the Okataina Volcanic Centre flows (19 ka - 5.5 ka age). Ben Lomond and Mayor Island flows are compared with overseas rhyolites. The range of morphology of rhyolite landforms is primarily dependent on effusive volume and effusion rate. No significant difference occurs in the viscosity of domes, coulees and flows of similar eruptive chemistry and temperature, from the same volcanic centre.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.Thesis