The stability of hazardous volcanic systems is strongly influenced by the uppermost magma storage depth and volatile exsolution1–3. Despite abundant evidence for an upper crustal magma reservoir beneath Yellowstone caldera4–7, its depth and the properties at its top have not been well constrained. New controlled-source seismic imaging illuminates a sharp reflective cap of the magma reservoir approximately 3.8 km beneath the northeastern caldera. Magma ascent to such low pressure is expected to drive volatile exsolution and potentially localized accumulation of bubbles near the top of the reservoir8,9, but this process typically remains hidden in contemporary volcanic systems. P-wave and P-to-S-wave reflections from the sharp top of the Yellowstone magma reservoir indicate that a mixture of supercritical fluid and magma fills the pore space at the cap of the approximately 3–8-km-deep low-shear-velocity layer imaged by seismic tomography6,7. The results are consistent with partial retention of bubbles exsolved from an upper crustal reservoir with ongoing magma supply from a volatile-enriched mantle source. Bubble accumulation can eventually lead to reservoir instability2,8, but the bubble volume fraction seismically estimated at the top of the reservoir today is lower than typical estimates of pre-eruptive conditions for rhyolites1,10,11, and measurements of the hydrothermal system document high fluxes of magmatic volatiles escaping to the surface12–15. We infer that the magma reservoir is in a stable state of efficient bubble ascent into the hydrothermal system on the basis of estimates that it is a crystal-rich (less than 30% porosity) reservoir for which dynamic modelling favours channelized bubble escape that prevents instability8.
