UBC Theses and Dissertations
The timescales and consequences of solid-state sintering in volcanic systems Ryan, Amy Grace
The permeability of rocks controls the movement of fluids and distribution of pore pressure in Earth’s crust. In volcanic systems the eruptive behavior of silicate magmas is governed by the volume and pressure state of magmatic gases. Reductions in permeability can promote explosivity. At the elevated pressure-temperature conditions endemic to volcanic systems, permeable pathways are ephemeral and can be destroyed by densification processes. The prediction of the intensity and timing of explosive eruptions depends on understanding the mechanisms that create and destroy permeable pathways, and their operational timescales. Solid-state sintering is a diffusion-driven process that converts unconsolidated, crystalline aggregates into dense, low-permeability composites. Elevated temperatures and pressures and substantial dwell times facilitate densification and lithification by solid-state sintering. However, solid-state sintering has largely been discounted in volcanic systems because densification timescales were assumed to be long. In this dissertation I show, for the first time, that solid-state sintering occurs in volcanic settings and on timescales short enough to influence volcanic activity. I use the chemistry, mineral contents, physical properties and microstructures of volcanic shear zones exhumed during two dome-building eruptions to show that they were densified and lithified within the volcanic conduit as a result of solid-state sintering. Reconstructions of each of the eruptions indicate sintering occurred on the timescale of magma ascent (months to years). I also use hot pressing experiments conducted at volcanic temperature-pressure conditions to further constrain the timescale of densification by solid-state sintering: the experiments produce dense, low-permeability rocks over periods of hours to days, and reproduce the dominant textures and properties of naturally-sintered volcanic rocks. The experimental dataset is the basis of a quantitative model that predicts the time-dependent evolution of material density as a result of solid-state sintering. Overall, I identify solid-state sintering as a viable, unrecognized mechanism driving rapid permeability loss and material strengthening in volcanic settings. Densification and lithification as a result of solid-state sintering hinder fluid flow and modulate eruptive behavior, including promoting cyclical explosivity. Finally, my work challenges the perception that crystal-rich aggregates in volcanic settings are persistently permeable and weak areas.
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