Dr. Antonio COSTA (ESSC, University of Reading, Reading, Regne Unit; Istituto Nazionale di Geofisica e Vulcanologia, Nàpols, Itàlia)
During extrusive eruptions viscosity of magmas is strongly affected by variations temperature, crystal fraction and water content. That implies that momentum equation in magma flows is intimately coupled with equations describing kinetics of crystal growth, conservation of dissolved water, and energy balance. Crystallization of microlites and gas loss by permeable flow are time-dependent processes that can introduce strong feedback mechanisms which greatly amplify the effect on extrusion rates of small changes of chamber pressure, conduit dimensions or magma viscosity. When timescales for magma ascent are comparable to timescales for crystallization, there can be multiple steady solutions. Such nonlinear dynamics can cause large changes in dome extrusion rate and pulsatory behaviour of dome growth. Time scales of this kind of ciclicity are also affected by wall-rock elasticity. In fact, two distinct periods can be associated to the elastic deformation of both magma cham

On the other hand conduit geometry and the coupling between magma pressure, flow, and conduit wall deformation have major controls on the dynamics of explosive eruptions. Fragmentation level is typically deeper in a dyke than in a cylinder. For flows in wide dykes pressure at the fragmentation depth can be lower than the surrounding lithostatic pressure by several tens of MPa, indicating that the wall-rocks of the dyke will be unstable, constraining the dyke width and eventually blocking the eruption. The behaviour changes drastically when we assume the conduit is a dyke at depth that evolves to a cylinder near the surface. In this case even very wide dykes can be stable because the fragmentation level moves into the cylindrical region where deformation is negligible.

A different behaviour is possible accounting for the combined effect of local crustal extension and magma chamber overpressure. This coupling can sustain linear dyke-fed explosive eruptions with mass fluxes in excess of 10^10 kg/s from shallow-seated (4–6 km depth) chambers during moderate extensional stresses.


Dia:07 de juny de 2012
Hora:12:00
Lloc:Sala d'actes de l'Institut Jaume Almera