THE SUBORDINATE ROLE OF BUOYANCY IN TRIGGERING THE ERUPTION OF LARGE MAGMA RESERVOIRS

Recent analytical investigations suggest that magma buoyancy is critical for triggering catastrophic caldera forming eruptions. Through detailed assessment of these approaches, we illustrate how analytical models have been misapplied for investigating buoyancy and are, therefore, incorrect and inconclusive. Nevertheless, the hypothesis that buoyancy is the critical trigger for large silicic eruptions warrants further investigation. As such, we utilize viscoelastic finite element models that incorporate buoyancy to test overpressure evolution and mechanical failure in the roof due to the coalescence of large buoyant magma bodies for two model cases. In the first case, we mimic empirical approaches and include buoyancy as an explicit boundary condition. In the second set of models, buoyancy is calculated implicitly due to the density contrast between the magma in the reservoir and the host rock. Results from these numerical experiments indicate that buoyancy does not promote overpressurization of large silicic magma reservoirs. Furthermore, no mode-1 failure is calculated along the magma chamber boundary due to buoyancy in large reservoirs. Rather, compressional stresses are observed due to buoyant magma focusing away from the edges of the reservoir and towards the center. Given the shortcomings of the analytical implementations and the results from the numerical experiments, we conclude that buoyancy does not provide an eruption triggering mechanism for large silicic systems. Claimed correlations of magma residence times, the eruption frequency-volume relationship, and the global observations of caldera dimensions are better explained by the model for thermomechanical evolution of calderas presented by Gregg et al. (2012).

Gregg et al. (2012), Catastrophic caldera-forming eruptions: Thermomechanics and implications for eruption triggering and maximum caldera dimensions on Earth, JVGR, 241-242, 1-12, doi: 10.1016/j.jvolgeores.2012.06.009.