Dynamics of dikes versus cone sheets in volcanic systems

Abstract

Igneous sheet intrusions of various shapes, such as dikes and cone sheets, coexist as parts of complex volcanic plumbing systems likely fed by common sources. How they form is fundamental regarding volcanic hazards, yet no dynamic model simulates and predicts satisfactorily their diversity. Here we present scaled laboratory experiments that reproduced dikes and cone sheets under controlled conditions. Our models show that their formation is governed by a dimensionless ratio (Π 1 ), which describes the geometry of the magma source, and a dynamic dimensionless ratio (Π 2 ), which compares the 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 dike and a cone sheet field, separated by a sharp transition that fits a power law. This result shows that dikes and cone sheets correspond to distinct physical regimes of magma emplacement in the crust. For a given host rock strength, cone sheets preferentially form when the source is shallow, relative to its lateral extent, or when the magma influx velocity (or viscosity) is high. Conversely, dikes form when the source is deep compared to its size, or when magma influx rate (or viscosity) is low. Both dikes 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 Π 2 . The extrapolated empirical dike-to-cone sheet transition is in good agreement with the occurrence of dikes and cone sheets in various natural volcanic settings.

How caldera collapse shapes the shallow emplacement and transfer of magma in active volcanoes

“…This geometry cannot be explained when considering a stress field resulting from a gravitationally loaded edifice in the absence of stress relaxation. Similar cone sheet geometries, albeit without the characteristic twisting observed at Fernandina, have been also experimentally obtained with shallow intrusions, depending on the geometrical configuration of the system and the dynamic interplay between host-rock properties and viscous stresses (Galland et al, 2014).…”

“…Both radial dikes and inclined sheets have been observed at several eroded volcanoes, with the latter also possibly associated to shallow viscous magma (Galland et al, 2014). In particular, inclined sheets have been identified in Scotland (e.g.…”

“…The dynamics of magma storage, transport and eruption are thought to be controlled by both bottom-up (e.g., magma supply rate, volume and composition; e.g., Poland et al, 2012;Galland et al, 2014) and top-down processes (e.g., the effect of edifice load on magma ascent; e.g., Pinel and Jaupart, 2000; Muller et al, 2001). Understanding the state of stress within a volcanic edifice is one of the key ingredients to improve our ability of forecasting magma propagation and eruption.…”

Interaction between Transform Faults and Rift Systems: A Combined Field and Experimental Approach

“…According to the Bukingam-theorem, or pi Theorem (Tibaldi, 1995;Galland et al, 2014;Merle, 2015, and references therein), seven variables minus three dimensions, equal to four independent dimensionless numbers ( Table 1) are needed to maintain a value as close as possible between the field and the experimental system, in order to warrant similarity. Time was not scaled in our experiments, because the brittle behavior of sand and other Mohr-Coulomb materials is generally regarded as independent of the strain rate (Holohan et al, 2005;Merle, 2015).…”

Mechanisms of overburden deformation associated with the emplacement of the Tulipan sill, mid-Norwegian margin

“…In recent years, distinct mechanisms controlling the formation of sill-associated doming have been proposed. The most common mechanism involves synemplacement uplift accommodating sill intrusion (e.g., Pollard and Johnson, 1973;Roman-Berdiel et al, 1995;Malthe-Sørenssen et al, 2004;Hansen and Cartwright, 2006;Kavanagh et al, 2006;Menand, 2008;Bunger and Cruden, 2011;Galerne et al, 2011;Galland, 2012;Galland et al, 2014Galland et al, , 2015. These models typically assume that uplift occurs via elastic bending, and sometimes failure, of the overburden, causing forced folding and fracturing due to the intrusion.…”