Logo IMG
HOME > PAST ISSUE > March-April 1998 > Article Detail


Creationism's Geologic Time Scale

Should the scientific community continue to fight rear-guard skirmishes with creationists, or insist that "young-earthers" defend their model in toto?

Donald Wise

Radiometric Dating

Modern geologists routinely date progressive evolution of the earth and its changing life forms by calculating ages of mineral samples containing concentrations of various radioisotopes and daughter products having known rates of decay. If "young earth" models have any hope of viability, creationists must somehow discredit almost all these methods of mineral dating.

Figure 3. Catastrophic phenomenaClick to Enlarge Image

One argument for doing so involves purported changes in the speed of light. Creationists argue that if so fundamental a property of nature as light can change, then all other properties, including rates of radioactive decay, could also change to yield false dates. Such arguments, though implausible, were possible up until about 20 years ago when improvements in instrumentation and methods greatly reduced the error bars on measurements of the speed of light. For all but the most committed, these refinements have laid to rest the idea that there are significant measurable changes in the speed of light (see discussion by Schadewald 1984).

Other creationist claims for a changing speed of light include redshifts, the lengthening of wavelengths of light from distant galaxies by up to four times that of ordinary light. They interpret this as a Doppler effect that should require the universe to be in violation of Einstein's equations by expanding faster than the speed of light. Consequently, they argue for a very young universe in which very rapid changes in the speed of light merely give the appearance of old age.

The whole redshift argument is based on the false premise that the redshift is a Doppler effect caused by the instantaneous movement of cosmic objects away from each other. In reality, some redshifts are caused by gravitational slowing of light as it escapes massive objects, but most reflect the fact that these wavelengths represent a kind of tape measure embedded in space itself. When the light started, its waves were of normal lengths. In the intervening eons, the cosmos expanded along the line of travel, stretching those embedded wavelengths as measures of the magnitude of long-term expansion of the cosmos itself.

A more down-to-earth creationist argument for unreliability of radiometric dating is R. V. Gentry's observation (recapitulated in Gentry 1992) of tiny halos of radiation damage around minerals embedded in Precambrian micas. Gentry argues that these halos had to be formed in primordial granites during the first few minutes of earth history by some unrecognized, now-extinct, very short-lived radioactive element.

The confused geology of Gentry's arguments and the geologic facts concerning his sampling site have been presented by J. R. Wakefield (1988). Wakefield points out that various kinds of halos of radiation damage around certain minerals in micas are common and that Gentry's samples came from dikes that cut Precambrian sedimentary rocks. Thus, field relations prove that his samples are younger than the sedimentary deposits and therefore cannot be of primordial origin.

Creationists commonly cite the "helium problem" as evidence for a young atmosphere that contradicts radiometric dates (Vardiman 1986 and J. Morris 1994). Scientists generally recognize that the rate of production of helium from the earth's crust and mantle exceeds by a factor of 2 to 5 the Jeans rate of strictly thermal escape from the upper atmosphere Harper and Jacobsen 1996). If these were the only factors in earth's helium balance, the creationists' claims of an atmosphere with a maximum age of only 2 million years might have some merit (J. Morris 1994).

However, a number of nonthermal processes are capable of accounting for steady-state atmospheric compositions of earth, Venus, and Mars, as outlined by B. C. Shizgal and G. C. Arkos (1996). These include exothermic exchanges of helium and nitrogen ions that can boost helium ions to escape velocity, as well as the solar wind, which sweeps helium ions outward along high-latitude open magnetic field lines. The helium problem remains under active investigation, but its scientific understanding has gone far beyond the simple thermal-escape models still cited by creationists.

Figure 4. Click to Enlarge Image

Another common creationist argument for the "unreliability" of radiometric dates is Austin's (1994) dating of the Uinkaret lava flows of the Grand Canyon region. It is generally recognized that some of these flows were so young that they cascaded over the canyon edges and dammed huge lakes within it. Austin cites a number of different analyses of these lavas that can be used to calculate a wide range of radiometric model ages, some older than the earth itself. He concludes by asking, "Has any Grand Canyon rock ever been successfully dated?"

There are many methods of deriving radiometric dates, some widely recognized as being far more accurate than others; the range of ages Austin cites merely shows the lack of precision among the cruder methods. For the most part, Austin used a ham-handed approach by dating whole rocks rather than individual minerals or parts of individual mineral grains. He then set up a straw man by culling other dates from the literature for comparison to complain about the wide spread of the results. In fact, Wenrich, Billingsley and Blackerby (1995) derived quite precise dates from these Uinkaret volcanics. Using some of the best potassium-argon methods, they found a regional pattern of younger and younger dates moving eastward with time to reach the youngest Grand Canyon volcanoes only 10,000 years ago, a date in good accord with erosional history.

Creationist claims about radiometric dates and their linkage to the supposedly unreliable fossil record fail to point out the rarity of locations where rocks with well-controlled fossil dates are closely associated with proper mineral material for very precise radiometric dates. A few hundred of these well-dated localities represent the primary control points on which the entire dating system of the geologic column is based.

Where such ideal conditions exist, the same horizons yield the same precise dates even if the locations are continents apart. For instance, Bowring et al. (1993) used the finest uranium-lead methods to date zircons associated with the base of the Cambrian in Siberia at 543.9 (± 0.2) million years, whereas in southern Africa, J. P. Grotzinger and his colleagues (1995) found a number of ash beds spanning the same fossil range, yielding dates from 545 to 539 (± 1) million years. In the Rocky Mountain region, about 20 Cretaceous (140–65 million years old) ash beds are interlayered with well-known fossil-bearing sequences recognized both here and in Europe. Precision dating by J. D. Obradovich (1993) of these ash beds differing by only 0.5 million to 1 million years confirmed the same 1, 2, 3... sequence as that derived both from fossils and field relationships.

comments powered by Disqus


Subscribe to American Scientist