Three‐Dimensional Failure Patterns Around an Inflating Magmatic Chamber

Abstract

Bedrock failure around an inflating magma chamber is an important factor that controls the occurrence of volcanic eruptions. Here, we employ 3‐D numerical models of elasto‐plastic shear failure around an inflating crustal reservoir, to study how the induced failure patterns depend on the geometry of the chamber, on the host rock strength and on the gravitational field. Our simulations show that either localized or diffuse plastic failure domains develop in 3 stages. Failure initiates (stage 1) after a critical overpressure is reached, the value of which depends on effective host rock strength. Next, and with increasing applied overpressure, either distributed (for zero friction angle) or localized plastic failure zones (for 30 ° friction angle) form (stage 2), until they finally connect to the surface (stage 3). Cylindrical chambers develop prismatic shear zones that merge from the surface and chamber walls. For spherical and prolate chambers, diffuse conical zones of failure develop from the chamber's crest, whereas for oblate symmetrical chambers, shear bands initiate at the horizontal tips but bend back above the center of the chamber to reach the surface. In contrast for asymmetrical oblate chambers, shear bands initiate in their cylindrical section and vanish along the elongated direction. Here, magmatic fluids may migrate both through diffuse elastic dilation zones at the tips, and through localized shear zones from the crest. Our results thus suggest how natural observations may be used to constrain the mode of failure occurring underneath a volcano. We discuss several natural examples in this context.

An Inside Perspective on Magma Intrusion: Quantifying 3D Displacement and Strain in Laboratory Experiments by Dynamic X-Ray Computed Tomography

“…In low-strength material, displacement of the overburden occurs mainly as uplift as strain localizes within a conical ring fault that propagates from the lateral edges of the cryptodome toward the surface (Figures 5, 9; Supplementary Material). This process is well-known both in laboratory and numerical models (Marti et al, 1994;Gudmundsson, 2007;Gerbault et al, 2018). In medium-strength material, the intrusion propagates laterally in a horizontal disk shape, until it reaches a critical diameter (Haug et al, 2018) and a conical mixed-mode ring fracture with a dominant openingmode component nucleates in the overburden (Figures 6, 10).…”

“…In medium-strength material, the intrusion propagates laterally in a horizontal disk shape, until it reaches a critical diameter (Haug et al, 2018) and a conical mixed-mode ring fracture with a dominant openingmode component nucleates in the overburden (Figures 6, 10). The observed opening-mode dominance is remarkable, because shear failure is predicted to dominate ring fault initiation in numerical models (Gerbault et al, 2018) and laboratory models using silica flour (Abdelmalak et al, 2012;Guldstrand et al, 2018). In high-strength material, displacement of the overburden is directed both upward and outward with strain localized around and ahead of the propagating dike tip and along the dike walls (Figures 7, 11).…”

“…Kavanagh et al, 2018). Host rock strength has been a particular point of interest for analog modeling studies (Mathieu et al, 2008;Abdelmalak et al, 2016;Guldstrand et al, 2017), but has also been shown to be a controlling factor in numerical models (e.g., Grosfils et al, 2013;Gerbault et al, 2018). With a varying ratio of sand to plaster and with other parameters held constant, our experiments confirm that thick cryptodome intrusions form in low-strength, loose or weakly consolidated sediments, while thin dikes form in highstrength materials such as limestones and other well-consolidated rocks (cfr.…”

Structural and Geochemical Interactions Between Magma and Sedimentary Host Rock: The Hovedøya Case, Oslo Rift, Norway

“…Mechanical indicators of LEFM, such as tapered intrusion fronts and cavities, were found in some cases in association with dike opening profiles that depart from the parabolic shape expected in elastic materials (Daniels et al, 2012;Vachon & Hieronymus, 2017) and blunt intrusion tips and shear-mode fractures. The dominant two-dimensional nature of outcrops and numerical models, including in this study, further hinders a better understanding of such complexities of the 3-D propagation process (e.g., Gerbault et al, 2018;Poppe et al, 2019;Stephens et al, 2017).…”

How Accurately Can We Model Magma Reservoir Failure With Uncertainties in Host Rock Rheology?

“…Numerical volcano models provide important evaluations of ground deformation signals using stress calculations and failure predictions, which are essential to give insight into the dynamic evolution of magma systems (e.g., Albino et al, ; Gerbault et al, , ; Gregg et al, , ; Grosfils, ; Hickey & Gottsmann, ; Hickey et al, ). Furthermore, the recent development of sequential data assimilation approaches provides near‐real‐time estimates of overpressure and stress state of magmatic systems, which is a promising step toward forecasting volcanic unrest with advanced observations (e.g., Albright et al, ; Bato et al, , ; Gregg et al, ; Gregg & Pettijohn, ; Zhan et al, ; Zhan & Gregg, ).…”