The Reykjanes Ridge became segmented following an abrupt plate motion change.
A persistent low-viscosity sub-axial conduit guided the reassembly of a linear axis.
Shallow mantle buoyant upwelling instabilities propagate along the sub-axial conduit.
Propagating instabilities replace ridge-orthogonal segmentation with V-shaped ridges.
Mantle plume flow is not required to explain Reykjanes Ridge crustal structures.