Snow and Language

C. P. Snow famously noted the inability of scientists to communicate with their liberal arts colleagues at Cambridge. As a scientist, he tended to blame the other side for not keeping up.

The problem isn’t so much that they have different interests and that those of science have become much more difficult to grasp since Einstein, but that the two groups use language differently.

In the liberal arts, there is some attempt to write in the current lingua franca, albeit, often a highly artificial and convoluted form. But words still tend to have meanings that can be deduced from context.

In science, there is a deliberate attempt to appropriate words and give them arcane meanings. As an example, they discuss secular trends in temperature in the mantle.

Now, to someone with any exposure to western civilization, secular is usually contrasted with religious, meaning that controlled by the civil or political world. Try to apply that to previous sentence.

After searching a bit in Google, I found economists had used the term secular to refer to long term trends, probably from some discussion of those events outside the control of the church hierarchy in the middle ages. That was enough. Scientists like to appear au courant. They adopted a word used by the movers and shakers of the world.

Graduate school is a time for adapting to new rules of discourse where many want, nay actively conspire to make an aspirant fail. Make a comment about the social background of Hawthorne to an English professor influenced by William Empson and you’re guaranteed a poor grade. Quote some fact from your historical geography text to an economics history professor who thinks it’s wrong and watch him look at you with dismay.

But these misadventures are nothing compared to what must happen when people enter into science. It begins in high school with never spelling out a word, but using abbreviations. One never says water when H₂O will do. One couldn’t possibly say the year 2000. Those zeros, that extraneous word thousand had to disappear. We all learned to say Y2K. For an historian it became the label for a technological crisis and the date remained a date. I’m not sure what happened in science, but so far I haven’t seen Y2K11.

It’s rather like science became captive to a group of men who never outgrew the thrill of using Pig Latin in their boyhood tree house sanctuaries. The primary initiation ritual was speaking obfuscation. And so, scientists and social scientists make proficiency in jargon their badge of identity.

Snow wasn’t the first to recognize the problems with language. It was an Oxford mathematician, Charles Ludwig Dodgson, who used the pen name Lewis Carroll to write:

'When I use a word,' Humpty Dumpty said, in rather a scornful tone, 'it means just what I choose it to mean - neither more nor less.'

'The question is,' said Alice, 'whether you can make words mean so many different things.'

'The question is,' said Humpty Dumpty, 'which is to be master - that's all.'

Notes:
Carroll, Lewis. Through the Looking Glass, 1871.

Snow, C. P. “The Two Cultures,”1959.

Photos: Jemez behind badlands with (top) far arroyo in front.

Mantle Structure

The mantle is the cream filling between a silicon-rich crustal meringue and an iron-rich core base. Unlike a common pie, however, it’s composed of layers detected by seismic wave tests and is not some separate ingredient like lemon or coconut, but a transition from silicon to iron in mixtures that includes oxygen, magnesium, aluminum, calcium, sodium, potassium and hydrogen in combinations that change by layer.

Starting at the top, the beginning depths, temperatures and distinguishing characteristics of these layers are:

Crust
Moho - 7 km - 500 C - increase in seismic velocity
Lithosphere - 50 km - 900 C - low viscosity, rigid
Asthenosphere - 200 km - 1100 C - flowing, low velocity
Transition Zone - 410 km - 1800 C - distinctive seismic patterns
Lower Mantle - 660 km - no agreement - high viscosity
Anomalies - 1700 km - no agreement - lateral velocity anomalies
D” Layer - 2891 km - 4000 C - ultralow velocity, distinctive seismic patterns
Core

Olivine, a compound of iron, silicon, magnesium and oxygen [(Mg,Fe)₂SiO₄], is identified as the most important component in the asthenosphere. In the lithosphere, it combines with elements like calcium and aluminum to produce pyroxene. Above that, in the Moho, pyroxene replaces the calcium and aluminum with potassium to become amphibole. That, in turn, replaces the potassium with calcium, sodium and hydrogen (water) to create biotite.

The ratio of iron and magnesium in olivine can vary, which is why they are represented by a comma, rather than a more specific number in the formula. The permutations of both olivine and pyroxene produce many of the silicon rocks found on the surface that have created the perception that the mantle is predominately composed of silicon.

