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