Evidence of seafloor spreading [1,2] proved the seafloor-spreading hypothesis [3] and led to the discovery of plate tectonics. A further observation-based analysis showed that seafloor-spreading results from downward pulling of the subducting slab [4], which drives plate tectonics and dictates the first order pattern of mantle convection [5] (Fig. 1a). Continental drift is understood as a passive response to trench retreat under gravity due to seafloor subduction [6]. All this, plus the understanding that ocean ridges are passive features [7], has completed the paradigm of the plate tectonics theory. However, a fundamental issue concerning mantle flow subjacent to the spreading oceanic lithosphere remains widely misperceived (Fig. 1b,c), which needs correction so as to better appreciate the efficacies of the plate tectonics theory and to correctly understand the origin and evolution of oceanic lithosphere as well as processes of chemical differentiation of the Earth. Correctly, materials in the seismic low velocity zone (LVZ, top portion of the asthenosphere) beneath ocean basins must flow towards ocean ridges against the spreading oceanic lithosphere above, especially beneath seafloors younger than ~ 70 Ma (Fig. 1d). Given the fundamental importance of this issue, I endeavor in this short communication to convince the reader in simple clarity that oceanic lithosphere-LVZ decoupling is required for the functioning of plate tectonics in terms of mass conservation and continuity principle of fluid mechanics, which is also revealed by geochemical studies [8]. Note that the discussion and illustrations presented here use understood geological and physical principles that are transparent and demonstrative rather than using interpretations based on dynamic/numerical modeling. The latter is useful and important but is often opaque and not unambiguous for understanding the key issue discussed in this paper (e.g., [9]). The central task of the plate tectonics theory at present is to understand mantle convection in general and mantle flow on varying scale in particular. Yet, one of the most popular perceptions in classrooms and textbooks gives an apparently simple picture of coupled flows between the oceanic lithosphere and the subjacent asthenosphere (Fig. 1b,c), following Arthur Holmes' concept of mantle convection in the form of "convective current" [10], which is useful in telling the reader that the Earth's mantle does convect, but has been used by many to argue that the seafloor spreading (the motion of oceanic lithosphere) is viscously dragged by subjacent mantle "convective current" (Fig. 1b,c). On the other hand, still others believed the other way round, i.e., the motion of the oceanic lithosphere in response to subducting slab pull induces the subjacent asthenosphere to flow with it. In either scenario, the lithosphere-asthenosphere motion is closely coupled because of the believed cause-and-effect connection between the two, moving in the same direction away from ocean ridges (Fig. 1b,c). Such coupled motion, as taught in classrooms and illustrated in textbooks, is still popular in current modelling research such as "plug flow in Earth's asthenosphere" [9]. According to the concept of mass conservation and continuity principle of fluid mechanics, it is straightforward that the oceanic lithosphere-LVZ movement is necessarily decoupled, especially beneath young (< 70 Ma) seafloors [11,12] (Fig. 1d). Because ocean ridges are passive features [7], plate separation creates a gravitational void to allow the asthenosphere to rise and melt by Accepted Sci Bull manuscript-20240815 < 2 > decompression to produce the magmatic ocean crust with the residues accreting to the growth of the oceanic lithosphere [13]. That is, continued seafloor spreading leads to continued oceanic lithosphere formation (magmatic crust + mantle lithosphere) and thus continued asthenosphere material supply. In simple words, the mass of the lithosphere accreted per unit time (mL) must be the same as the mass of the asthenosphere supply per unit time (mA; mL= mA). In addition to the contribution of ridge melting residues, the oceanic lithosphere thickens with age by accreting asthenosphere material from below until reaching its full thickness (L) of ~ 90 km at the age (t) of ~ 70 Ma as the result of conductive heat loss to the seafloor, which is expressed by L ∝ t 1/2 [5,7,11]. It follows that the lithosphere accretion