Rheological effects of microlites on the Plinian eruption of basaltic magma

We study the rheological effects of high concentrations of microlites on the Plinian eruption of basaltic magma. We characterize the non-Newtonian rheology of concentrated suspensions that are analogous to the magmas from two of the best-studied basaltic Plinian eruptions, Mt. Etna, Italy in 122 BC and Mt. Tarawera, New Zealand in 1886. Pyroclasts from both eruptions exhibit a very high abundance of microlites (60 -90 vol. %) in the matrix surrounding irregular shaped vesicles. Our experimental suspensions consist of silicone oil with viscosities similar to basalt magma (10-100 Pa s) with micron-size particles at concentrations of > 60 vol. %. We find that at the lower end of the range of the observed microlite (experimental particle) concentrations, the magma (suspension) has a yield strength of approximately 1,000 Pa, which is sufficient to affect bubble coalescence and textural evolution of the vesiculating magma. We also find an increase in the zero shear-rate viscosity by at least 103 orders magnitude. Furthermore, we observe that viscosity is both strain and strain-rate dependent. We integrate textural observations from pyrcolasts with experimental results to show how yield strength, viscosity increase, as well as strain and strain-rate dependence of viscosity modulate bubble growth, bubble coalescence and magma ascent during the Plinian eruptions of Mt. Etna and Mt. Tarawera.