We investigate the three-dimensional interaction of mantle plumes and migrating mid-ocean ridges with variable viscosity numerical models. Numerical models predict that along-axis plume width W and maximum distance of plume-ridge interaction xmax scale with (Q/U)1/2, where Q is plume source volume flux and U is ridge full spreading rate. Both W and xmax increase with buoyancy number &Pgr;b which reflects the strength of gravitational- versus plate-driven spreading. Scaling laws derived for stationary ridges in steady-state with near-ridge plumes are consistent with those obtained from independent studies of Ribe <1996>. In the case of a migrating ridge, the distance of plume-ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate. Given the mildly buoyant and relatively viscous plumes investigated here, the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow. From observed plume widths of the Gal¿pagos plume-migrating ridge system, our scaling laws yield estimates of Gal¿pagos plume volume flux of 5--16¿106 km3 m.y.-1 and a buoyancy flux of -2¿103 kg s-1. Model results suggest that the observed increase in bathymetric and mantle-Bouguer gravity anomalies along Cocos Plate isochrons with increasing isochron age is due to higher crustal production when the Gal¿pagos ridge axis was closer to the plume several million years ago. The anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly (50¿--100 ¿C) and moderate radius (100--200 km). Predictions of the along-axis geochemical signature of the plume suggest that mixing between the plume and ambient mantle sources may not occur in the asthenosphere but, instead, may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source.¿ 1997 American Geophysical Union