We investigate the generation of megaplumes by the release of buoyant hydrothermal fluid from the seafloor. We show that megaplumes may be generated from various modes of venting, including both the instantaneous and continuous release of hydrothermal effluent from either a point or line source. The hydrothermal effluent forms a buoyant plume, which rises through the water column to its neutral buoyancy height and then intrudes laterally to form a neutral cloud. Owing to the influence of the Earth's rotation f, whose magnitude is &OHgr;=f/2, the neutral cloud eventually becomes unstable, giving rise to geostrophic vortices that propagate away from the source. By combining the scaling laws governing turbulent plumes and geostrophic vortices, we establish new relationships between the megaplume geometry and the source conditions. We find that megaplumes whose radius greatly exceeds their height of rise are formed from sources that persist for at least several days, since, in the deep ocean, the radii of eddies produced by short-lived releases of buoyant fluid are comparable to their rise height. Our model predicts the total buoyancy B of the hydrothermal effluent released in forming such megaplume structures. We also calculate the total megaplume heat content in terms of the total buoyancy release and the thermal anomaly of the megaplume, by considering the effects of the ambient stratification in both temperature and salinity on plume properties. Finally, we apply the model to data from three historic megaplume events at the Juan de Fuca ridge. ¿ 1999 American Geophysical Union