RHEOLOGICAL ESTIMATES OF RHYOLITE LAVA FLOWS FROM THE OKATAINA VOLCANIC CENTER, NEW-ZEALAND

The lava flows of the Okataina Volcanic Centre (OVC) include the young est calc-alkaline rhyolitic lavas of the Taupo Volcanic Zone, New Zeal and. The textures of the Haumingi and Waiti flows, two of the most vol uminous OVC lavas, are interpreted using a flow textural stratigraphy model developed for North American rhyolite flows. Textures on the flo w carapace developed in response to vesiculation throughout eruption a nd flowage, followed by brittle fracture associated with the developme nt of flow folds and ridges. Late-stage vesiculation occurred in respo nse to spherulitisation within the upper obsidian layer and stress rel ease associated with avalanching of the flow front. Unlike some North American calc-alkaline rhyolite flows, coarsely vesicular pumice is ab sent. Two methods were used to determine the viscosity of the lava: (1 ) a physicochemical method using new microprobe analyses of glass: (et a = 10(10) Pa s, Haumingi flow; 10(11) Pa s, Waiti flow), which repres ents the viscosity of the interior of the flow during lava emplacement ; and (2) a morphological method representing the viscosity of the arr ested flow: (eta = 10(10)-10(11) Pa s for both flows). Yield strengths for OVC flows lie in the range 2.4-5.3 x 10(5) Pa. Mean velocity of b oth lavas is c. 10(-4) m/s, giving between 0.5 and 2.0 years for the H aumingi 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 fl ow centre to cool below the brittle glass transition temperature (T(g) = 670-degrees-C) is 3 years and >30 years, respectively. These estima tes constrain the length of time that vesiculation could have occurred within the cooling lava flow. Although few explosion pits were identi fied on the Haumingi and Waiti flows, they do occur on other OVC flows and are a potential hazard for viscous rhyolite lavas. Hazardous expl osion pit formation and small-scale pyroclastic flows triggered by a c ollapsing flow front can occur between 2 and 10 years after the lava e merged from the vent.