A volcanic eruption at the summit of Axial Volcano on January 25, 1998, instantaneously created extensive and vigorous hydrothermal discharge. Moorings 2 km apart along the eruption fissure recorded temperature increases of ~0.6 ¿C up to 115 m above bottom within hours of initial seismic activity. Water temperatures at the mooring sites remained high for about 5 days, then declined steadily over the next 2 weeks. A response cruise 18 days after the eruption found hydrothermal temperature anomalies of ~0.1 ¿C over the eruption site, and a more intense and much thicker plume 20 km downstream of the eruption. We estimate the steady-state heat flux required to produce this distal plume, evidence of discharge conditions perhaps 1--13 days after the eruption, as 60--230 GW. The Axial eruption thus produced the largest vent field heat flux yet measured, but these high levels lasted less than 3 weeks. ¿ 1999 American Geophysical Union