Numerical model of magmatically driven hydrothermal system THM(C) in volcanic settings
(2025) 36th ALERT Workshop — Location: Aussois, France (29.September.2025)
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Poulet, Thomas 
- Abstract
- (en) Volcanic flank collapse is a recurrent natural disaster documented at volcanoes worldwide, including Mount St. Helens (1980), Bezymianny (1956), Bandai (1888), and Unzen (1792) ([1], [2]). These large-scale instabilities are often linked to hydrothermal alteration, in which circulating fluids and heat interact with volcanic rocks, altering their mineral composition and weakening their mechanical properties [3]. However, numerical investigation of mineral alteration and deposit formation in volcanic hydrothermal systems remains largely undeveloped. Current models of magmatically driven hydrothermal systems primarily address fluid and heat transport, often neglecting the mechanical response of host rocks. This limits their usefulness in assessing volcanic stability. In this context, modeling the coupled thermal, hydraulic, mechanical, and chemical processes offers a new way to identify zones prone to alteration and potential flank instability. We constructed a two-dimensional numerical model of a magmatically driven hydrothermal system using the finite element method (FEM) within the open-source MOOSE framework, which is a multiphysics environment for solving coupled nonlinear problems. The PorousFlow module was used to simulate fluid flow, heat transfer, mechanical behavior, and chemical processes. The model couples heat from a magmatic source with fluid circulation in the surrounding porous medium, as well as the resulting stress changes in the host rock. Chemical processes are represented through indicators of conditions favorable to species transport rather than through explicit solute tracking. Currently, the mechanical response is limited to linear elasticity. This new model, still under development, offers insights into the dynamics of magmatically driven hydrothermal systems. Permeability is the main factor determining the driving heat transfer mechanism between conduction and advection ([4], [5]). Heat conduction coefficient affects the driving heat transfer mechanism less because its range of values in rock is smaller than that of permeability. Consequently, a variation in the order of magnitude of permeability between rock layers might cause heat accumulation and vaporization, or ,conversely, provide an easy escape route. Similarly, faults or other vertical heterogeneities, change the entire dynamic by creating a water freeway from deep within the earth to the surface. Classifying hydrothermal systems as advective or conductive, also provides insight into how volcanic settings respond to regional stresses and strains. A low-permeability, conductive system, will experience a high stress peak when subjected to a high strain rate, such as during an earthquake. On the other hand, a high permeability, advective system, will easily dissipate an increase in stress when subjected to a high regional strain rate. In volcanic edifices, cold meteoric water flows from the head at the center to the toes on the sides ([4], [5]). This flow shields the volcanic edifice from the hot mineralized (magmatic) water from deep below. This creates relatively sharp temperature variations underneath and near the sides of the volcanic edifice. This process also facilitates the accumulation of high-temperature areas near the bottom of the volcanic slopes and mineral transport. Thermal stresses around the volcanic edifice can reach 20 MPa of tension when assessed elastically. Volcanic rocks do not have a tension strength of 20 MPa, so such elastic assessment is physically incorrect. Nevertheless, the thermal stresses are sufficient to explain the nucleation of faults and the opening of existing ones. Since this swelling behavior creates tension near the surface due to temperature variation alone, it could certainly explain magma rising. Indeed, heat travels faster than the magmatic and clearly creates faults. These faults cause depressions that suck magma to the top. As the magma come closer to the surface, it heats on its turn the area, creating even more faulting and depression. The presence of the necessary conditions for the dissolution or precipitation of minerals in the hydrothermal system is used to track the transport of chemical species. Due to the shielding effect of the cold downward flow, the chemical species are not transported to or from the body of the volcanic edifice. Instead, they are transported on the sides at the base of the volcanic edifice’s slopes. Since hydrothermal alteration decreases rock mechanical strength, it could explain volcanic flank instabilities, especially since it occurs at the critical slope toe. The numerical model is still being developed mechanically to couple the opening of existing faults, the nucleation of faults, and plastic computations with the other physics. References: [1] L. Siebert, “Large volcanic debris avalanches: Characteristics of source areas, deposits, and associated eruptions,” Journal of Volcanology and Geothermal Research, vol. 22, no. 3–4, pp. 163–197, Oct. 1984, doi: 10.1016/0377-0273(84)90002-7. [2] L. Siebert, H. Glicken, and T. Ui, “Volcanic hazards from Bezymianny- and Bandai-type eruptions,” Bull Volcanol, vol. 49, no. 1, pp. 435–459, Feb. 1987, doi: 10.1007/BF01046635. [3] M. Detienne, “Unravelling the role of hydrothermal alteration in volcanic flank and sector collapses using combined mineralogical, experimental, and numerical modelling studies,” Université catholique de Louvain, Louvain-la-Neuve, 2016. [4] S. Scott, T. Driesner, and P. Weis, “Geologic controls on supercritical geothermal resources above magmatic intrusions,” Nat Commun, vol. 6, no. 1, p. 7837, July 2015, doi: 10.1038/ncomms8837. [5] S. Scott, T. Driesner, and P. Weis, “Boiling and condensation of saline geothermal fluids above magmatic intrusions,” Geophysical Research Letters, vol. 44, no. 4, pp. 1696–1705, Feb. 2017, doi: 10.1002/2016GL071891.
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Citations
Niclaes, J., Poulet, T., Delmelle, P., & Rattez, H. (2025). Numerical model of magmatically driven hydrothermal system THM(C) in volcanic settings. 36th ALERT Workshop, Aussois, France. https://hdl.handle.net/2078.5/256259