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 typical InSAR resolution thresholds. In felsic crust, viscoelastic relaxation further dampens overpressure and surface deformation, consistent with globally observed reduced deformation at arc volcanoes. Our results define a regime of “stealth” pressurisation in shallow hydrous reservoirs, in which magma emplaced centuries earlier may accumulate eruption-scale overpressure with little or no unrest detectable by short temporal baseline geodetic monitoring. This mechanism can help explain unexpected explosive eruptions with little precursory deformation.