Lithosphere–asthenosphere boundary
The lithosphere–asthenosphere boundary (referred to as the LAB by geophysicists) represents a mechanical difference between layers in Earth's inner structure.
[3] This criterion works particularly well in oceanic lithosphere, where it is reasonably simple to estimate the temperature at depth based upon the age of the rocks.
The definition of the LAB as a thermal boundary layer (TBL) comes not from temperature, but instead from the dominant mechanism of heat transport.
The depth to the base of the CBL can be determined from the amount of forsterite within samples of olivine extracted from the mantle.
[5] Seismic tomographic studies suggests that the LAB is not purely thermal, but rather is affected by partial melt.
[5] One way to determine if the LVZ is generated by partial melt is to measure the electrical conductivity of the Earth as a function of depth using magnetotelluric (MT) methods.
[5] Because mantle flow induces the alignment of minerals (such as olivine) to generate observable anisotropy in seismic waves, another definition of the seismic LAB is the boundary between the anisotropic asthenosphere and the isotropic (or a different pattern of anisotropy) lithosphere.
[8] Evidence from converted seismic phases indicates a sharp decrease in shear wave velocity 90–110 km below continental crust.
[9] Recent seismological studies indicate a 5 to 10 percent reduction in shear-wave velocity in the depth range of 50 to 140 km beneath ocean basins.
Data from ocean seismometers indicate a sharp age-dependent LAB beneath the Pacific and Philippine plates and has been interpreted as evidence for a thermal control of oceanic-lithosphere thickness.
The LAB is particularly difficult to study in these regions, with evidence suggesting that the lithosphere within this old part of the continent is at it thickest and even appears to exhibit large variations in thickness beneath the cratons,[13] thus supporting the theory that lithosphere thickness and LAB depth are age-dependent.