Below the asthenosphere, in the transition zone, the forsterite-fayalite form of olivine changes its crystalline structure into wadsleyite [(Mg,Fe²⁺)₂(SiO₄)], which in turn becomes ringwoodite [(Mg,Fe²⁺)₂(SiO₄)]. The important feature of their structures is that changes in oxygen create spaces for water molecules which are being released as parts of the crust sink into the transition zone. Eli Ohtani thinks the presence of water trapped in these two forms may explain the layer’s seismic properties.

In the lower mantle, the crystalline structure changes again, this time to magnesium silicate perovskite [(Mg,Fe)SiO₃]. Water is lost and iron becomes more important. With an atomic weight of 26, it has a partially filled third orbit. In its ferrous form [F²⁺], it has two electrons available to bond with other atoms; in its ferric form [F³⁺] it has three.

As pressure increases, the spin speed within the atom changes from high to low. Ferric iron changes its spin at 70 gigapascals which is about 1700 kilometers down in the area where seismic anomalies have been detected. Ferrous iron changes its spin pattern at 120 gigapascals which is about 2600 kilometers down, just above the D” layer where the crystalline structure changes again to one called post-perovskite.

The two valences can appear in the same compound in different ratios, while the amount of iron relative to magnesium also varies. Such differences explain why detectable seismic patterns are hard to replicate with current technology: the deeper one goes into the mantle, the greater the amount of iron that has changed its spin cycle.

Neither of the transition zone compounds are known from samples from the mantle. Wadsleyite was found in the Peace River meteorite from Canada. Ringwoodite was first identified in the Tenham meteorite in Australia and has since been seen in other meteorites from other parts of the world. It’s the laboratory conditions under which the two were synthesized that suggests they are the primary components of the transition zone.

A team headed by James Brado put samples of magnesium silicate perovskite through laboratory tests to produce evidence of spin changes and the deep mantle conditions that could produce them. They didn’t indicate where they obtained their raw material.

Notes:
Brado, James, Guillaume Fiquet, and François Guyot. Thermochemical State of the Lower Mantle: New Insights from Mineral Physics.

Ohtani, Eli. Recent Progress in Experimental Mineral Physics: Phase Relations of Hydrous Systems and the Role of Water in Slab Dynamics.

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.

Diamonds

I’m sure this is a simple minded question, but if the mantle is homogenous why aren’t diamonds found everywhere?

When I was a child, this wasn’t a question. I was told in fourth grade that all the ferns in the swamps got covered by debris and turned into coal in the Pennsylvanian period. I don’t know if I was also told that all you had to do to create a diamond was add more pressure, or if I make the connection later when I heard a diamond is simply a form of carbon.

I was wrong about coal and diamonds, but it’s a common enough conclusion. All you have to do is enter the two words in a Google search to see how many others have the same idea. Indeed some of the first men to try to make synthetic diamonds began with charcoal. The ones who succeeded used graphite in a belt press able to producing pressures beyond 10 gigapascals at temperatures above 2000 degrees Celsius.

Once scientists at General Electric had produced that first synthetic stone it became possible to know, fairly precisely, the conditions required to make a diamond. But first, it was necessary to understand more about carbon.

The carbon atom is created from helium in a giant or supergiant star which is then scattered as dust in a supernova explosion. That dust is then coalesced into planets in third generation stars like our solar system.

The number of carbon atoms on Earth was set at creation, although some have since been introduced by meteorites. The carbon atom is particularly promiscuous, able to join with other elements like oxygen and hydrogen in long chained molecules. Perhaps ten million have been identified so far, and more are possible.

However, carbon atoms can also combine with themselves into crystalline structures. Graphite has a hexagonal structure, diamond a cubic one. These are the two main allotropes of carbon. Moving between them, means converting the crystalline pattern of one into the structure of the other.

I suppose it made little sense to believe coal could somehow become a diamond. Coal is mined. The first diamonds were found in alluvial expanses in India, then in Brazil. It wasn’t until the late nineteenth century that men in South Africa found diamonds beneath the surface deposits they were exhausting.

