Ultra-low velocity zones (ULVZs) above the core-mantle boundary (CMB) are observed structures that may be related to the liquid outer-core. As ”thin patches” of dramatically low seismic-wave velocity, they are occasionally found near the base of mantle plumes or in/near high seismic-wave speed regions above the CMB. The causes of their morphology and geodynamics remain unclear, and simulation of high-density melts diverge from observations. We introduce a two-dimensional time-dependent Stokes two-phase flow (with melt migration) numerical model to investigate the evolution of high melt-fraction regions affected by CMB-mantle tangential flows, amidst a hot mantle plume and an optional neighboring cold downwelling. We find that (a) the participation of cold sources with temperature differences between ~4000 K at the plume central regions to <~3900 K at the plume-cooling flanks, separated by horizontal distances of approximately 100 ±50 km are necessary for dense melts (fractional mass-density difference >+1–2%, <+10%) to attain total-mass quasi-stability, (b) an enhanced tangential-horizontal flow in reverse circulation within the broad plume base (with speeds >1–3 times the lowermost-mantle characteristic flow speeds); are necessary for high aspect-ratio-morphology melt-lenses to be compatible with seismic observations. Stresses exerted on variable CMB topography may arise from lateral motion of localized outer-core rigidity-zone structures. The CMB-mantle tangential flow and/or outer-core interacting with CMB-topography may help generate mega-ULVZs, particularly if they appear along the edges of large low-shear-wave-velocity provinces (LLSVPs) or in/near high-seismic-speed “cold” zones. A strong link exists between ULVZ morphology-evolution and the dynamic and heterogeneous environment in-around the CMB.