Geomorphic development of White Island Volcano based on slope stability modelling

Abstract

White Island shows many geomorphic features associated with stratovolcanoes that have undergone major sector collapse, most notably the flat-floored amphitheatre comprising the present crater. ArcGIS is used to develop a reconstructed pre-failure cone and 2D limiting-equilibrium stability analysis is undertaken of the reconstructed cone. Water table ranges from 0% (dry) to 100% (saturated) are assumed and earthquake accelerations of 0–0.5 g bracket the likely range of accelerations expected in a volcanic setting; geotechnical data from Moon et al. [Moon, V., Bradshaw, J., Smith, R. and de Lange, W., 2005. Geotechnical characterization of stratocone crater wall sequences, White Island Volcano, New Zealand. Engineering Geology, 81: 146–178] are used. The models suggest that flank failure is a feasible explanation for the present-day geomorphology, and indicate that there were at least two retrogressive failures. The best model sees a core of hydrothermally altered material overlying relatively unaltered andesite lavas and breccias, where accelerations of 0.17 g (saturated slope) to 0.45 g (dry slope) are required to initiate failure (F = 1.0). The failure modelled was relatively small (volume of 0.21 km3), and would have mostly involved hydrothermally altered materials with some fresh rock mass. Two toreva blocks are recognised occupying the seaward margin of the amphitheatre, with a lobe of debris offshore suggested as the debris avalanche deposits. Modification of both the outer slopes of the island and the inner crater walls has occurred since the failure event, especially on the southwestern margin of the crater. Modelling suggests that further failure of the inner walls is likely and poses a significant hazard to tourists visiting the island; further outer slope failures are unlikely.