The growth of dykes from magma chambers

Failure of the margins of magma chambers produces dykes which may breach the ground surface and cause an eruption. The pressurization and failure of liquid-filled cavities is investigated with experiments in which gelatine is used as the analogue for a crustal medium. A theoretical analysis is derived which considers dyke nucleation to occur by the failure of a pre-existing magma-filled crack in the chamber wall. Flow of magma from the chamber into the crack gradually pressurizes and widens the crack. Eventually, the stress conditions for failure of the crack-tip are attained and dyke propagation commences. Magma viscosity influences the flux into the crack and hence the rate at which the crack-pressure increases. Higher viscosity magmas consequently require greater chamber overpressures and/or longer time delays before dyke nucleation occurs. The theoretical analysis is in approximate agreement with experimental results and provides estimates of the critical conditions required for a magma chamber to rupture. If a basaltic magma chamber is suddenly pressurized (e.g., by replenishment), the delay before dyke nucleation is predicted to be on the order of hours to days. For rhyolitic magmas, the delays are typically many years. Where chambers are gradually becoming pressurized (e.g., by exsolution during magmatic crystallization), cracks in the walls of relatively silicic magma chambers are likely to be further from pressure equilibration with the chamber, resulting in the development of higher chamber overpressures before dyke nucleation occurs. Solidification in silicic cracks which are slowly pressurizing may inhibit crack-tip failure; greater chamber overpressures are consequently required before dykes eventually form. The implications of higher chamber overpressures during dyke nucleation are that repose periods are longer, dykes propagate further before freezing, which allows dykes to transport silicic magmas over substantial distances, and a more vigorous eruption occurs if the dyke breaches the surface.