Geodetic Insights into the Magmatic System of Askja Caldera, Iceland

Over recent decades, the conceptual model of magmatic systems has shifted from simple magma chambers to complex crystal mush zones, whose physical properties and mechanical behaviour remain poorly constrained. This thesis examines the mechanisms by which mush zones drive surface deformation at Askja by integrating InSAR, GNSS and microgravity data with numerical modelling.

Askja's most recent uplift began in July 2021, following almost four decades of subsidence, and continues to the present. The uplift is attributed to magma injection into shallow sills. When intrusion occurs within a compressible mush, part of the new magma volume is accommodated, leading to a trade‑off between magma density and mush compressibility that prevents direct density estimation from geodetic and gravity data alone. By adopting petrologically constrained density estimates from previous eruptions, I constrain mush compressibility to 0.7–1.2 × 10⁻¹⁰ Pa⁻¹, consistent with a poorly degassed basaltic mush.

To characterise the 2021–2023 uplift, I developed a poroelastic Finite Element Model (FEM) comprising two sills embedded within a mush framework, simulating magma transport and pore‑pressure diffusion during and after injection. Results indicate that the initial deformation was driven by an injection rate of 6 m³/s, combining contributions from shallow sills and a deeper reservoir, decaying to 0.45 m³/s by September 2021.

Finally, I investigate long-term subsidence, observed since at least 1985, using a poroviscoelastic FEM that couples regional plate spreading with the mechanical response of the mush. Results suggest that plate spreading alone is insufficient to generate the pressure gradients required to drive porous flow. Instead, the observed deformation is primarily governed by the viscoelastic response of the mush, supplemented by magma withdrawal triggered by initial high-pressure gradients.