Rheological estimates of rhyolite lava flows from the Okataina Volcanic Centre, New Zealand

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Abstract
The lava flows of the Okataina Volcanic Centre (OVC) include the youngest calc‐alkaline rhyolitic lavas of the Taupo Volcanic Zone, New Zealand. The textures of the Haumingi and Waiti flows, two of the most voluminous OVC lavas, are interpreted using a flow textural stratigraphy model developed for North American rhyolite flows. Textures on the flow carapace developed in response to vesiculation throughout eruption and flowage, followed by brittle fracture associated with the development of flow folds and ridges. Late‐stage vesiculation occurred in response to spherulitisation within the upper obsidian layer and stress release associated with avalanching of the flow front. Unlike some North American calc‐alkaline rhyolite flows, coarsely vesicular pumice is absent. Two methods were used to determine the viscosity of the lava: (1) a physicochemical method using new microprobe analyses of glass: (η ‐ 1010 Pa s, Haumingi flow; 1011 Pa s, Waiti flow), which represents the viscosity of the interior of the flow during lava emplacement; and (2) a morphological method representing the viscosity of the arrested flow: (η = 1010‐1011 Pa s for both flows). Yield strengths for OVC flows lie in the range 2.4–5.3 × 105 Pa. Mean velocity of both lavas is c. 104 m/s, giving between 0.5 and 2.0 years for the Haumingi and Waiti flows to advance 6.7 and 5 km, respectively. Using a cooling model based on a finite difference approximation to the heat conduction equation, the time for the carapace (upper 20 m) and the flow centre to cool below the brittle glass transition temperature (T 8 = 670°C) is 3 years and >30 years, respectively. These estimates constrain the length of time that vesiculation could have occurred within the cooling lava flow. Although few explosion pits were identified on the Haumingi and Waiti flows, they do occur on other OVC flows and are a potential hazard for viscous rhyolite lavas. Hazardous explosion pit formation and small‐scale pyroclastic flows triggered by a collapsing flow front can occur between 2 and 10 years after the lava emerged from the vent.