Fig. 5: Excess pressure in a magma plumbing system.

Pressure history during inflation and deflation. Combined overpressure from magma flow and buoyancy pressure in a magma body (red, solid line considering a constant magma density and dashed line taking into consideration the magma density changes induced by gas exsolution) and in a location higher within a sustained magma channel (blue, solid line considering a constant magma density and dashed line taking into consideration the magma density changes induced by gas exsolution). The two curves are separated by ΔρgH (which is a constant value when volatiles are not considered). Note that volatiles are not expected to influence the overpressure evolution for a magma body located at 8 km or larger depth. Gas exsolution may cause the two curves to slightly diverge as deflation evolves, as such a process will always have a larger influence at shallower depth. Failure of the magma body occurs at t = 0 and caldera collapse (failure of caldera faults) begins in this example at day 8 after the failure of the magma body. For a long time prior to that (years), the pressure conditions are close to the magma body failure limit due to magma buoyancy. Overpressure in the magma body is limited according to the general failure criterion, here considered to correspond to be effective tensile strength of 2.5 MPa (black horizontal dotted line). In this example, slip on caldera ring faults begins at day 8 when critical conditions for caldera failure are reached (green horizontal line). Following that, there is small change in pressure as piston collapse drives out a large volume of magma over months. See text for discussion.