We address the question of formation of event plumes following dike emplacement in a hydrothermal upflow zone at a mid-ocean ridge. We assume a preexisting low- to moderate-temperature single-pass hydrothermal system and suggest that dike emplacement provides a damaged zone of high permeability along its margins as well as the heat required to drive the event plume. We also consider the role of thermoelastic stresses in limiting the heat output of the event plume. Our calculations show that event plumes can result from dike emplacement into a preexisting, moderate-temperature hydrothermal system, provided the local permeability generated by the dike is ~10-8--10-10 m2. These values are consistent with our limited field observations from the Susanville ophiolite, suggesting that the permeability near dike margins attributed to dike emplacement results mainly from open fractures mostly aligned parallel to the margins of dike. We estimate the porosity in the damaged zone to be of the order 0.1--1%. If the high-permeability zone has relatively low porosity (~0.1%), thermoelastic stresses close the fractures sufficiently to reduce the heat output, giving rise to a weak event plume (~1014 J). If the porosity is higher (≥1%), however, thermoelastic stresses become unimportant, and a similar dike can generate an event plume of ~1017 J. Following event plume emission, the circulation decays rapidly to its original temperature; however, the heat output from the chronic plume is greater because of the increased permeability resulting from dike emplacement. The decay of heat output to prevent plume levels requires that the newly created permeability be sealed, perhaps as a result of chemical precipitation in the cracks. ¿ 2000 American Geophysical Union