Caldera collapse unloading volcanoes: the textbook case of Fernandina, Galapagos

Abstract:

Calderas are topographical depressions resulting from the yielding of magma chambers roof after large eruptions or intrusive events. On the outer slope, most calderas display radial fissures and, in limited cases, also circumferential fissures. Despite many hypotheses, the conditions controlling the formation of radial and/or circumferential fissures, and thus the shallow magma transfer within the volcano slopes, are still poorly understood. Here we demonstrate with numerical and analog models that the mass redistribution associated with caldera formation promotes shallow sill-shaped magma chambers and controls the orientation of eruptive fissures. We find that depending on the initial injection depth, dikes will bend or twist about an axis parallel to propagation resulting in circumferential and radial eruptive fissures, respectively. This mechanism is governed by the competition between gravitational unloading pressure and dike overpressure. We apply our results to Fernandina (Galapagos, Ecuador), the best case of caldera with radial and circumferential fissures, showing that the predicted stress field caused by the caldera unloading is consistent with the pattern of eruptive fissures and the dynamics of magma propagation.