Thermal boundary layer (TBL)
The definition of the LAB (lithosphere–asthenosphere boundary) as a thermal boundary layer (TBL) comes not from temperature, but instead from the dominant mechanism of heat transport. The lithosphere is unable to support convection cells because it is strong, but the convecting mantle beneath is much weaker. In this framework, the LAB separates the two heat transport regimes [conduction vs. convection]. However, the transition from a domain that transports heat primarily through convection in the asthenosphere to the conducting lithosphere is not necessarily abrupt and instead encompasses a broad zone of mixed or temporally variable heat transport. The top of the thermal boundary layer is the maximum depth at which heat is transported only by conduction. The bottom of the TBL is the shallowest depth at which heat is transported only by convection. At depths internal to the TBL, heat is transported by a combination of both conduction and convection.
The definition of the LAB (lithosphere–asthenosphere boundary) as a thermal boundary layer (TBL) comes not from temperature, but instead from the dominant mechanism of heat transport. The lithosphere is unable to support convection cells because it is strong, but the convecting mantle beneath is much weaker. In this framework, the LAB separates the two heat transport regimes [conduction vs. convection]. However, the transition from a domain that transports heat primarily through convection in the asthenosphere to the conducting lithosphere is not necessarily abrupt and instead encompasses a broad zone of mixed or temporally variable heat transport. The top of the thermal boundary layer is the maximum depth at which heat is transported only by conduction. The bottom of the TBL is the shallowest depth at which heat is transported only by convection. At depths internal to the TBL, heat is transported by a combination of both conduction and convection.