NEW APPLICATION FOR EVALUATING THE INFLUENCE OF CRYSTALLIZATION ON MAGMA RHEOLOGY

Viscosity strongly influences the rheological behavior of magmas, which is a key determinant of magma transport process and volcanic eruption style. Understanding the factors controlling viscosity is important in terms of characterizing risks posed by active volcanoes and forecasting hazards. Natural magmas can show a very wide range in viscosity, from 10^-1 – 10¹⁴ Pa s, depending on composition, temperature, volatile content and crystal fraction. In particular, the suspended crystal fraction can induce a significant increase in viscosity through crystal-crystal interaction and complex flow dynamics of the melt.

Here, we test the relative viscosity increase induced by crystallization of natural andesitic (57 wt.% SiO₂) and basaltic (48 wt.% SiO₂) melts, using the method developed by Vona (2011) for the concentric cylinder apparatus. The viscosity is investigated for each sample (1) above its liquidus temperature (1400 °C) and (2) at sub-liquidus conditions. For each temperature increment, thermal equilibrium is achieved over a period of days under constant stirring. Simultaneous monitoring of the torque is used to calculate the apparent viscosity of the suspension. After the experiment, the mineralogical assemblage, the crystal fraction and the preferential crystal alignment are quantitatively analyzed.

At temperatures below the liquidus, all melts show a continuous increase of relative shear viscosity due to increases in crystal fraction. First, at a given sub-liquidus temperature, small, equant oxides crystallize, causing a slight increase in viscosity. After some delay, crystallization of large, tabular plagioclase begins, inducing a much larger asymptotic increase in apparent viscosity until equilibration is achieved. Furthermore, a decrease in strain rates results in an increase in viscosity, evidencing the non-Newtonian character of the magmatic suspension. At lower sub-liquidus temperatures, additional crystallization is promoted with amplified rheological consequences. In conclusion, these experiments indicate that natural intermediate and basic magmas undergo significant rheological changes at the onset of crystallization, which deserve consideration in magma migration models.