Your Argument About Sinking Uranium is Flawed

For Dean Sessions, the fact that radioactive elements are thought to be more abundant in the crust of the Earth than the lower layers is significant for several reasons, one of which is that he thinks this supports his idea that the Earth gets less dense toward its center.  In fact, he thinks it is the last nail in the coffin of the “accretion theory,” which holds that the Earth was originally formed by the gravitational accretion of space junk (meteors, asteroids, and so on), which smashed together and generated enough frictional heat to at least partially melt the Earth, after which differentiation into layers according to density occurred.

According to the accretion theory, heavier elements like Uranium and Thorium, both radioactive, sunk to the center of the melted, spherical magmaplanet Earth, but according to the geology book, The Heart of the Earth, this is not reality:

<b>”</b>Again,<b> why s</b><b>hould </b><b>the radio</b><b>active materials be concentrated </b><b>in the surface layer? The </b><b>elements </b><b>involved </b><b>are very dense; </b>if the earth cooled from a liquid mass, one would expect them to settle to the center. <b>But no: they are apparently found </b><b>almost entirely at the surfac</b><b>e–</b><b>why?”</b>

One important point to consider here is that the heavy elements should not even be on the surface if the Earth was once completely melted.  Because they are the heaviest, these elements should have sunk to the center of the Earth, but apparently, they did not.  Later, in the Ore Mark subchapter, 18.13, we discuss why we find the heaviest elements near the Earth’s surface.  (UM, Vol. 1, pp. 100-101.)

The Heart of the Earth is a book published in 1968 by geophysicist O.M. Phillips.  It seems to be one of Dean Sessions’ favorite geological sources, even though it is almost 50 years old.  Although it is somewhat dated, it’s actually a pretty good book.  Sessions sometimes pulls quotations out of context from the book, but in this case Phillips was plainly wrong.  What’s needed to explain this is some geochemistry, not just geophysics.  (Oh, wait… I’M a geochemist!)

Suppose I take some lead tetrachloride (PbCl4) and dissolve it in water.  If I let the solution sit long enough, will the lead separate out and sink to the bottom?  No, it won’t.  The lead isn’t just lead–it’s now part of the water, tied to an intricate network of chemical bonds.

Now suppose I dissolve more and more lead tetrachloride into the water, heating it a little bit, so that when it cools down it will be supersaturated and start to form lead tetrachloride crystals out of the water again.  Since the PbCl4 crystals are more dense than the surrounding liquid, it will drop to the bottom of the container.

Finally, suppose I mix some water (H2O) and vegetable oil (a mixture of molecules made of the elements carbon, hydrogen, and oxygen).  Will the oxygen atoms sink to the bottom, underneath the carbon atoms, which will congregate underneath the hydrogen atoms?  Of course not.  The oil and water will separate, but the individual atoms will not separate from the water and oil molecules.

The reason the elements won’t separate from one another is that the chemical bonds holding them in the compounds where they reside are much stronger than the force of gravity, although the bonds operate over much shorter distances.

When we are discussing how the Earth could have separated into layers of different density, therefore, we have to think about what COMPOUNDS it would be most favorable for different elements to join under different conditions, and how dense those COMPOUNDS are.

Geochemists classify uranium, for instance, as an “incompatible element” with respect to minerals in the Earth’s mantle.  This means that if there is some uranium incorporated into the structures of the high-pressure and temperature silicate and oxide minerals in the mantle, and the pressure and temperature conditions are right for partially melting some of these minerals, elements like Uranium will preferentially jump ship from the minerals into the melt.  Since the melt is less dense than the surrounding rock, it tends to rise above the rocks to be incorporated in the crust.  Voilà!  The Uranium becomes more concentrated in the crust than in the mantle!

A similar process, in the reverse direction, can be observed when sea ice forms on the ocean.  Seawater is salty, and the salt ions are easily incorporated into the structure of liquid water.  (Water is actually really good at dissolving salts.)  The National Snow and Ice Data Center explains what happens when it gets cold enough for sea ice to form.

When sea ice forms, most of the salt is pushed into the ocean water below the ice, although some salt may become trapped in small pockets between ice crystals. Water below sea ice has a higher concentration of salt and is more dense than surrounding ocean water, and so it sinks. In this way, sea ice contributes to the ocean’s global “conveyor-belt” circulation.

What starts as a single substance (salty water), is transformed into two substances (ice and even saltier water), and the salt is unequally divided between the two.  The ice is less dense, so it floats on top of the water.

Another reason Sessions thinks it’s significant that radioactive elements like uranium are more concentrated in the crust is that he believes it supports his idea that the Earth is cooler in the center.  I explained in another article that if a spherical body had heat generated in an outer layer, heat would still build up in the center until there could be a net heat flow toward the outside.  In other words, the center would have to heat up at least as hot as the outer layer.  If there were some (but not as many) heat sources (like radioactive elements) closer to the center of the body, it would end up hotter in the center.

I don’t want to beat a dead horse, but I can’t resist pointing out that one of Dean Sessions’ favorite geological sources, The Heart of the Earth, could have enlightened him about this point if he hadn’t been so bent on quote-mining his sources.

These considerations, then, force us to abandon the idea of a gradually cooling earth without heat generation, in spite of the fact that the “cooling time” was not, on the face of it, either absurdly short or absurdly long.  The model that emerges is one in which the internal temperature of the earth is governed largely by the generation of heat in the earth’s crust and possibly a little below it.  This generation may not exactly balance the heat flow outwards through the surface;  if the present generation rate is slightly less than the outwards flux, then the average temperature of the interior may be decreasing very gradually, at a rate slower than the one we calculated in the previous section, since part of the heat loss is offset by the internal generation.  On the other hand, the generation may be rather in excess of the present rate of loss; in that event the interior is gradually becoming hotter.  There is at present no way of telling with certainty which of these alternatives is the correct one; each possibility has its advocates.

This general conclusion of a shallow heat source provides a solution to another part of the puzzle that we encountered earlier.  It was pointed out that the temperature gradient measured at the surface could not possibly continue to the center of the earth, since this would lead to absurdly high temperatures there.  Suppose, for the sake of discussion, we again take a definite model; that all the heat is generated in a layer near the surface and that this exactly balances the measured surface heat flux–the interior is at a steady temperature, neither heating nor cooling.  Since there is no temperature change in the interior, then there is no net heat flux there either, and from Fourier’s law, there is no temperature gradient.  The internal temperature, below the depth of the heat source, is quite uniform.  This model is exactly analogous to the inside of a furnace that is heated by elements on all sides.  The temperature at all points inside the furnace is the same; outside the elements, however, (toward the surface) the temperature drops rapidly through the walls to room temperature as the heat leaks out.  For the earth, this simple model indicates that the relatively rapid temperature gradient found near the surface, continues only to the depth over which the radioactive sources are present and beneath that, the temperature is, in essence, constant.  We know that at 30 km the temperature is about 1100 °C; if the “heating elements” are all above this, the temperature throughout the mantle and core is the same.

One should not, however, expect this simple picture that we have postulated to be literally exact.  There may be some degree of radioactivity at great depths even in the core of the earth, so that the deep temperature gradient, though small, may not be exactly zero.  Nevertheless, the fact remains that most of the radioactive decay appears to be concentrated near the surface, so that the temperature of the core of the earth, if not as small as 1100 °C, may be, say, 3000 °C (Figure 50), but almost certainly not, say, 10,000 °C.  (O.M. Phillips, The Heart of the Earth, pp. 150-151.)