An Extended Rheological Map of Pāhoehoe—‘A‘ā Transition

Abstract

The pāhoehoe—‘a‘ā morphological transition involves a change in the rheological response of the magmatic suspension from pure viscous to complex rheological behavior, resulting in the development of tear-apart features. Here, we present a suite of concentric cylinder experiments aimed at studying the effects of cooling and shear rates on the rheology of a phonotephrite melt in response to crystallization. Experiments were conducted at: (a) isothermal subliquidus temperatures of 1,167–1,189°C and shear rates of 1–3 s−1; (b) constant cooling rates of 1–10°C/min and shear rates of 1–20 s−1. We defined the viscosity-temperature-time window of lava solidification, as well as the transition from coherent flow to shear localization and physical separation (i.e., viscous rupture). Through this approach, we mapped the processes and timescales affecting pāhoehoe—‘a‘ā transition in natural lavas at variable cooling and shear rates. Under disequilibrium conditions, as the cooling rate increases, both crystallization onset and viscous rupture occur at lower temperature and earlier in time. Moreover, the time to reach the crystallization onset and viscous rupture also decreases with increasing shear rate. Both increasing cooling and shear rate reduces the critical crystallinity required for viscous rupture, a consequence of the non-linear interplay between temperature, crystallization kinetics, and melt viscosity. This outcome expands our knowledge on compositional, thermal, and rheological changes in phonotephritic systems. In addition to shear rate and apparent viscosity, comparison with previous measurements on basaltic systems indicates that the pāhoehoe—‘a‘ā transition is sensitive to the composition and cooling path of lavas.