At a passive spreading center the motion of the fluid rising in the conduit between diverging lithosphere plates is generated by the motion and weight of the plates. Combined gravity and viscous forces exerted by the rising material on the overhanging conduit walls deform the adjacent lithosphere to produce sea floor topography. If viscous forces in the conduit and long-term strength of the young lithosphere were negligible, topography could be in approximate isostatic balance, consistent with the observation that the gravity field is relatively undisturbed. However, deep axial valleys for slow spreading suggest that viscous forces are not negligible; they evidently suppress the rise in the conduit and support the gravitationally unstable lithosphere. For a general class of conduits that could occur in nature (those with a bottleneck near the top) the upward viscous force equals the weight of the mass deficiency in the conduit. Dynamically supported topography above such conduits could account for the gross mass balance without limiting the role of viscous forces. A static pressure deficiency at the base of the conduit drives the upflow against viscous resistance; for given physical properties this driving pressure is determined by the elevation of the sea floor at the axis. When V changes from one steady value to another, the rate of upflow in the conduit must change to balance the new rate of lithosphere production. The upflow may be adjusted by varying the static driving pressure through changes in axial elevation ('damping') or by changing the resistance to upflow by varying the conduit width ('throttling'). However, the mechanism by which conduit fluid changes to lithosphere solid determined conduit width as a function of velocity. Hence as velocity changes, the axial elevation must adjust to assure compatibility among the dynamics of conduit flow, the dynamics of accretion, and the condition of mass continuity. If conduit width is proportional to velocity (an approximation for the case of isothermal conduit walls), axial elevation must decrease sharply as small velocities decrease. This is consistent with observed deep axial valleys in slow spreading and small axial ridges in fast spreading; it is illustrated with analytical results for Newtonian flow in a wedge-shaped conduit. At small velocities, slower spreading plates will be harder to separate. If body forces and tractions elsewhere on the plates cannot equilibrate the horizontal component of suction caused by deep valleys required at very small velocities, steady spreading at these velocities is impossible, and the plates will coalesce. This could be why the earth's lithosphere is so well organized and not fragmented into many small plates. Resistance to plate separation associated with dissipation of mechanical energy in the axial region can be expressed in terms of the dynamic model, and hence so can the stability conditions for oblique spreading and transform faults. Oblique spreading and absence of the characteristic axial valley on the Reykjanes Ridge can be attributed to anomalously low asthenosphere viscosity associated with the Iceland volcanic center. In the Reykjanes conduit the reduced viscous suppression could eliminate the axial valley, and reduced dissipation of mechanical energy could lead to stable spreading in the oblique mode. The passive dynamic model is easily generalized to simple active cases.