Stealth pressurisation of boiling magma limits geodetic detectability of volcanic eruptions

Crystallisation-driven volatile exsolution (second boiling) is widely invoked to pressurise shallow magma reservoirs, yet its ability to generate eruption-triggering overpressure and detectable ground deformation is poorly quantified. We couple phase equilibria with thermal and viscoelastic models of sill-like intrusions to map pressure evolution and geodetic detectability of second boiling. Thermodynamic calculations for basaltic and rhyolitic magmas from Askja (Iceland) and Puyehue-Cordón Caulle (Chile) are embedded in conductive thermal models for thin (20 m) and thick (200 m) sills at 2-4 km depth, and the resulting volume changes and magma compressibility are propagated into pressurisation histories and volume-change rates. Hydrous magmatic sills can develop overpressures of 20-30 MPa on timescales of months to centuries depending on initial sill size and crustal parameters, whereas relatively dry basalt undergoes cyclic behaviour with contraction followed by modest re-pressurisation. For a given composition and depth, thin and thick sills attain similar overpressures but differ by an order of magnitude in volume-change rate. Only small volatile-rich sills exceed geodetic resolution thresholds. In felsic crust, viscoelastic relaxation further dampens surface deformation, consistent with globally observed reduced deformation at arc volcanoes. Applied to Askja, cooling of a basaltic sill can explain the subsidence from 1983-2021, while second boiling in a small rhyolitic body can plausibly contribute to rapid uplift observed starting in mid-2021. Our results define a regime of “stealth” pressurisation in shallow hydrous reservoirs in mafic-intermediate crust, where eruption-scale overpressure can accumulate with undetectable geodetic unrest and help explain unexpected explosive eruptions with little precursory deformation.