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.

Earthly Beginnings

The early history of the earth is more theory than fact, and those theories are taken more from astrophysics than other disciplines. Those with other interests tend to pick through the available information in hopes of arriving at some early history for their subject.

A great many look for the thread that explains the origin of life. Others want to know how the moon was made.

My focus has been the creation of the conditions that made possible the emergence of that part of the North American plate where New Mexico sits between 1710 and 1600 million years ago, with a certain inclination to pay some attention to the formation of Michigan, the state where I was raised.

For me, this means envisioning how a 4600 million year old cloud of gas and dust evolved into a stratified ball whose layers have been defined by the workings of heat and cold.

The most important element has been iron. According to Wikipedia, iron begins in stars hot enough to burn silicon and initiate a chain of reactions with helium that transform matter from silicon to calcium to titanium to chromium to an unstable iron. That iron fuses with a helium nucleus to create ⁵⁶nickel, after which the star collapses and the ⁵⁶nickle transforms into ⁵⁶iron via ⁵⁶cobalt.

During the earliest millennia of the planet, radioactive decay continued in particles of nickle in the cloud which created conditions warm enough to heat the iron dust whose melting point is 1535 degrees centigrade. Following one basic law of physics, liquids are heavier than gases, the molten iron would have begun to isolate itself from the heat generating gases.

As it moved away from the source of heat, the molten iron would have gone through several structural phases. When the temperature fell below 1540c degrees, it would have begun to solidify and at 770c degrees it became magnetic.

Following another simple rule of physics, heat rises and cold falls, the cooler materials would have drifted towards the center of the ball, and the gases remained on the surface. Once enough iron had fallen below 770c degrees, the magnetic core could form. This process took about 50 million years and was complete around 4535 million years ago.

Once the nickle and iron coalesced into a ball divided into two parts, the magnetic, solid inner core and the molten outer core where temperatures today range from 5000c to 2200c degrees, silicon, magnesium and similar elements were segregated into an outer layer. Steve Kershaw suggests the mantle emerged around 4000 million years ago, and that it took about 2000 million years for the separation to be completed sometime around 2000 million years ago.

While the interior was still evolving, the outermost layer cooled into a skin that became the precursor of the crust. The oldest rocks found so far on the North American plate are from the Nuvvuagittuq greenstone belt near Hudson Bay. They are estimated to be between 3800 and 4280 years old.

The skin created a barrier between the warmer interior and the lighter gases which allowed water to condense without turning into steam. Oceans existed more than 3900 years ago, according to Kershaw. Scientists argue whether all the water was native or was increased by collisions with meteors and comets which could still easily penetrate the surface gases.

At this point, the history of the earth diverges into four separate narratives - the mantle and its skin, the oceans, the gases, and the crust and its plates - which rejoin sometime before New Mexico makes its appearance on the stage.

Notes:
Chandler, Harry. Metallurgy for the Non-Metallurgist, 1998, on properties of iron.

Kershaw, Steve. “Precambrian Ocean Change” in Oceanography: an Earth Science Perspective, 2000, with contributions from Andy Cundy.

Wikipedia entries on Earth’s history and iron.