Many people suppose that the evidence from radioactive dating of minerals in rocks proves the earth must be of great age, at least a few billion years, but the concentration of radioactive isotopes in the rocks of the crust may be too high for an ancient earth. As the atoms of radioactive materials disintegrate, heat is generated and temperatures rise. Heat conduction in rocks is very slow. If the abundance of the long-lived isotopes uranium (U235 and U238), potassium (K40), and thorium (Th232) were similar in the earth's mantle and the continental crust, the heat generated would have soon melted the entire earth.
So, geologists have to assume these radioactive isotopes which occur in crustal rocks are absent (or at least, much less abundant) in the earth's interior, in order to support their idea that the earth is billions of years old. It is assumed the radioactive isotopes are confined to rocks of the crust, and particularly the continental rocks, especially granite. The mechanism of differentiation is questionable, as well as the evidence for such a process. Differentiation mechanisms that have been proposed assumed the earth was once completely molten, but this now seems to be incorrect.
I see the assumption about the depletion of long-lived radioactive isotopes in the earth's interior as the big weakness of radioactive dating theories that are generally regarded as very persuasive arguments for an ancient earth.
The curious thing is, plate tectonics theory says the rocks of the crust and of the ocean floor are being recycled back down into the deepest parts of the mantle. Would not the subduction process, if it occurs, carry down radioactive isotopes from the surface rocks as well?
Far from providing evidence for an earth that is billions of year old, radioactive isotope distributions within the earth may show the earth's formation could not have been very long ago!
In the last century William Thompson (Lord Kelvin) [See bio] investigated the time required for the earth to cool to its present temperature, assuming it was originally molten. His papers challenged the demands of uniformitarian geologists for the immense age of the earth by placing upper limits on the amount of time available. Lord Kelvin announced that not more than 40 million years could have passed since the earth solidified. Because of Kelvin's enormous prestige and high standing as an authority in science, uniformitarian geologists were seriously embarrassed by this result. A long controversy followed.
The phenomenon of radioactivity was discovered by French physicist A. Henri Becquerel in 1896. In 1905, Ernest Rutherford suggested that radioactive minerals could be used to date rocks. It seemed that radioactivity also provided an additional source for heat within the earth that could remove the constraints on the earth's age Lord Kelvin's calculations had imposed on the uniformitarians. T. Mellard Reade expressed some of the relief geologists now felt when he wrote: "The bugbear of a narrow physical limit to geological time being got rid of, we are free to move in our own field of science."
In the early days after the discovery of radioactivity no one knew whether it would provide evidence that the earth is heating up, or in a steady state, or cooling from a former molten condition as Lord Kelvin had postulated for his calculations. This would depend on how much radioactive material exists within the earth, and this information was unavailable. English geologist Arthur Holmes figured that if one knew the earth's age, (and he thought he did) he could estimate how much radioactive material the earth contains. He made measurements of rates of various geologic processes, such as sediment accumulation, assuming the validity of Lyell's principle of uniformitarianism.
Holmes was largely responsible for the inflation of the earth's age in this century. In a paper on radioactivity he quoted the following statement about his method of reasoning, by his friend L.H. Adams [Holmes, A. 1925, p. 508]:
It remained for Holmes to make the bold step which reconciles in a satisfactory manner all the factors involved. Instead of attempting to calculate the age of the earth, and thence the interior temperatures, from the surface gradient, he started with the age of the earth as determined by other means, and from that datum calculated the total amount of radioactive material in the earth and thence the temperatures at various depths.
One of these "other means" was evidently the time required for evolution to occur, as Holmes was quite concerned to show that the earth had remained favourable for life. He wrote [Holmes, A. 1937. p. 28]:
The work of Darwin and Wallace showed that the time required for the progress of organic evolution must have been very great... The panorama of life in the past implies a vast stretch of time...
Data on average radioactive isotope content and heat production rates in various rocks is available in many geology and geochemistry texts.
Geophysicist H. Jeffries said each cubic cm of average granite supplies heat at the rate of 5 x 10-13 cal/sec. [Jeffries, 1976, p. 402] The total observed heat flow from the earth (about 10-6 cal/cm2 sec) could be supplied by the heat produced by the radioactive isotopes in a layer of average granite 20 km thick. This can be interpreted as evidence the earth is young, and is heating up.
At present, however, that possibility is automatically ruled out by most geologists, who prefer to believe instead that the radioactive isotopes U235, U238, K40, and Th232 are absent or much less abundant in the earth's interior than in rocks of the crust. Radioactive isotopes in basalts are far less abundant than in granite.
What could explain the accumulation of the minerals containing radioactive isotopes in the continental crust? In 1930 V.M Goldschmidt advanced a theory to address this problem; he proposed that large atoms of uranium and thorium in a cooling melt (of a formerly molten earth) could not easily fit into crystal lattices of silicate minerals so remained in the melt, ascending higher and higher, eventually becoming incorporated into the crust. More recent workers have abandoned the assumption of a formerly molten earth, but still rely on Goldschmidt's differentiaton mechanism.
V.N. Zharkov [Zharkov, 1986, p. 106] quotes Vernadskii:
It is unquestionably the case that the observed thermal regime of our planet can only exist under conditions such that the amount of uranium and thorium decreases rapidly for even very small depths below the surface.
This must be so, they say, (assuming the earth is of great age), because if heat producing radioactive isotopes existed in the minerals of the earth's interior in the high concentrations we find in crustal rocks of the continents, the heat would have already melted the earth. The alternative of a young earth, of course is unacceptable, except for creationists.
The question remains, how come the hypothetical "subduction" mechanism of plate tectonics theory does not return the uranium, thorium, and potassium-40 of crustal rocks down into the mantle?
And, the mechanism by which the radioactive isotopes could become differentiated into the crust remains a problem. Verhoogen wrote [Verhoogen, 1979 p. 27]:
There is no generally recognized mechanism by which radioactive elements become heavily concentrated in the uppermost continental crust.
Heat production rates due to radioactivity may well be an excellent argument for a young earth, once we penetrate the web of evolutionary assumptions.