Vibro-agitation of chambered magma

Abstract

We present the results of a novel set of calculations into the effect of in situ pressure reduction of a crystal-rich, basaltic magma layer by propagating seismic ( P) waves. Three stages in the process are identified. Critically, an instability can arise such that a low pressure melt layer develops close to the floor in initially densely packed magma (with mean crystal volume fraction ϕ ¯ = 0.6 ) on near-instantaneous timescales. The role of particle pressure, p˜( z), 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. The quasi-static particle pressure that develops as a result of the spatially-decaying oscillations leads to two effects: (1) a rapid reduction in interstitial melt pressure (c. 10 to 20% ambient pressure) leading to bubble formation, and (2) contraction of a thin magma layer at the base of the magma chamber. Where the basalt layer is overlain by silicic magma, the resulting gas phase may promote local gravitational interaction leading to chamber unrest. The proposed mechanism has implications for the timescales of crystal–liquid separation in magmas, which during the seismic event could be on the order of seconds.