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It is generally accepted that tectonic earthquakes may trigger volcanic activity, although the underlying mechanisms are poorly constrained. Here, we review current knowledge, and introduce a novel framework to help characterize earthquake-triggering processes. This framework outlines three parameters observable at volcanoes, namely magma viscosity, open- or closed-system degassing and the presence or absence of an active hydrothermal system. Our classification illustrates that most types of volcanoes may be seismically-triggered, though require different combinations of volcanic and seismic conditions, and triggering is unlikely unless the system is primed for eruption. Seismically-triggered unrest is more common, and particularly associated with hydrothermal systems.

In the current paradigm, magma primarily exists in the crust as a crystalline mush containing distributed melt lenses. If a melt-rich (or fluid) lens is less dense than the overlying mush, then Rayleigh-Taylor (RT) instabilities will develop and could evolve into spheroids of ascending melt. Due to contrasting melt-mush rheologies, the theoretical RT instability wavelength can be orders of magnitude larger than the magmatic system. We explored how this confinement affects the gravitational stability of melt lenses through laboratory experiments with pairs of liquids with one layer much thinner and up to 2.2 ⋅ 10 5 times less viscous than the other; we extended the viscosity ratio to 10 6 with linear stability analysis. We found the growth rate of a bounded RT instability is approximately Δ gD 6 2 , where Δ is the difference in density between the fluids, g is gravity, D is the container diameter, and 2 is the viscosity of the thicker viscous layer. This differs from the unbounded case, where the growth rate also depends on the thickness and viscosity of the thin, low-viscosity layer. Applying the results to melt lenses in magmatic mushes, we find that for the ranges of expected rheologies, the timescales for development of the instability, and the volumes of packets of rising melt generated span very wide ranges. They are comparable with the frequencies and sizes of volcanic eruptions and episodes of unrest and so suggest that RT instabilities in mush systems can cause episodic volcanism. Key Points:• Melt-mush Rayleigh-Taylor instabilities are generally laterally confined, which reduces the growth rate • The confined instability growth rate only depends on the mush viscosity, melt lens diameter, and density difference • Mush rheology is a key control on size and frequency of eruptions related to buoyancy instabilities

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