Recent measurements of large water column plumes in association with seismic activity and evidence of a fissure release of magma on the sea floor at the CoAxial Segment, northern Juan de Fuca Ridge, initiate questions about the response of the benthic ocean to large and sudden line discharges of heat. Here these issues are investigated using a three-dimensional non-hydrostatic hydrodynamical model. The model treats the case of a starting plume from a finite-length, narrow line source of heat ascending into a rotating environment stratified in temperature and salinity. The developing water column plume is studied over an initial time period of approximately f-1, where f is the Coriolis frequency. Midway into this interval the plume attains its maximum rise height of ~900 m, for given buoyancy flux and stratification conditions. Thereafter, the plume spreads laterally and the effects of the earth's rotation come into evidence. Outward flow at 400-900 m above bottom is primarily transverse to the line source, quickly making the plume at those heights more circular than line-like.