Rheological Properties of Ascending Magma at a Basaltic Andesitic System: Puy De La Vache, France As a Case Study

Abstract:

Eruptions at basaltic andesitic systems are often characterized by effusive behavior. However, an increasing number of examples around the World indicate that these magmas, with low degrees of differentiation and basic compositions, have the capacity to produce explosive eruptions; thus presenting a considerable hazard.

Volcanoes in the Chaîne des Puys (France), more specically the Puy de la Vache volcano, are good examples of explosive activity at basaltic andesitic centers. A combination of factors can explain this behavior: high magma ascent rate, high content of dissolved gases and high crystal contents. The latter leads to an increase in magma viscosity. The rheological properties of the magma control the eruptive style; viscosity itself strongly depends on composition, temperature, crystal fraction and vesicularity. In particular, an increase in crystal content may drastically change the composition of the residual melt and adds rigid particles to the mixture, inducing a complex strain rate dependence.

Here, we are interested in the rheology of basaltic andesitic magmas as a function of crystal content. To quantify the effect of crystals on the viscosity, we measured the stress-strain relationship of the remelted Puy de la Vache lava by concentric cylinder viscometry at super and subliquidus conditions. Concurrently, we performed a textural analysis to determine the crystal fraction at each temperature. We also measured viscosities close to the glass transition using a parallel plate viscometer. We then compared the measured viscosities with those obtained from calculations using existing models. With the experimentally obtained stress-strain rate relationship of the magma at different temperatures and crystal fractions, our goal is to design a model where viscosity varies across the conduit depending on the shear stress and strain rate conditions. This will allow us to characterize the potential explosive behavior of basaltic to andesitic magmas.