Since the mid-1980s, eight eruptions have occurred along the Juan de Fuca and Gorda Ridges in the NE Pacific. MBARI has examined seven of them, including the April 2015 eruption at Axial Seamount, using autonomous underwater vehicles (AUVs) to map at 1-m lateral resolution and remotely operated vehicles (ROVs) to observe and sample the lava flows and hydrothermal deposits. We have done similar work on the Alarcon Rise at the mouth of the Gulf of California, where all eruptions pre-date our studies of the ridge system, but are recent enough to host impressive active hydrothermal systems.AUV sonar data were processed with MB-System software and the resulting maps were brought into a geographical information system (GIS) for display and analysis. The highresolution AUV maps were used at sea as basemaps in a real-time GIS to guide our ROV dives with greater efficiency than possible before. From the maps and observations, we defined lava flow boundaries and channel systems, assessed fault activity, calculated flow sizes, evaluated age relationships between flows, and then sampled lavas for chemistry. Those data were coupled with age dates from sediment samples to place changes in eruption styles, lava chemistry, and hydrothermal systems on the ridges into the previously elusive framework of time. Using these tools, we have constructed geologic field maps and volcanic histories of the spreading ridges, much like volcanologists do on land, but all unprecedented for submarine volcanoes. The synergistic high-resolution mapping and targeted ROV sampling of the ridges has permitted better understanding of when, how, how much, and how often spreading ridges erupt, and how their magmatic and hydrothermal systems change over time.