Stress-driven catastrophic failure of volcanic conduits in explosive eruptions

Abstract:

In most explosive eruption models, the wall rock of the volcanic conduit is portrayed as a passive boundary condition for the transport of magma from depth. We argue that in many cases the volcanic conduit can play an active role in determining eruption progression, and, in particular, the termination of an eruption. This understanding matches well with actual observations of explosive eruptions, which often terminate abruptly. We couple (one way) a multiphase conduit flow model to a solid mechanics model of the conduit wall rock, and determine the controls (shear and normal tractions and in situ host rock stress stress) sufficient to trigger wall-slumping shear failure which could lead to the termination of an eruption. Failure is significantly influenced by shear tractions exerted on the conduit wall by the viscous flow of magma, and we derive a critical shear traction which guarantees conduit failure. Because shear tractions and gradients in normal stress are greatest near the fragmentation depth, failure is generally incipient at the depth of fragmentation. In addition to conduit flow tractions, the in situ host rock stress state is an essential control on the nature of conduit failure. Informed by analogous work in borehole stability analysis, we show that catastrophic conduit failure, characterized by radially-inward normal faulting possibly disrupting flow, is only possible in certain ambient stress states. Finally, we propose a general model for the understanding of the evolution of explosive eruptions. This model is defined by conduit widening earlier in an eruption accompanied by decreasing chamber pressure. These two phenomena work together to change the distribution and orientation of stresses along the conduit wall. We show that as an eruption progresses, the conditions shift to favor wall-slumping conduit failure, which can explain the abrupt termination of many explosive eruptions.