Speaker: Benoit Taisne
Magma ascent towards surficial eruptive vents or fissures occurs through dyke propagation in the vast majority of cases. Thus a failed eruption is such that the dyke cannot extend to the surface. We have investigated conditions for this using laboratory experiments and numerical calculations. When magma-filled cracks propagate close to the Earth’s surface, host rock temperature is well below the magma solidus. Solidification and substantial increase in magma viscosity can occur, are most pronounced near the propagating tip and can slow or arrest the progress of the dyke. We describe the propagation behaviour of such a hydraulic fracture using a laboratory experimental system of a crack fed by a constant flux of paraffin wax from a source reservoir propagating through gelatin below the solidus of the wax. The most novel behaviour is an intermittent regime in which cracks periodically stop advancing due to solidification, then swell at constant length while enhancing the elastic deformation in the surrounding solid before propagation resumes. Dyke propagation through a density stratified medium was studied numerically using a new calculation code that solves for viscous flow within the dyke and elastic deformation in the surrounding medium. We have studied how magma traverses a low density layer. The dyke continues to rise through the layer even though magma is negatively buoyant and develops increasingly large magma overpressures at the base of the layer. This overpressure may eventually exceed the threshold for fracturing in the horizontal direction, leading to the injection of a sill. Vertical dyke propagation can therefore proceed to the surface if the thickness of the low density layer is smaller than a threshold value. We have determined a scaling law for this threshold thickness as a function of magma buoyancy. Our theoretical model reproduces intermittent propagation with precise behaviour depending on the controlling stress balances, and provides a tool to analyse natural systems.