Lava domes: growth phases and deformation under variable effusion rate

Lava domes typically form during the eruption of highly viscous lava from a volcanic vent. Due to their high viscosity, they spread slowly over a limited spatial area, unlike less viscous lava flows. However, lava domes are potentially lethal because they cyclically collapse, generating pyroclastic flows, or explode. To date, these latter events are only partially understood and are linked to various sources of overpressure in a context, often overlooked, of variable effusion rates. Here, we present 3D numerical simulations of the growth of a viscous lava dome, allowing us to determine the total and dynamic pressure, as well as the components of the strain rate and stresses within the dome, and to study the influence of the flow rate on pressure, strain rate, and stresses.  Using a non-dimensional scale analysis involving the dimensions of the vent, we show the different growth phases of a lava dome during a sequence involving (i) a phase of constant input flow rate through the vent; followed by (ii) the cessation of discharge (i.e. zero input flow rate through the vent). Considering the radial, hoop and vertical shear strain rate components, respectively, e_rr, e_θθ and e_rz , as well as the corresponding stresses and comparing the magnitudes of the latter to typical yield strengths, we examine through space and time where ring fractures, radial tensile fractures, and shear fractures may occur.  We show that the location of these different fracture mechanisms depend on the growth phase and the time at which the eruption ceases (i.e. the time when the imposed flow rate is set to zero).  Lastly, the arrest of lava discharge is found to lead to rapid dome depressurization and subsidence.  We will discuss the implications of sudden lava dome depressurization as triggers for the breakdown and explosion of lava domes.