Alteration-driven permeability evolution in volcanic hydrothermal systems revealed by coupled THM(C) numerical modeling

Volcanic flank collapse is a recurrent natural disaster documented at volcanoes worldwide, including Mount St. Helens (1980), Bezymianny (1956), Bandai (1888), and Unzen (1792). 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. However, numerical investigation of mineral alteration and deposit formation in volcanic hydrothermal systems remains largely undeveloped. Current models of magmatically driven hydrothermal systems primarily focus on fluid and heat transport, often neglecting the mechanical response of the host rocks. Additionally, they typically consider the constant physical properties of the host rock, such as porosity and permeability. 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. Chemical processes are represented through indicators of conditions favorable to species transport rather than through explicit solute tracking. Such indicators are used to update the porosity and the permeability of the host rock.

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. Permeability heterogeneities 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.

In volcanic edifices, cold meteoric water flows from the head at the center to the toes on the sides. 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.

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, closing the pores and decreasing the permeability.

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.