Time scales of shallow magma chamber replenishment at Campi Flegrei caldera.

Abstract

Ascent of primitive magmas from depth into shallow, partially degassed reservoirs is commonly assumed to be a viable eruption trigger. The resulting processes of convection and mixing have played an important role both in pre- and syn-eruptive stages in many eruptions of different sizes at the unrest Campi Flegrei caldera in Southern Italy. We performed numerical simulations of magma chamber replenishment referring to an archetypal case whereby a shallow, small magma chamber containing degassed phonolite is invaded by volatile-rich shoshonitic magma coming from a deeper, larger reservoir. The system evolution is driven by buoyancy, as the magma entering the shallower chamber is less dense than the degassed, resident phonolite. The evolution in space and time of physical quantities such as pressure, gas content and density is highly heterogeneous; nonetheless, an overall decreasing exponential trend in time can be observed and characterizes the efficiency of the whole process. The same exponentially decreasing trend can be observed in the amplitude of the synthetic ground deformation signals (seismicity over the whole frequency spectrum) calculated from the results of the magmatic dynamics. Depending on the initial and boundary conditions explored, such as chamber geometry or density contrast, the time constant thus the inferred duration of the process can vary. An initial vigorous phase of convection and mixing among the two magma types reaches an asymptotic stage after a few hours to half a day. Independently, the evolution of pressure in the magmatic system also depends on the initial and boundary conditions, leading either to eruption-favorable conditions or not. Relating the time scales for convective processes to be effective with their outcomes in terms of stresses at the boundaries of the magmatic system can substantially improve our ability to forecast the evolution of volcanic unrest crises worldwide.