# 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

## 1 Answer

Reading this question, I'm not sure if you realize what an isotherm is - in this case, imagine peeling rock from the surface of the earth until you reached a temperature of 1300 degrees Celsius. Now, in some areas, such as mid ocean ridges, you wouldn't have to peel back much rock at all, perhaps only a few dozen kilometers. Whereas in regions of old, continental crust, you would have to peel off hundreds of kilometers of rock to get down to rock at 1300 degrees centigrade.

The result would be a lumpy sphere at a constant 1300 degrees, which is your isothermal surface or isotherm. The thickness of the lithosphere is very variable.

Interestingly, the Asthenosphere - Mesosphere boundary is created by a mineral phase transition, which is very pressure-dependent; this makes it far more of a smooth sphere.

• I understand what an isotherm is. My point is that the melting point of the mantle changes with depth, and so the temperature at which the lithosphere transitions to asthenosphere should too (as the change in creep mechanism depends on the mantle temperature relative to it's melting temperature)? If this is the case then surely the LAB is not an isotherm? – Benjamin Rogers-Newsome Dec 20 '18 at 19:59
• The onset of plastic deformation isn't very temperature dependent. Generally speaking, transition to a liquid means an increase in volume, and so is strongly affected by pressure. Plastic creep in solid rock is not associated with volume changes, so won't be strongly affected by pressure. – Andrew Jon Dodds Dec 21 '18 at 9:40