Radioactivity and the Age of the Rocks

Radiometric (or radioactive) dating of rocks involves the decay of some "parent" element into its stable (nonradioactive) end product. As an example, uranium is a parent element which decays to its end product, radiogenic lead. (It is called radiogenic lead to distinguish it from other lead which is not derived from radioactive decay.) By measuring (1) how much of a parent element in a rock has decayed into its end product, and (2) the current rate of this decay, most geologists believe they can assess the date when the parent was incorporated into the rock, or equivalently, the period of time that has elapsed since the rock formed.

My attention turned to the question of whether the decay rates of different radioactive elements have always been what they are at present. A uniform decay rate would mean, for example, that the amount of uranium in a rock would constantly diminish while the end product, radiogenic lead, would constantly increase. In this instance the ratio of uranium to radiogenic lead would be a measure of the time since the rock solidified. If, however, [p. 15] the decay rate was much higher sometime in the past, then the radiogenic lead would have rapidly accumulated in the rock—what normally would have taken eons would have been accomplished in a short period.

On the basis of the uniform decay rate assumption, the rock would be falsely judged to be quite ancient, not because the data (meaning the ratio of uranium to radiogenic lead) was wrong, but because of an erroneous premise. How important, then, it was to know the truth about this matter. My university physics courses had taught me to believe the assumption of uniform decay was beyond question, but no proof was given. Did such proof actually exist? If so, I needed to find it, for some weighty matters about the evolutionary scenario hung in the balance.

The assumption of constant decay rates is an integral part of the evolutionary premise that all physical laws have remained unchanged throughout the history of the universe. This is the uniformitarian principle, the glue that holds all the pieces in the evolutionary mosaic together. If it is wrong, all the pieces become unglued and evolution disintegrates. Understandably, scientists who are convinced that evolution is beyond question might have difficulty in considering variable decay rates. To do this would be equivalent to admitting that the uniformitarian principle might be in error, which would be tantamount to agreeing that evolution could be wrong. My acceptance of evolution had been quite firm; yet I always remained willing to consider new evidence. Thus I didn't feel any inhibitions about continuing my inquiry into radiometric dating and the crucial question about decay rates.

In the summer of 1962 I was awarded a National Science Foundation Fellowship for three months to attend the Oak Ridge Institute of Nuclear Studies Summer Institute in Oak Ridge, Tennessee. My free time was devoted to studying about radioactivity and the age of the earth. The following fall I taught physics full-time and concurrently pursued graduate studies in physics at the Georgia Institute of Technology in Atlanta. The investigation of radioactive dating techniques was sandwiched between teaching duties and course work. My attention was increasingly drawn to a tiny radioactive phenomenon found in certain rocks because it was thought to be the evidence for the constancy of radioactive decay rates throughout earth history. It occurred to me that a reinvestigation of this phenomenon might serve as an appropriate thesis topic for the doctoral degree. Before approaching the physics department chairman with this suggestion, I perused most of the important scientific reports on the subject. The next three sections are a summary of my initial findings. [p. 16]

The Puzzle of the Rings in the Rocks

The scientific literature revealed a fascinating story that began to unfold in the late 1800's, when improved microscopes became available. Mineralogists realized the microscope could be a powerful tool to examine many features of rocks and minerals, hidden from normal view. They especially wanted to see through pieces of rock to learn how the different minerals were interlaced. To accomplish this they learned to prepare thin, translucent slices of minerals. Mineral specimens that appeared clear of defects with the unaided eye were now often seen to contain tiny grains of other minerals. Most of these tiny grains aroused little interest; mineralogists just assumed they were embedded in the host mineral when it crystallized.

Some of the tiny grains attracted attention, not because of their own appearance, but because of what appeared around them. Mineralogists saw that these grains were surrounded by a series of beautifully colored, concentric rings. Under the microscope the tiny ring patterns resembled a miniature archery target, with the grain at the center as the bull's eye. Because of their halo-like appearance and because they exhibited color variations known as pleochroism in certain minerals, these concentric ring patterns came to be known as pleochroic halos.

Upon further study mineralogists found that what appeared as a series of flat, concentric rings under the microscope was actually a cross section of a group of spherical shells. To illustrate: If an onion is thinly sliced from top to bottom, the onion rings with the largest diameter will be in the slice through the center. The off-center onion slices will still show the ring pattern, but the diameters of the rings will be smaller. This is similar to what mineralogists found when they examined adjacent slices of a mineral containing a pleochroic halo. Thin slices immediately above and below the center grain showed reduced ring sizes when compared to the slice through the center. This proved that the two-dimensional pleochroic halo seen under the microscope was actually a slice of a group of tiny, concentric microspheres.

The presence of the tiny grain in the center was thought to hold the key to the origin of the halos. Some mineralogists speculated that an organic pigment might have been trapped in the halo center when the mineral formed, only to diffuse out later to form tiny colored spheres. However, no one could identify the pigment or satisfactorily explain how diffusion could produce multiple spheres. Pleochroic halos defied explanation until, about the turn of last century, uranium and some other elements were discovered to be radioactive. [p. 17]