A simple hydrologic model of seawater circulation at ocean ridge axes implies that the transient occurrence of large volumes of buoyant, heated water in the oceanic water column (megaplumes) can be attributed to the emplacement of dikes in oceanic crust. For dikes to generate megaplume flow, the permeability of both the recharge areas and the upflow zone must be greater than that required for ordinary black smokers. An increase in permeability in the upflow zone by several orders of magnitude results from dike emplacement, and megaplume discharge ceases as the dike cools. Vigorous black smoker venting may not persist very long at a megaplume site after the event occurs.