The electrical conductivity of sea floor materials is a strong function of porosity, pore geometry, fluid content, temperature and percentage of partial melt. These material properties are particularly volatile near the midocean ridge. Tracking electromagnetic eddy currents generated on the sea floor as they diffuse across the ridge can provide images of the subaxial electrical and through interpretation the geological structure. We have computed numerically in 2D the response of a crustal magma chamber located beneath the East Pacific Rise to excitation in two fundamental orthogonal modes, rise--normal (TE) and rise--parallel (TM) electric field. A finite element representation of the electromagnetic (EM) diffusion equations is solved in the Laplace domain and the results inverted into the time domain via the Gaver--Stehfest algorithm. A sequence of snapshots for each mode for the time range 0.1 to 15 s shows the complete progression of the eddy currents into, through and around the magma chamber. The TE and TM signals at a receiver site above the conductive magma chamber are delayed and attenuated in comparison with those expected for normal layered oceanic crust. ¿ American Geophysical Union 1989