Toward a unified dynamic model for dykes and cone sheets in volcanic systems

International audienceIgneous sheet intrusions, such as dykes and cone sheets, represent various geometries of magma channels throughthe crust. In many volcanoes, they coexist as parts of complex plumbing systems and are likely fed by commonsources. How they form is fundamental regarding volcanic hazards, but yet no dynamic model simulates and predictssatisfactorily the diversity of sheet intrusions observed in volcanic systems. Here we present scaled laboratoryexperiments that reproduced dyke and cone sheet intrusion geometries under controlled conditions. Combined to aparametric study, a dimensional analysis shows that two dimensionless numbers 1 and 2 govern the formationof these intrusions. 1 is geometrical and describes the geometry of the magma source; 2 is dynamical and comparesthe local viscous stresses in the flowing magma to the host-rock strength. Plotting our experiments againstthese two numbers results in a phase diagram evidencing a dyke and a cone-sheet field, separated by a sharp transitionthat fits a power law. This result shows that dykes and cone sheets correspond to two distinct physical regimesof magma emplacement in the Earth’s crust. Cone sheets preferentially form when their source is shallow relativeto their size, when the magma influx (or viscosity) is large, or when the host rock is weak. In addition, both dykesand cone sheets may form from the same source, the shift from one regime to the other being then controlled bymagma dynamics, i.e. different values of 2. We compare our phase diagram to geological data and show that theextrapolated empirical dyke-to-cone sheet transition predicts the occurrence of dykes and cone sheets in variousnatural volcanic settings. This study thus provides a unified dynamic model of sheet intrusions emplacement andcaptures fundamental mechanisms of magma transport in the Earth’s crust