Dimensions and dynamics of megaplumes

We investigate the generation of megaplumes by the release of buoyant hydro thermal fluid from the seafloor. We show that megaplumes may be generated f rom various modes of venting, including both the instantaneous and continuo us 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 ma gnitude is Omega = f/2, the neutral cloud eventually becomes unstable, givi ng rise to geostrophic vortices that propagate away from the source. By com bining the scaling laws governing turbulent plumes and geostrophic vortices , we establish new relationships between the megaplume geometry and the sou rce conditions. We find that megaplumes whose radius greatly exceeds their height of rise are formed from sources that persist for at least several da ys, since, in the deep ocean, the radii of eddies produced by short-lived r eleases of buoyant fluid are comparable to their rise height. Our model pre dicts the total buoyancy B of the hydrothermal effluent released in forming such megaplume structures. We also calculate the total megaplume heat cont ent in terms of the total buoyancy release and the thermal anomaly of the m egaplume, by considering the effects of the ambient stratification in both temperature and salinity on plume properties. Finally, we apply the model t o data from three historic megaplume events at the Juan de Fuca ridge.