Red-ite Rate Mistakes

At times of late folks have decided to term volumes of the mantle (in particular) as being “red-ite” and “blue-ite” to avoid over interpreting such bodies as being hot or of some material.  Even so, the general assumption in the upper mantle is that red-ite is hot and blue-ite cold.  So what does this tell you about surface uplift rates?

Precisely nothing.

And yet there is quite a literature where the presence of a red blob in tomography is taken to mean that overlying crust is rising, or a blue blob means it is sinking.  This is nonsense for multiple reasons.  (GG is here refraining from identifying some guilty parties, but it shouldn’t be hard to find some).

First, it would be the rate of change of buoyancy that would matter to start with.  A present-day hot body (say, for instance, a pluton) would be in isostatic balance (as much as the flexural strength of the lithosphere would allow).  If the pluton were simply sitting there, slowly cooling, little would happen until the thermal front from the pluton were to fade out enough that the whole volume of pluton and surrounding rock was losing heat. There would be no uplift; eventually there would even be subsidence, even as the pluton might remain somewhat hotter than its surroundings. For there to be uplift, the pluton either needs to get hotter or bigger. Seismic tomography has no temporal history; if you want to go there, you have to make a bunch of assumptions and then model processes.

Second, the assumption of buoyancy for a red blob, while defensible, is hardly certain.

Third, there are processes that can interfere with the expression of a mantle anomaly’s buoyancy at the surface.  Several papers studying Rayleigh-Taylor instabilities have shown that the crust can flow in above a growing instability to produce uplift even as the anti-buoyant drip grows below.

Mistaking a rate for a level value is a blunder that earns rapid and widespread approbation in economics; perhaps similar blunders should be called out in earth science.

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