We have studied the development of multiscalar mantle flow beneath mid-ocean ridges using a three-dimensional numerical technique. Modeling includes various modes of heating and the shear flow induced by plate motion away from a spreading center with and without transform offsets. The computational box consists of two layers differing in their viscosities. It is shown than when flow is generated by heat sources located for the most part in the lower layer, convective cells have horizontal dimensions proportional to the entire thickness of the box. It is only when flow is driven by buoyancy forces in the top layer, or by transient cooling along the top interface, that transverse rolls are found to be confined within the top low-viscosity layer. These rolls remain generally located far from the ridge plane, except in the case of a viscosity contrast of 3 orders of magnitude between the bottom and top layers. We conclude that segmentation beneath mid-ocean ridges requires both strong viscosity contrasts and the lack of deep convective flow. Marine geophysical observations suggest that mantle upwelling beneath mid-ocean ridges is two-dimensional (sheetlike) beneath intermediate, fast and hotspot-influenced spreading centers and three-dimensional (plumelike) beneath slow-spreading centers and ridges overlying cold regions of the mantle, such as the Australian-Antarctic Discordance. We propose that these observations can be explained by variations in the viscosity structure (and temperature) of the mantle along the mid-ocean ridge system. Models including a transform offset show that the active portion of the transform is a region of mantle downwelling. The toroidal flow induced by the offset shifts the plumes away from the ridge crests. The effects of the toroidal field are strongly magnified when the return flow cannot penetrate in the lower layer due to a large viscosity contrast. Stresses generated by the flow partly explains the development of inside corner highs and the evolution of transform faults. ¿ American Geophysical Union 1993