Seafloor volcanic eruptions are difficult to directly observe due to lengthy eruption cycles and the remote location of mid‐ocean ridges. Volcanic eruptions in 2005–2006 at 9°50′N on the East Pacific Rise have been well documented, but the lava volume and flow extent remain uncertain because of the limited near‐bottom bathymetric data. We present near‐bottom data collected during 19 autonomous underwater vehicle (AUV) Sentry dives at 9°50′N in 2018, 2019, and 2021. The resulting 1 m‐resolution bathymetric grid and 20 cm‐resolution sidescan sonar images cover 115 km2, and span the entire area of the 2005–2006 eruptions, including an 8 km2 pre‐eruption survey collected with AUV ABE in 2001. Pre‐ and post‐eruption surveys, combined with sidescan sonar images and seismo‐acoustic impulsive events recorded during the eruptions, are used to quantify the lava flow extent and to estimate changes in seafloor depth caused by lava emplacement. During the 2005–2006 eruptions, lava flowed up to ∼3 km away from the axial summit trough, covering an area of ∼20.8 km2; ∼50% larger than previously thought. Where pre‐ and post‐eruption surveys overlap, individual flow lobes can be resolved, confirming that lava thickness varies from ∼1 to 10 m, and increases with distance from eruptive fissures. The resulting lava volume estimate indicates that ∼57% of the melt extracted from the axial melt lens probably remained in the subsurface as dikes. These observations provide insights into recharge cycles in the subsurface magma system, and are a baseline for studying future eruptions at the 9°50′N area.

Partitioning of seafloor spreading between slip on faults and magmatic diking varies considerably, and is thought to be partly controlled by spreading rate (e.g.,

Magnetic anomaly variations near mid‐ocean ridge spreading centers are sensitive to a variety of crustal accretionary processes as well as geomagnetic field variations when the crust forms. We collected near‐bottom vector magnetic anomaly data during a series of 21 autonomous underwater vehicle Sentry dives near 9°50′N on the East Pacific Rise (EPR) covering ∼26 km along‐axis. These data document the 2–3 km wide axial anomaly high that is commonly observed at fast‐spreading ridges but also reveal the presence of a superimposed ∼800 m full wavelength anomaly low. The anomaly low is continuous for ≥13 km along axis and may extend over the entire survey region. A more detailed survey of hydrothermal vents near 9°50.3′N reveals ∼100 m diameter magnetic lows, which are misaligned relative to active vents and therefore cannot explain the continuous axial low. The axial magnetization low persists in magnetic inversions with variable extrusive source thickness, indicating that to the extent to which layer 2A constitutes the sole magnetic source, variations in its thickness alone cannot account for the axial low. Lava accumulation models illustrate that high geomagnetic intensity over the past ∼2.5 kyr, and decreasing intensity over the past ∼900 years, are both consistent with the broad axial anomaly high and the superimposed shorter wavelength low. The continuity of this axial low, and similar features elsewhere on the EPR suggests, that either crustal accretionary processes responsible for this anomaly are common among fast‐spread ridges, or that the observed magnetization low may partially reflect global geomagnetic intensity fluctuations.