2026-08-12 –, Room 3
JustRelax.jl (de Montserrat et al., (2026)) is an open-source, highly portable, and high-performance package designed for geodynamic modeling. It employs the Accelerated Pseudo-Transient (APT) method to solve the Stokes and diffusion equations, making it well-suited to exploit Graphics Processing Units (GPUs). It incorporates a wide range of features critical to computational geodynamics, including complex and highly non-linear rheologies, free surface, and a particle-in-cell method to advect material information.
Simulating the solid Earth's thermo-mechanical evolution requires solving coupled, non-linear Stokes and heat-diffusion problems across large domains with sharp material contrasts,complex nonlinear visco-elasto-plastic rheologies, and long time scales. Traditionally, these simulations run on High-Performance Computing (HPC) machines due to their substantial memory and computational demands. As HPC centres transition away from CPU-only architectures toward GPU-accelerated systems, large legacy codes written originally for CPU architectures face extensive rewriting and optimisation efforts.
Here, we present JustRelax.jl, an open-source, highly portable Julia package for geodynamic modelling. It employs the matrix-free Accelerated Pseudo-Transient (APT) method to solve the Stokes and heat-diffusion equations, making it well-suited to exploit modern GPU hardware while remaining fully functional on CPUs. JustRelax.jl exposes a high-level API while remaining highly modular, building upon a suite of packages developed within the (∂)GPU4GEO project. GeoParams.jl handles solver-agnostic rheology calculations, while JustPIC.jl manages advection via a multi-XPU Particles-in-Cell scheme specialised for staggered grids. Backend portability is achieved through ParallelStencil.jl's architecture-agnostic kernel abstractions, and distributed-memory communication is managed by ImplicitGlobalGrid.jl via MPI. JustRelax.jl demonstrates near-perfect algorithmic weak scaling in both 2D and 3D up to 512 GPUs, enabling us to perform large-scale geodynamic simulations on modern HPC machines.
Recently, the code has been utilised for a domain specific benchmark of plastic shear band localization using 64 Nvidia Grace-Hopper GPUs achieving a global resolution of 41'000 x 41'000. This resolution opens the door to simulate a lithospheric scale geodynamic model at the metre scale.
Reference
de Montserrat et al., (2026). JustRelax.jl: A Julia package for geodynamic modeling with matrix-free solvers. Journal of Open Source Software, 11(118), 9365, https://doi.org/10.21105/joss.09365
Pascal Aellig is a PhD student in computational geosciences at the Johannes Gutenberg University Mainz. He co-develops JustRelax.jl and other packages in the framework of geodynamics with the focus on the evolution of magmatic systems of various scales.
I am a Computational Geoscientist at the University of Lausanne developing scalable high-performance solvers for geodynamic modelling. My previous work explored the dynamics of interacting subduction zones and associated plate motion in the Mediterranean, while contributing to the development of computational tools for geodynamic applications using automatic differentiation and Julia.