The brittle-ductile transition in active volcanoes

Abstract

Contrasting deformation mechanisms precede volcanic eruptions and control precursory signals. Density increase and high uplifts consistent with magma intrusion and pressurization are in contrast with dilatant responses and reduced surface uplifts observed before eruptions. We investigate the impact that the rheology of rocks constituting the volcanic edifice has on the deformation mechanisms preceding eruptions. We propose a model for the pressure and temperature dependent brittle-ductile transition through which we build a strength profile of the shallow crust in two idealized volcanic settings (igneous and sedimentary basement). We have performed finite element analyses in coupled thermo-hydro-mechanical conditions to investigate the influence of static diking on the local brittle-ductile transition. Our results show that in active volcanoes: (i) dilatancy is an appropriate indicator for the brittle-ductile transition; (ii) the predicted depth of the brittle-ductile transition agrees with the observed attenuated seismicity; (iii) seismicity associated with diking is likely to be affected by ductile deformation mode caused by the local temperature increase; (iv) if failure occurs within the edifice, it is likely to be brittle-dilatant with strength and stiffness reduction that blocks stress transfers within the volcanic edifice, ultimately damping surface uplifts.

Intense Seismicity During the 2014–2015 Bárðarbunga‐Holuhraun Rifting Event, Iceland, Reveals the Nature of Dike‐Induced Earthquakes and Caldera Collapse Mechanisms

“…If we interpret the base of the seismicity to represent the depth of the brittle‐ductile transition, we can infer that the top of the zone of melt accumulation (where it is too hot for brittle failure to occur, except at very high strain rates) is below this depth (e.g., Parisio et al, ). At the neighboring Askja volcano, the maximum depth of crustal seismicity shallows from ∼8 km b.s.l.…”

The risks of long-term re-injection in supercritical geothermal systems

“…Our approach is a first attempt to model the complex geomechanical behaviour of ESGS. Several open questions remain at present to be addressed, and future studies should focus on the influence of inelastic behaviour (rock strength decays at high temperature 70 ), on the role of geological structures and multiple faults, on the full three dimensional reservoir behaviour, on the presence of fluid mixtures (), on modelling the frictional behaviour of faults and dynamic rupture. Nevertheless, we have provided an important piece of evidence to support the role of cooling in connection to induced seismicity and fault destabilization.…”

A Model of Failure and Localization of Basalt at Temperature and Pressure Conditions Spanning the Brittle‐Ductile Transition

“…The plastic yield surface is a function of the invariants of the effective stress tensor, namely, the deviatoric (von Mises equivalent) and the mean stress , which are defined as where . The expression of the yield surface (Figure 1) is an extension of the Hoek‐Brown model (Grassl & Jirásek, 2006), which was previously employed to characterize the BDT of rocks (Parisio et al., 2019), and reads …”

“…The mechanisms that control the deformation of intact rock under shearing depend on environmental variables, such as the acting temperature, stress, and strain rate (Kato et al., 2004; Kumari et al., 2017; Odedra et al., 2001; Ohnaka, 1995; Violay et al., 2012; Wong, 1982). As temperature rises with depth, it influences the solid and fluid rheology and their interaction, and as a result the environmental effects are even more relevant in the deep part of the crust and in regions where the geothermal gradient is above average, such as volcanic areas (Parisio et al., 2019). The deformation characteristics of rocks can determine the (potential) maximum depth at which earthquakes can be expected: While several have placed the limit at 600°C (references within Molnar, 2020), evidence suggests that such a limit is likely to be arbitrary and that earthquakes could be expected in the hot crust at ∼800°C (Molnar, 2020).…”