Plug and chug: The effects of volatile exsolution, and disequilibrium transport on cyclically erupting silicic volcanoes

Eruptions at silicic volcanos are often observed to be cyclical. Episodes of elevated degassing and magma extrusion arise with periods of several hours to days. Changes in eruption rates have been explained by stick-slip behavior of a viscous plug of magma. Yet, the mechanism by which the viscous plug is periodically perturbed, over a narrow, regular frequency range remains a topic of debate. Previous studies suggest that a permeable, gas-rich, viscous magma may select pressurized gas waves with specific periods due to competition between magma compaction and the expansion of compressible gas upon ascent. Such pressurized gas waves are inferred to induce cyclical eruptive behavior by interaction with the viscous plug of 1-100 hours, vaguely consistent with observations at active silicic volcanos. To refine the predicted range of periodicity for cyclical eruptions we present a model where volatiles dissolved in the magma exsolve during ascent of the volcanic conduit. Well known experimental studies show that volatile-free magma is several orders of magnitude more viscous than magma containing dissolved volatiles. Furthermore, volatile exsolution from magma during decompression and ascent results in supercooling and is associated with the rapid nucleation of microlite crystals causing further increases in viscosity. Therefore, changes in the volatile content of magma during extraction is key to understanding the necessary time and length scales required for plug formation and simultaneously leads to changes in the dynamics of the previously studied gas-magma mixture. Using two-phase flow theory, we construct a conduit model that accounts for changes in magma viscosity as a volatiles are transferred from the magma to the gas phase. We find that increases in viscosity during ascent selects longer wave-length, higher velocity gas waves thereby reducing the range of predicted periodicity for vulcanian eruptions. Longer eruption cycles imply that the magma and gas drift further away from thermodynamic equilibrium during ascent, yielding pressurized gas waves of greater amplitude and thus greater overpressure. Gas waves with greater overpressure are more likely to lead to catastrophic failure of the viscous plug and present greater potential for explosive volcanic hazard.