Authors: Heriberto Patino Luna

Mentors: Raphael Affinito, Christelle Wauthier, Tushar Mittal

Abstract

Understanding the rheology of volcanic rocks, i.e., the relationship between stress (applied load) and strain (resulting deformation), is crucial for inferring magmatic system behavior from observations like ground deformation and predicting hazards such as flank collapse. For instance, Pacaya Volcano in Guatemala is prone to flank instability. Magmatic systems and flank stability models typically assume a (visco)-elastic rheology model with stress/strain/damage independent material properties. However, lab experiments for other rocks like granite and sandstone suggest that damage and compaction can significantly alter material properties.

We conducted uniaxial deformation experiments on Pacaya lava flows with samples representing different parts of a lava flow (base, core, and crust) with multiple stress-strain steps (stress-controlled and strain-controlled setups) to assess if the existing models are sufficient. Also, active acoustic data was collected to probe the rock’s internal structure evolution (analogous to ambient noise tomography). Using a Maxwell viscoelastic model with the experimental strain rate, we found a significant mismatch between the data and the model, even after optimizing elastic modulus and viscosity parameters. By allowing the elastic properties to be strongly stress-dependent by a factor of 30 (change in elastic modulus between 10 to ~100 MPa), we get a better model fit, but it still doesn’t fully represent the samples’ mechanical behavior. We describe the results of new rheological model fits using fractional visco-elastic models coupled with acoustic waveform features (wave velocity, frequency, coda structure) to parameterize the experimental data and provide insights into the sample’s microstructural evolution. Our results emphasize the importance of using lab-validated rheological models for understanding magmatic processes and improving hazard modeling.