Towards a General Model for Volcanic Caldera Dynamics

Abstract:

Volcanic calderas often show a behavior different from that of other volcanoes. In caldera complexes, it is not unusual to record long-term unrests, with remarkable ground deformation, seismicity, and geochemical changes, that do not culminate in an eruption. On the contrary, in certain cases, an unrest accompanied by minor geophysical changes can be followed in few months by an eruption, as in the case of Rabaul Caldera in 1994. Those behaviors are not simple to interpret. The dramatic advances in volcano monitoring over the last years has allowed us to record the dynamic phenomena of several calderas with great detail. This, highlighted characteristics that are typical of a single caldera, but also some features common to several calderas. The main common features are remarkable ground deformation with intense uplift episodes, that are often followed by subsidence. The ground deformations are generally characterized by strong horizontal components. The seismicity is almost always in swarms and has a spatial distribution that often shows seismic gaps. Moreover, calderas are the largest geothermal systems in the world. We think that a process of sill intrusion can explain the common features highlighted by many observations carried out on different calderas. We developed a dynamic model of sill intrusion in a shallow volcanic environment. In our model, the sill, fed by a deeper magma reservoir, intrudes below a horizontal elastic plate, representing the overlying rocks, and expands with axisymmetric geometry. The model is based on the numerical solution of the equation for the elastic plate, coupled with a Navier-Stokes equation for simulating the dynamics of the sill intrusion. We performed a number of simulations, with the objective of showing the main features of the model. In the experiments, when the feeding process stops, the vertical movement reverses its trend and the area of maximum uplift undergoes subsidence. Under certain conditions the subsidence can occur even during the intrusion of the sill. The stress field produced by the intrusion is mainly concentrated in a circular zone that follows the sill intrusion front. Results are consistent with the characteristics of calderas, described above.