Lithosphere-Asthenosphere boundary

The Lithosphere- Asthenosphere boundary represents a mechanical difference between layers in Earth’s inner structure. Earth’s inner structure can be described both chemically (crust, mantle, core) and mechanically. The Lithosphere-Asthenosphere boundary (referred to as the LAB by geophysicists) lies between Earth's cooler, rigid lithosphere and the warmer, ductile asthenosphere. The actual depth of the boundary is still a topic of debate and study, although it is known to vary according to the environment.

The LAB is determined from the differences in the lithosphere and asthenosphere including, but not limited to, differences in grain size, chemical composition, thermal properties, and extent of partial melt; these are factors that affect the rheological differences in the lithosphere and asthenosphere.

The lithosphere-asthenosphere boundary represents a rheological boundary. Colder temperatures at Earth's shallower depths affect the viscosity and strength of the lithosphere. Colder material in the lithosphere resists flow while the "warmer" material in the asthenosphere contributes to its lower viscosity. The increase in temperature with increasing depth is known as the geothermal gradient and is gradual within the rheological boundary layer. The lithosphere is the portion of the thermal boundary layer commonly defined by its purely conductive heat transport. Throughout the rheological boundary, the geotherm gradually transitions from the conductive nature of the lithospheric geotherm to the convective (adiabatic) nature of the underlying asthenosphere.

The LAB is often observed and imaged via signal processing techniques and seismic waves. Seismic tomographic studies suggests that the LAB is not determined by a purely thermal model, but rather it is affected by the presence of partial melt material in the asthenosphere. Evidence from converted seismic phases indicates a sharp decrease in shear-wave velocity 90–110 km below continental crust. Recent seismological studies indicate a 5 to 10 percent reduction in shear-wave velocity in the depth range of 35 to 120 km beneath ocean basins. The seismic discontinuity often associated with this sharp contrast in wave velocity and presence of partial melt is known as the Gutenberg discontinuity or "G" to many geophysicists. The Gutenberg discontinuity coincides with the expected LAB depth in many studies and has also been found to become deeper under older crust, thus supporting the suggestion that the discontinuity is closely interrelated to the LAB.

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