Finding Earth's Age and Other Developments in Geochronology

Overview

The discovery of radioactivity and radioactive decay has helped to solve many problems that have plagued geologist for centuries, especially the question of absolute ages of rocks, fossils, and Earth itself. These answers have provided a more rigorous science of geology and have given scientists in a large variety of fields firm data upon which to base their studies and hypotheses. In addition, by giving a scientifically unassailable age for Earth, isotopic methods have been used by many to argue against a literal interpretation of the Bible, and it is not uncommon for practitioners of this science to be called upon to testify in legal cases involving the teaching of evolution or variants of creationism.

Background

For uncounted centuries, man either had no idea of the age of Earth or, based on a literal reading of religious works, felt Earth to have been created not more than a few thousand years ago. During the nineteenth century, as geologists gained a better understanding of geologic processes, most scientists became certain of Earth's antiquity, but still lacked any real knowledge as to what that meant. Estimates of Earth's age ranged from a few million years to many billions of years, all based on different methods of age determination.

One of the driving factors behind efforts to determine this age was the introduction of (and controversy surrounding) evolutionary theory. Evolution required vast amounts of time for species to gradually form, die off, or transform one into another. The incredible variety of life found in the fossil record simply could not occur in an Earth of only a few million, or even a few tens of millions of years old. If Earth could be shown to be old, but "only" a few million years old, evolution might yet be shown false, and man might retain a special place in creation.

The single most influential estimate of Earth's age was put forth by Lord Kelvin, William Thomson (1824-1907), the preeminent physicist of his day. Kelvin's estimates were all based to some extent on the amount of time it would take Earth to cool from an initially molten state to its current temperature. They ranged from a few tens of millions of years to nearly half a billion years. Because of Kelvin's prestige, few dared to challenge his calculations or the premise upon which they were based, even when it became apparent that Earth was, instead, much older.

In 1895, Wilhelm Röntgen (1845-1923) discovered x rays, and, in the following year, Henri Becquerel (1852-1908) discovered radioactivity in uranium. In the next few years, uranium was discovered to be present in trace amounts in virtually all rocks and soils on Earth. It was also quickly discovered by Marie and Pierre Curie (1867-1934 and 1859-1906) and others that uranium decays through a long series of radioactive elements to finally become lead, which is not radioactive. These radioactive intermediary nuclides include radium, radon, and thorium, all of which occur naturally. Work by, among others, Ernest Rutherford (1871-1937) and Frederick Soddy (1877-1956) showed that heat was released during radioactive decay while Bertram Boltwood (1870-1927) noticed that all minerals containing uranium also contained lead and helium.

These last two discoveries were of particular importance. If radioactive decay released heat, then this meant that the premise upon which all of Kelvin's calculations were based was incorrect because Earth would be cooling at a slower rate than otherwise. In addition, the invariable correlation between uranium, lead, and helium meant that uranium likely turned into helium and lead through radioactive decay. Since Rutherford had shown that the rate of radioactive decay changes predictably over time, this gave a way to construct a "clock" for determining the age of rocks. One of the first such age estimates, about 500 million years, was made by Rutherford and was based on the ratio of helium to uranium. Helium is given off during the decay of heavy elements in the form of alpha particles, which are simply the nuclei of helium atoms emitted from heavy, radioactive elements. However, Rutherford soon found his calculations to be in error because of the many alpha-emitting nuclides present in the uranium decay chains and because helium atoms escape mineral crystals with relative ease. It turned out that lead was a far better "end-point" to use for this dating. By the 1940s age estimates were converging on the current figure of 4.6 billion years of age for Earth. Later work dating meteorites indicated the solar system to be slightly older and, when the Apollo program returned with lunar rocks, we found that the Moon is a few hundred years younger.

Over the next few decades, increasingly sophisticated isotopic dating methods were developed that used a variety of radioactive elements. Some of the more widely used of these are the rubidium-strontium method, the potassiumargon and argon-argon methods, but a number of other geochronometers have been developed for specific purposes. For example, examination of isotopes of iodine and xenon in meteorites tells us about the conditions leading to the formation of the solar system, while analyzing the ratio of neodymium and samarium isotopes can help us trace the geochemical history of mountain ranges.

Impact

Today, the field of isotope geology and geochronology is far more advanced than in 1907. Geochemists routinely use mass spectroscopy equipment, including the latest advance, the tandem accelerator mass spectrometer (TAMS), to analyze isotope with an amazing degree of precision and accuracy. Since its inception, isotopic methods have had a profound impact in the scientific fields of geology, paleontology, evolutionary theory, biology, botany, and (using carbon-14 methods) in the fields of anthropology, archaeology, and history. In addition, by providing a solid and scientific basis for determining the age of Earth and its inhabitants, isotope geology has also resolved the debate over the origin of Earth for all but a handful of biblical literalists.

The chief scientific impact of isotopic dating methods has been to give an absolute timetable for events on Earth. The importance of this can scarcely be overstated. Do you want to know how long dinosaurs dominated Earth? Find the rocks with the first dinosaur fossils and the last fossils and date them. In a few days, you'll have a firm date telling you that they reigned for over 150 million years. How about determining when oxygen first appeared in the atmosphere? In this case, find the oldest rocks that can form only under conditions of low oxygen—their age tells you the last date the atmosphere could have been oxygendeficient. Geologic dating has told us how quickly life can evolve, exactly when the dinosaurs, trilobites, ammonites, and other fossil species went extinct, when life first colonized the land, when the Gondwana supercontinent last broke up, when the Moon formed, and much more.

Having this information is interesting from the standpoint of sheer intellectual curiosity. However, it is also important because it can give us some sort of framework upon which to hang our concept of geologic and evolutionary time. Imagine trying to go to class if you can only say with certainty that social studies comes sometime after home room but before gym and that you go home sometime after track practice, which comes after lunch and before dinner. Without a clock, we might know what happened on the early Earth, but we have no idea of how fast it might have happened or what might have happened at the same time in various parts of Earth. We can construct a history of Earth based only on relative dates (that is, what happened before what); it just isn't very interesting or very informative. Virtually every historical science depends to some greater or lesser extent on isotopic methods of dating past events, and those dating methods have given us, in effect, Earth's clock and calendar.

In the social realm, isotopic dating methods have proven to be of some intrinsic interest as well as providing scientists with an outstanding tool to use in debates against those who believe in a literal interpretation of religious documents.

The intrinsic interest of non-scientists in geologic dating has a great deal to do with the general interest that most people have in trying to better understand our world. As shown by the continuing popularity of natural history museums, newspaper and other media articles on scientific topics, and the popularity of books explaining science, a large portion of the population has some interest in learning more about the world in which they live. A large part of that understanding, as with scientists, is in finding out when significant events took place, even if the time scale is almost incomprehensible in magnitude. This is especially true regarding research on our own origins.

However, it is likely that the most significant social impact of isotopic dating lies in its utility in the perennial debate over the origin of Earth and its inhabitants. So potent an argument, in fact, that virtually every court case involving the teaching of evolution versus creationism (or variants such as "scientific" creationism) at some point sees testimony by a prominent isotope geologist who explains the science behind isotopic dating methods and their results.

P. ANDREW KARAM