Solid-State Sintering Can Cause Explosivity and Seismogenic Unstable Sliding During Dome-Building Eruptions

The hydraulic and rheological properties of shear zones that develop in dome-building eruptions govern the potential for explosive and seismic behavior at those volcanoes. Previous isostatic hot-pressing experiments demonstrated that crystalline fault gouge undergoes solid-state sintering at high temperature and pressure, and sintering causes lithification and permeability loss within volcanic shear zones over years. We present results of torsion experiments in which we document, for the first time, gouge rheological behavior at the temperature-normal stress-strain rate conditions expected in volcanic shear zones, and determine the effect of shear on solid-state sintering rate. Gouge sheared at 500°C, 50 or 100 MPa normal stress and 10⁻⁴ to 10⁻³ s⁻¹ does not sinter, exhibits strain-independent behavior, and deforms by distributed granular flow. In contrast, gouge sheared at 900°C, 50 or 100 MPa normal stress and 10⁻⁵ to 10⁻³ s⁻¹ sinters, strain-weakens, and exhibits microstructural evidence of strain localization and significant lithification. Samples sheared at 900°C for <1 hr have final porosities and permeabilities comparable to gouge samples hot-pressed for 60 hr, indicating sintering is enhanced by a shear stress acting on grain boundaries in addition to a normal stress. These results are used to develop a model for time-dependent densification by solid-state sintering that is applicable when gouge is static or shearing. Finally, we propose that sintering-driven lithification can cause deforming shear zones to transition from aseismic stable sliding to seismogenic sliding, potentially resulting in the drumbeat seismicity frequently observed at dome-building volcanoes.