The Mantle

The mantle is the great terra incognita. Contra Jules Verne, no one has actually visited it. We have no rocks pulled directly from it, only those thrown out through volcanos or those that have seeped from cracks in ocean bottoms.

What we absolutely know has come from modern technology. We know its approximate thickness, 2890 kilometers or 1800 miles, which is the difference between the calculated size of the core and the external perimeter observed from space.

Seismic wave tests have identified areas with radically different patterns. One, the D” or D double prime layer, sits between the core and the mantle. Another, the Moho or Mohorovičić discontinuity, lies between the mantle and the crust.

Beyond that, nothing is agreed upon.

In the face of such massive unknowns, scientists, like the rest of us, have fallen back on what they do know, in this case, that the laws of physics never change. If they ever yielded on the absolute truth of the patterns of heat and effects of temperature, there would be no way to calculate or predict anything. They would be thrown back into something akin to medieval wonders.

Geophysicists have created an image of the mantle as an homogenous mixture slowly stirred by convection. The only disruption comes from the crust of the earth sinking down in a subduction zone. The materials are absorbed into the mix, and prepared for the next eruption. The mantle remains unchanged from its primordial state.

The second absolute for geologists is that rocks never lie. They may tease and mislead, but they never outright lie. Therefore, the actual components of rocks summarized in the periodic table have not changed. The proportions and distributions may have changed, one may have been transformed into another, but oxygen has always been oxygen, samarium has always been samarium.

They focus on the mantle rocks the earth is currently displaying, the basalt from mid-ocean ridges and the basalt from ocean islands. They have different chemical signatures, so therefore must have different sources. Seismology has defined two boundaries. Therefore, the first is thought to come from the top of the mantle, the other from somewhere near the bottom generated somehow from crust that has fallen there from the surface. Whatever exists between is so much unimportant, dark matter, rather like the liquid that survives in a vacuum pack of pickles.

Since seismologists revealed more discontinuities in the mantle that correlate with changes in the structure of olivine, geochemists have begun to apply the laws of physics to rocks themselves and have been asking the effects of heat and temperature on the atomic structure of the matter that makes up the mantle. They are developing a view of the mantle as one of layers, albeit each composed of the same matter under different conditions. So far, they can only define the first thousand kilometers. The rest, nearly two-thirds of the mantle, awaits future research.

All these views are ahistorical. When asked how the mantle evolved from that gaseous mass we’ve always been shown into the stable planet we know, they tend to fall into the von Däniken trap. Erich von Däniken is the one who, when confronted with evidence of ancient new world civilizations that contradicted the long held view that man has been progressing in a straight line from primitive life to the present with no lost knowledge or relapses, argued in Chariots of the Gods? that monuments like those found in Peru were the result of contacts with aliens from outer space.

The outer space concept now is a bit more sophisticated. Since we’ve brought back rocks from the moon, scientists have become aware of the effects of the constant bombardment that must have been occurring before the atmosphere developed to protect us. But, it’s still a bit of a deus ex machina, a type of solution even Horace knew was suspect.

None of their views ultimately make sense.

The only model I have for the effects of heat is boiling chicken soup. The globs of fat in water break into smaller units until they turn into an edible suspension. As soon as you turn off the heat, the fat begins reseparating out. Continents form and unform, as it were. The underlying soup is irrelevant. Plate tectonics makes sense.

The only model I have for mixing is making a cake. Even with the most efficient mixer, lumps of flour will remain if you don’t keep smashing them and redirecting the pieces back toward the beaters. A complete blend cannot be left to a machine.

I have a hard time visualizing how the mantle became completely homogenous. But, I’m willing to accept that’s the case, if someone can suggest when that occurred. Scientists have been trying. The best have created mathematical models, but most end projecting absurdities. One group has suggested how two reservoirs of magma could have been formed 2500 million years ago in conditions that existed prior to the development of plate tectonics.

If I latch onto their explanation, it’s because I want to know what are events that occurred before the North American plate began forming after the break up of the Kenorland supercontinent 2500 million years ago.

However, I know that accepting an answer that meets my expectations is dangerously naive. The alternative is to accept ignorance which is always difficult, but apparently necessary in this case.

I can only assume the mantle was evolving, that something happened between the iron catastrophe that accompanied the formation of the core and the formation of Laurentia, and move on to trying to understand the origin of the next layer, the oceans.

Notes:
Anderson, Don. “Self-Gravity, Self-Consistency, and Self-Organization in Geodynamics and Geochemistry,” has a readable explanation of the various views of the mantle, with a chart showing the layers in the first thousand kilometers.

van Thienen, Peter, J. van Summeren, Robert D. van der Hilst, A. P. van den Berg and N. J. Vlaar. “Numerical Study of the Origin and Stability of Chemically Distinct Reservoirs Deep in Earth’s Mantle.”

Both appear in Earth’s Deep Mantle: Structure, Composition and Evolution, 2005, edited by Robert D. van der Hilst, Jay D. Bass, Jan Matas and Jean Trampert for the American Geophysical Union.