# Why is the Lithosphere-Asthenosphere Boundary an Isotherm?

My understanding of the LAB is that it represents the point in the earth where the mantle undergoes a temperature activated mechanical change. The temperature of the boundary depends on the melting point of the mantle and so one could say that the LAB is an isotherm with temperature given by: $$T=kT_m,$$ where $$T_m$$ is the melting point of the mantle and $$k$$ is some constant.

This is how I have had the situation explained to me, and in sources that I have read that the LAB is an isotherm at $$\pu{1300 ^\circ C}$$.

My only problem with this is that surely $$T_m$$ changes with depth, and therefore the Lithosphere-Asthenosphere boundary is not an isotherm at all, but depends on the depth at which the boundary is at (for example at a mid-ocean ridge where the boundary becomes shallower towards the ridge axis).

• Tm would change with pressure, not depth. The two are closely related, but the density of continental crust is much lower than oceanic crust. Think isostasy. I think. Not sure. – Gimelist Dec 10 '18 at 23:28
• @MichaelWallace there is no evidence for LENR exisiting, it has no basis in physics, and if it was happening we would expect to see things that are not observed. There is no hidden heat source that requires an imaginary cold fusion reaction. – Gimelist Dec 18 '18 at 21:03