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Permeability controls energy and matter fluxes in deep‐sea hydrothermal systems fueling a 'deep biosphere' of microorganisms. Here, we indirectly measure changes in sub‐seafloor crustal permeability, based on the tidal response of high‐temperature hydrothermal vents at the East Pacific Rise 9°50'N preceding the last phase of volcanic eruptions during 2005–2006. Ten months before the last phase of the eruptions, permeability decreased, first rapidly, and then steadily as the stress built up, until hydrothermal flow stopped altogether ∼2 weeks prior to the January 2006 eruption phase. This trend was interrupted by abrupt permeability increases, attributable to dike injection during last phase of the eruptions, which released crustal stress, allowing hydrothermal flow to resume. These observations and models suggest that abrupt changes in crustal permeability caused by magmatic intrusion and volcanic eruption can control first‐order hydrothermal circulation processes. This methodology has the potential to aid eruption forecasting along the global mid‐ocean ridge network. Plain Language Summary: Permeability governs how fluids such as water and melt flow beneath the seafloor, and is influenced by diverse magmatic, volcanic, and tectonic forces as new oceanic crust is formed and altered over time. Despite being a master variable controlling the fluxes of energy and matter in deep‐sea hydrothermal systems, permeability remains a poorly constrained hydrologic parameter of the oceanic crust. Here, we indirectly measure changes in crustal permeability for the first time, using the tidal response of high‐temperature hydrothermal vents on the East Pacific Rise at 9°50'N preceding the last phase of volcanic eruptions in 2005–2006. We show that abrupt changes in crustal permeability caused by magmatic intrusion and volcanic eruption can control first‐order hydrothermal circulation processes. Key Points: Hydrothermal vent fluid temperature data modeled with ocean tides reveal permeability changes associated with seafloor volcanic eruptionsChanges in crustal permeability caused by magmatic intrusion and volcanic eruption control first‐order hydrothermal circulation processesThis methodology expands the potential of forecasting seafloor eruptions along the global mid‐ocean ridge network [ABSTRACT FROM AUTHOR]

Two-thirds of Earth's surface is formed at mid-ocean ridges, yet sea-floor spreading events are poorly understood because they occur far beneath the ocean surface. At 9°50'N on the East Pacific Rise, ocean-bottom seismometers recently recorded the microearthquake character of a mid-ocean ridge eruption, including precursory activity. A gradual ramp-up in activity rates since seismic monitoring began at this site in October 2003 suggests that eruptions may be forecast in the fast-spreading environment. The pattern culminates in an intense but brief (∼6-hour) inferred diking event on 22 January 2006, followed by rapid tapering to markedly decreased levels of seismicity. [ABSTRACT FROM AUTHOR]