Then it became possible to know something about their origins. They had come up to the surface through volcanic pipes made of kimberlite. The ones in India and Brazil had eroded away, leaving a false impression of their provenance. The ones in South Africa still existed.

Diamonds are believed to be formed at pressures between 4.5 and 6 gigapascals and at temperatures between 900 and 1300 degrees Celsius, both lower than conditions in the first belt press.

The only parts of the mantle that meet the conditions for a diamond lie between 140 and 190 kilometers beneath the thickest, oldest parts of continental crust. Ocean bottom crust is thinner so temperatures rise more quickly with depth. The requisite condition never materializes.

Diamonds today are found in India, Brazil, South Africa, Siberia, and parts of Canada and Australia. These lie under the Indian, Amazonian, African, Angaran, Canadian and Australian shields. The last two are near areas where the oldest known rocks have been found.

Neither a view of a homogenous mantle nor one with two reservoirs explains the appearance of diamonds. The first view would have to be amended to suggest the chemistry was the same throughout the mantle, but varied by location from weight from above. The other looks for the reservoirs near the upper and lower heat sources, not somewhere above the boundary between the upper and lower mantles.

Note: Most of the information came from Wikipedia entries on carbon, diamond, kimberlite and synthetic diamond.

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.

A Rock Is a Rock Is a Rock

The young man in the mineral shop who dismissed my gray rock as “just a rock” with none of the special characteristics of a recognized mineral was wrong.

Rocks aren’t just rocks. To begin with, there are only eight elements that go into the composition of most: oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium. They represent great permutations on matter.

Basalt is some combination of oxygen, silicon, aluminum, calcium, sodium and magnesium. The percentages of silicon and sodium are often used to define the type.

Granite is formed from magma and contains oxygen, silicon, aluminum, sodium and potassium.

Quartz is essentially oxygen and silicon.

Rocks don’t limit themselves to just the eight fundamental elements. Basalt, pyroxene and perovskite can contain titanium. Zircons are silicon and oxygen with zirconium.

More important for geologists interested in the history of the planet are their trace minerals. The half-lives of radioactive isotopes of elements like uranium, neodymium and samarium are used to date them. Zircon crystals are the oldest found above ground.

Rocks, as pieces of matter, do have definable characteristics. In the early twentieth century, Norman Bowen crushed them so he could heat the powders until they melted, then heat them more until they boiled. He let them cool in controlled steps.

He ended with iron-magnesium bearing olivine which was formed at a high temperature and pressure. At a specific lower temperature it becomes pyroxene. At another temperature, pyroxene becomes amphibole, and that, in turn, becomes biotite. In contrast, plagioclase gradually transitions from a rock rich in calcium to one rich in sodium without the abrupt phase breaks.

At a set point, they become potassium feldspar, which becomes muscovite, which becomes quartz. These are the three major components of granite, with quartz the least likely to weather away because it was formed in conditions closest to those of the present.

In the past few decades, new technologies like diamond-anvil presses have allowed geologists to return to experiments like those of Bowen, only now they are looking at how rocks are altered at the higher pressures and temperatures deep in the mantle. Instead of changes in elemental chemistry they’re interested in changes in crystalline structure.

When pressure increases, the O^2- ions in olivine are altered into a spinel structure. At even higher pressures, the spinel structure converts to one found in perovskite and another found in periclase.

Near the earth’s surface, enstatite combines silicon, oxygen and magnesium. It too converts to a perovskite-like structure at higher pressures, then near the bottom of the lower mantle to a post-perovskite structure.

All of this makes terminology a bit confusing, since the same thing is identified by its elements, its crystalline structure and its origins. What one thought was just a rock isn’t. There are at least 11 forms of olivine, including basalt which is further broken into five groups. There are 24 types of pyroxene, including enstatite, and 37 varieties of mica, including biotite and muscovite.

It doesn’t help that only some of these can be seen. Anything with a post-perovskite structure immediate changes to something else when the pressure relents which it must if it’s to make a journey from the center of the earth where it may end up as simple quartz in my arroyo.

Mica (top) from near Albuquerque; granite with bands of gray quartz and shining specks of mica (middle) and a thin piece of quartz (bottom) from the far arroyo.