The dynamics of the dome at Santiaguito volcano, Guatemala

The in situ measurement of gas-jet dynamics at Santiaguito volcano, Guatemala, revealed that explosive degassing events are composed of single explosions (or pulses) that represent the sudden ejection of fast particles from one or more vents.We use a combined 2-D crosscorrelation and frequency analysis to explore the regularity of such pulses.During an event pulses occur regularly every 3 s.Here, we present a simple conceptual model to explain the pulsed nature of explosive events.In our model, magma rise through the conduit is subject to shear-fragmentation, which means that the whole magma column rises at once when the shear rate near the conduit walls exceeds the magmas yield strength, that is, the brittle failure criterion is reached.The sudden displacement of the magma column either leads to longitudinal oscillations of the column itself (given that the magma is compressible, magma spring model), or compresses a layer of bubble rich magma at shallow depth (gas cushion model).Pressure builds up inside this layer until the cap-rock is uplifted and gas can escape.The uplift is subject to friction along the walls.Because of its weight, the cap sinks back, thereby compressing the remaining gas giving rise to an oscillation of the dome surface.In the gas cushion modelusing a gas layer of 0.65 m at ∼80 m depth-those gas driven oscillations occur at about the same frequency as observed with our in-situ measurement.Using the magma spring model, the frequency highly depends on the bulk modulus of the magma (10 7 -10 9 Pa) and the length of the displaced magma column (here 50-400 m).