Express Letter Radial variations of melt viscosity around growing bubbles and gas overpressure in vesiculating magmas Nadav G. Lensky *, Vladimir Lyakhovsky, Oded Navon Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel Received 16 October 2000; accepted 3 January 2001 Abstract The viscosity of silicic melts depends strongly on their water content. As bubbles grow in a supersaturated melt, water evaporates from the bubble^melt interface. A diffusive profile develops and leads to steep viscosity gradients across the melt shell. Here we investigate the effects of radial viscosity profiles on the dynamics of bubble growth. We find that the effective melt viscosity resisting gas overpressure in the bubbles is close to the viscosity at the dehydrated rind, and may be higher than that of the surrounding melt by more than an order of magnitude. As a result, bubbles may retain pressures that are higher than ambient pressure for longer times, magma degassing is delayed to shallower depth, and fragmentation of magma due to gas overpressure may occur over a wider range of conditions. Measured water content in eruption products yields information on the average melt viscosity, however additional information about the concentration profiles is needed for estimating the effective viscosity that controlled the evolution of bubble growth in the ejecta. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: viscosity; bubbles; magmas; pressure 1. Introduction The diversity of volcanic eruptions is widely attributed to the extreme range of viscosities of the erupting magma [1]. During an eruption, as highly viscous silicic magma ascends and decom- presses, its dissolved volatiles become supersatu- rated and gas bubbles nucleate and grow. Gas overpressure in the bubbles is maintained by fall- ing ambient pressure, continuous exsolution of the volatiles, and the viscous resistance of the melt around the bubbles. At high ascent rates, when the strength of the melt is exceeded either by gas overpressure in the bubbles [2^4] or by high strain rates of the £ow [5,6], the bubble-bearing magma fragments and erupts explosively [7,8]. The gas overpressure in the bubbles is sup- ported by viscous stresses in the surrounding melt. When melt viscosity is uniform, gas over- pressure is proportional to the viscosity of the melt ; high-viscosity melts preserve higher over- pressures and are more likely to fragment. How- ever, over a wide range of volcanic conditions, narrow rinds of highly viscous melt develop around the growing bubbles due to dehydration of the melt. This e¡ect of variable melt viscosity 0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII:S0012-821X(01)00227-8 * Corresponding author. Tel: +972-2-658-6513; Fax: +972-2-566-2581; E-mail: nadavl@vms.huji.ac.il Earth and Planetary Science Letters 186 (2001) 1^6 www.elsevier.com/locate/epsl