September 12, 2005
Subjectivity in Radiometric Dating
Rick Lanser
This article was first published in the June 2003 ABR E-Newsletter.
The acceptance of radiometric dating has had a major influence on people's willingness to accept the theory of
evolution. Most people feel that radiometric dating, having a reputation for scientific respectability, proves
that the earth is many millions of years old, thus providing the time framework required for the theory of evolution
to have plausibility.
As explained by physicist Walt Pasedag, radiometric dating uses the principle of radioactive decay, a physical
process which can be measured with great precision. Radioactive decay is described by a simple mathematical relationship,
which states that the amount of a specific isotope that remains in a sample declines exponentially with time, according
to a fixed rate constant, the "decay constant" which is unique for the isotope. Decay constants have
been measured for all known isotopes ever since Madam Curie discovered radium in 1898. When the isotope decays,
it turns into a different isotope, the so-called 'daughter' product. By measuring the quantities of parent and
daughter isotopes, the time that has elapsed to generate the measured amount of the daughter product can be
calculated.
As far as the technical aspects go, the process lives up to its reputation of great accuracy. Problems arise,
though in INTERPRETING this measurement. The decay time calculated from the measurements, no matter how precise,
cannot be interpreted as the age of the rock the sample was taken from without making a number of assumptions,
and that's where the problems arise. The basic assumptions that need to be made are:
-- that the initial quantity of the daughter product is known (e.g. equal to zero),
-- that the measured quantities of the isotopes have the same origin, and
-- that the sample was isolated from any and all physical and chemical processes which would affect the quantities
present in the sample.
In other words, there was no migration of isotopes, no leaching, influx, or chemical reaction which could disturb
the balance between parent and daughter isotopes throughout the long time spans usually postulated. This assumption
would be totally untenable for sedimentary rock, which, by definition, is formed by processes which would disturb
this balance, so radiometric dating cannot be used for this type of rock. Unfortunately, fossils, the other approach
to dating rock, only occur in sedimentary rock.
The above weaknesses of radioisotope dating are well known. Less recognized, however, is an additional difficulty
pointed out by geologist C. Allen Roy in an e-mail to a creation listserv. Even if one minimizes the above-mentioned
weaknesses and accepts the methodology of radioisotope dating as valid, it is still fraught with a subjectivity which
disqualifies it as a truly scientific approach to dating rocks. He made three basic observations, which are summarized
below.
1. The data-gathering process is assumed to be objective
We are given the impression that: 1, radiometric dating is science; 2, science is objective; therefore 3, dating of
rocks using radiometric methodology is objective science. In other words, it is assumed that the methods of radiometric
dating yield valid, objective data. The process of measuring isotopes is now performed at the level of technology rather
than scientific experiment; rock samples are simply submitted to labs which have the technology to process the rock
to measure contained isotopes. The methodology, accuracy and precision with which isotopes are measured, and the
principles by which ages are derived from those measurements, are not debated even by most creation scientists.
Thus, it is assumed that the technological methods are not faulty, and therefore we expect any scientific results
thus obtained to determine our conclusions -- that the data gathered are as objective as the methods used to obtain
it. This is not the case with radiometric dating, however.
2. Yet, objective data is overruled by other criteria
An example from the Grand Canyon is instructive. Radiometic dating is done primarily on igneous rock
(although some work is done on volcanic ash). Within the Grand Canyon are several outcrops of volcanic rock,
two of which are noteworthy -- the Cardenas Basalts of the Grand Canyon Supergroup, at the bottom of the Grand Canyon,
and the Uinkaret plateau lava flows. These outcrops are of interest because the Cardenas Basalts represent some of
the oldest rocks in the Grand Canyon, existing below all the horizontal layers which make the canyon so famous.
On the other hand, the Uinkaret Plateau lava flows are extruded on top of the plateaus above the canyon, some of
which also flowed down inside the Grand Canyon. Everyone agrees that the Uinkaret lava flows must be younger than
the formation of the Grand Canyon, and that the Grand Canyon must be younger than the Cardenas basalts. Ergo, the
Uinkaret lava must be younger than the Cadenas basalts. It is a simple process of logic: what lies beneath must be
older than what lies on top.
Contrary to this expectation, when Rb/Sr (Rubidium/Strontium) radioisotope measurements were made of these rocks,
the Uinkaret lava flows actually compute to be OLDER (~1.3 billion years old) than the Cardenas Basalts
(~1.0 billion years old). This is a conundrum to the geologists, which they solve by accepting the Cardenas basalt
results, and discarding the Uinkaret lava flow Rb/Sr results -- not on the basis of any fault with the measurements,
but simply because the results do not fit the logically derived conclusions! Although the methodology and
technological processes were faithfully and precisely followed on rock samples from both sources, the dating for
the Uinkaret is rejected as being older than the date for the Cardenas.
3. Thus, objectivity takes a back seat to subjective expectations
What we see is that, just because the process of radiometric dating is done with great precision and great
care, it does not automatically mean that the results are going to be accepted as valid. In most geological papers
where radiometric samples are involved, there is usually a "discussion" about whether to accept or reject
the ages so acquired. That choice comes about through factors other than the accuracy of the radiometric process.
In the case of the Uinkaret/Cardenas igneous rock, non-scientifically acquired data actually takes precedence
over the scientifically (technologically?) acquired radiometric data. It is strictly observation and logic,
not scientific experiment, that shows that the Uinkaret must be younger than the Cardenas. This fact outweighs
the scientific data acquired through the radiometric dating process.
The important point here is that at the very moment that a radiometric age is rejected (or even accepted) based
on other data, the radiometric dating method itself becomes immediately subject to that other information.
Thus, no matter how precise the radiometric process has been, the results are still handled subjectively. In the
case of the Unikaret, the computed ages are rejected because it is impossible for them to be correct. In the case
of the Cardenas, the computed ages are accepted because they agree with what is expected.
In conclusion
From the above points, we see that: 1, The acceptance or rejection of radiometrically acquired ages for rock
depends upon factors other than the science/technology of radiometric dating; 2, the dating process of rock units
is subjective even when radiometric dates are available; and 3, if any rock unit is believed to be young, then any
old radiometric age is rejected, not as inaccurate, but as irrelevant.
Thus, we are left with no other option than to discard radiometric dating as a determinative, scientific approach
for obtaining objective dating about the earth; and therefore, it is also irrelevant as a support for the theory
of evolution. It provides no support for the theory, having been shown to be ultimately subjective, and unable
to supply the desired objective scientific support for the long ages required by the theory of evolution.
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