Abstract

We present the results of a novel set of calculations into the effect of in-situ pressure reduction of a crystal-rich, crustal magma chamber by propagating seismic (P) waves. Three stages in the process are identified. Critically, an instability can arise such that a fluidized, low pressure melt layer develops close to the floor in initially densely packed magma (Φ = 0.6) on near-instantaneous timescales. The role of particle pressure, a newly identified force arising from interactions between adjacent crystals in the magma, is fundamental to the development of the instability, which will not arise in crystal-free liquids. Key variables governing the instability are identified and include the mean particle diameter, the excitation frequency (1-10 Hz), interstitial melt viscosity and melt compressibility. Small penetration depths (high particle pressures) develop most readily in compressible, volatile-bearing magmas at higher frequencies. The quasi-static particle pressure that develops as a result of the spatially-decaying oscillations leads to two effects: 1) a rapid reduction in the interstitial melt pressure (c.0.16 MPa/s) resulting in heterogeneous bubble nucleation, and 2) fluidization of a thin layer at the base of the magma chamber. Both these effects in turn increase the particle pressure. The fluidization effect provides a way of rapidly segregating crystals from interstitial liquid (mechanical differentiation) to produce a potentially unstable melt-rich layer at the chamber floor with a viscosity interval (η) in the range 102 < η <103 Pa s. A non-linear runaway effect is identified, driven by positive feedback between particle pressure, melt viscosity and degassing.