The effect of particle size on the rheology of liquid-solid mixtures with application to lava flows: Results from analogue experiments

We investigate the effect of crystal size on the rheology of basaltic magmas by means of a rheometer
and suspensions of silicon oil with natural magmatic crystals of variable size (from 63 to 0.5 mm) and
volume fraction fi (from 0.03 to 0.6). At constant fi, finer suspensions display higher viscosities than
coarser ones. Shear thinning (flow index n < 1) occurs at fi > 0.1–0.2 and is more pronounced (stronger
departure from the Newtonian behavior) in finer suspensions. Maximum packing and average crystal size
displays a nonlinear, positive correlation, while yield stress develops at fi > 0.2–0.3 irrespective of the
crystal size. We incorporate our results into physical models for flow of lava and show that, with respect
to lava flows containing coarser crystals, those with smaller crystals are expected to: 1) flow at lower
velocity, 2) have a lower velocity gradient, and 3) be more prone to develop a region of plug flow. Our
experimental results explain the observation that phenocryst-bearing and microlite-bearing lavas at Etna
volcano (Italy) show smooth pahoehoe and rough aa’ surfaces, respectively.

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