Mid-ocean ridge volcanism forms two-thirds of the Earth’s surface yet our knowledge of the dynamics of mid-ocean ridge eruptions is limited due to the difficulty of long-term monitoring on the seafloor. I will present results from in situ geophysical observation of two mid-ocean ridge eruptions. At the East Pacific Rise, a typical fast-spreading ridge, we find that the eruption was primarily controlled by the buildup of tectonic stress to a critical level. At Axial Seamount, a ridge-based hotspot volcano, we observe increased rate of magma influx into a laterally-discontinuous shallow magma reservoir weeks before the eruption. This suggests the eruption might have been triggered by magma overpressure in the shallow reservoir. Our observations present two end-member models of how mid-ocean ridge systems might evolve towards an eruption.