The rheology of crystal-bearing basaltic magmas from Stromboli and Etna A. Vona a,⇑ , C. Romano a , D.B. Dingwell b , D. Giordano c a Dipartimento di Scienze Geologiche, Universita ` degli Studi Roma Tre, L.go San leonardo Murialdo 1, 00146 Rome, Italy b Department of Earth and Environmental Sciences, Ludwig Maximilians University, Theresienstr. 41/III, 80333 Munich, Germany c Institute of Earth Sciences “Jaume Almera”, CSIC, Lluis Sole ´ i Sabaris s/n, 08028 Barcelona, Spain Received 23 July 2010; accepted in revised form 10 March 2011; available online 25 March 2011 Abstract Our ability to forecast volcanic hazards at active volcanoes stems from our knowledge of parameters affecting eruption dynamics. Persistent activity displayed by basaltic volcanic systems, such as Etna and Stromboli, is governed mainly by the textural evolution and rheological properties of eruptive products. Here, we investigate the high temperature (1131 °C< T < 1187 °C) rheological behavior and textural evolution of remelted natural basaltic magmas from Stromboli and Etna volcanic systems upon cooling and crystallization at atmospheric conditions. The pure liquid and subliquidus iso- thermal viscosities are investigated using a concentric cylinder apparatus by varying strain rate applied to the investigated materials. Detailed textural analysis has been performed in order to evaluate the effect of crystal distribution on the rheolog- ical evolution of magmas. At subliquidus conditions, the mineralogical assemblage is dominated by the presence of spinel and plagioclase with the occurrence of stable clinopyroxene only at the lowest temperatures (1157 and 1131 °C for the Stromboli and the Etna samples, respectively). The overall crystal fraction (/) varies between 0.06 and 0.27. In the high T regime the viscous response to applied stress and strain rate is strongly affected by the presence of even small amounts of crystals. Large discrepancies between measured and predicted viscosities obtained using the Einstein–Roscoe (ER) equation are also found at low crystal fractions. We find here that, at the investigated conditions, the physical effect caused by the presence of elongated crystals is orders of magnitude higher than that predicted by existing models which only account for spherical particles. It also appears that a weak shear thinning behavior occurs at the lowest temperatures investigated. Crystal shape and, with it, the strain-rate dependence of the rheological properties appear to play primary roles in influencing the transport properties of these basaltic magmas. Ó 2011 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Viscosity is a critical property governing the efficiency of transport processes of molten geomaterials. It controls eruption dynamics and affects physico-chemical processes such as degassing and crystallization in magmas (e.g., Car- icchi et al., 2007; Giordano et al., 2008, 2010). The viscosity of naturally-occurring magmas can span in fact more than 15 orders of magnitude (10 1 –10 14 Pa s) primarily in re- sponse to variations in melt composition (X), volatile con- tent, temperature (T), pressure (P), as well as variation in the proportions and distributions of suspended solids and vesicles (Webb and Dingwell, 1990; Dingwell et al., 1992; Dingwell, 1996; Papale, 1999; Llewellin et al., 2002; Llewel- lin and Manga, 2005; Vetere et al., 2006; Ardia et al., 2008; Giordano et al., 2009). Heat production (viscous heating) or loss, due to either friction or cooling at the conduit wall during ascent of magma toward the surface, are also fea- tures observed in natural environments strongly affecting magma viscosity (Rosi et al., 2004; Costa et al., 2007; Giordano et al., 2007). A large number of experimental 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.03.031 ⇑ Corresponding author. Tel.: +39 06 57338082; fax: +39 06 57338201. E-mail address: avona@uniroma3.it (A. Vona). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 3214–3236