IMAGING THE INTERIOR OF MULTI-PHASE ANALOG LAVA FLOWS: MRI MEASUREMENTS OF VELOCITY AND RHEOLOGY

Lavas and magmas are complex, multi-phase materials whose rheology depends on the dynamic interactions between silicate melt, crystals, and vapor bubbles. Lava rheology is a key parameter in understanding the dynamics of flow emplacement, but measurements are challenging to make, and choosing appropriate constitutive relationships between lava texture and rheology is an ongoing challenge in volcano science. Measurements in the field and under many laboratory conditions are limited to the exterior of flows due to the opaque nature of suspensions, and estimates of rheology and interior dynamics rely on theoretical and numerical models.

We address this challenge by applying phase-contrast velocimetry nuclear magnetic resonance imaging (PCV-MRI) to directly measure the velocity field in the interior of analog lava flows. Using a dam-break geometry, the suspension is initially confined to a rectangular reservoir and instantaneously released to slump under its own weight down a confined channel. As a lava analog, we use silicone oil with viscosities of ~1 and 5 Pas and create suspensions of 0-0.45 volume fraction solid particles (sesame seeds) and 0-0.2 volume fraction bubbles.

We compare the velocity field measured using MRI to optical imaging techniques and velocities predicted by the commonly-applied shallow-water equations. We highlight where the flow conforms to shallow-water theory and where edge effects become important. We utilize the fact that in dam-break geometries the flow experiences a range of strain rates simultaneously (from 0 up to 2 1/s in our experiments). We calculate shear rates in the flow interior from the measured velocities and estimate stresses generated by the free surface to obtain rheology curves for suspensions in the low to intermediate capillary regime. Finally, we fit a Herschel-Bulkley model to observed rheology and compare our results to existing empirical models of suspension rheology.