is fastest towards the ridge with ~ 50% of the full thickness completed in the first ~17.5 Myrs (i.e., t1/2 = [0.5*70 1/2 ] 2). All this demonstrates in simple clarity that the LVZ, the top portion of the asthenosphere, beneath ocean ridges represents regions of the lowest pressure in the entire mantle that drives the ridgeward flow both locally and globally. The latter is described physically as ridge suction [8] in the context of discussing plume-ridge interactions. The asthenosphere materials needed for the lithosphere accretion at and near ridges can be supplied both vertically from below at great depths and transported laterally by ridge suction (Fig. 1d). However, the LVZ has the lowest viscosity with the top defined by the LAB (lithosphere-asthenosphere boundary) [11] and the base defined by the Lehmann Discontinuity at ~220±30 km (Fig. 1d) that marks the sudden seismic velocity and viscosity increase at depths [14], which informs that the seismic LVZ is also a zone of rheological low viscosity (lvz) [12]. These observations and reasoning, together with the presence of a melt-rich layer close beneath the LAB [11], make the seafloor spreading possible with little resistance [11,12] and make the ridgeward LVZ flow against plate motion physically straightforward while also satisfying the principles of mass conservation and flow continuity. Hence, the lithosphere-LVZ decoupling is required for the functioning of plate tectonics [8]. It follows from the above that the extent of lithosphere-LVZ decoupling must increase with increasing ridge spreading rate because the material needs to form the lithosphere increase with increasing spreading rate. As the full thickness of mature oceanic lithosphere (LFull = ~ 90 km) for seafloors > 70 Ma is independent of spreading rate, the volume of the lithosphere (V) formed per unit time per unit length parallel to the ridge can be calculated using a globally valid form of V = R1/2 * LFull, where R1/2 is the half-spreading rate. As LFull is constant, V ∝ R1/2, and LFull is thus a simple proportionality. By using L = 11t 1/2 [15], it is straightforward to calculate the thickness and volume of the lithosphere formed per unit ridge length as a function of spreading rate for seafloors < 70 Ma. Fig. 2a, as an example, compares V = f(R1/2) formed for the first one million years for scenarios of R1/2 = 10 and 60 mm/yr, respectively. As expected, V[R1/2 = 60 mm/a]/V[R1/2 = 10 mm/a] = 440 km 3 /73.33 km 3 = 6, confirming the linear relationship V ∝ R1/2 that is valid for lithosphere growth globally for active accreting for t < 70 Ma and for the overall net growth calculated from mature lithosphere for t > 70 Ma. Fig. 2b shows such linearity in terms of material supply required for lithosphere formation due to cooling in the first one million years as a function of R1/2. If a ~ 5 km thick magmatic crust is assumed, the material supply would be much greater. In summary, the discovery of plate tectonics and its further understanding have proved the long-held view of mantle convection, in which the subducting oceanic lithosphere dictates the first order pattern of mantle convection [5] (Fig. 1a) although details of mantle convection remain speculative. Influenced by the idea of "convective current" [10], the apparently sensible and popular speculation is the coupled motion of the spreading seafloor lithosphere with the subjacent asthenosphere (Fig. 1b,c), which is erroneous. In terms of basic physics such as mass conservation and the principle of flow continuity, the materials in the LVZ must flow towards the ridge against the spreading seafloor lithosphere to supply the materials needed to form the magmatic ocean crust at the ridge and for continued mantle lithosphere accretion beneath young (t < 70 Ma) lithosphere. The extent of this lithosphere-asthenosphere decoupling increases with increasing seafloor spreading rate, which is illustrated with demonstrations by geochemical systematics of near-ridge hotline seamount lavas as a function of distance to the ridge axis (e.g., The Easter hotline and Foundation hotline seamount chains in the southeast Pacific) [8]. The nature of the low viscosity of the seismic LVZ (i.e., low viscosity zone, lvz) [12] and the presence of a melt-rich layer close beneath the LAB [11,12] makes seafloor spreading possible with little shear friction, which establishes a fundamental condition for the functioning of plate tectonics.