In the 1880's there were three hypotheses about the age of the earth. The view from biblical genealogies was that the earth was a few thousand years old. Kelvin's hypothesis, based on cooling rates, was something in the range of 20-100 million years. Geologists argued from rates of erosion that the earth was several hundred million years old.
To distinguish between these hypotheses, an alternative test not based on any of the
hypotheses was needed. Radioactive dating of rocks emerged as a fair alternative
test. Radioactive dating might have supported any of the hypotheses. However,
radioactive dating shows that the oldest rocks are four billion years old. There are
many additional physical tests which support an age of billions of years. The
combined force of all these tests is compelling. The earth is assuredly billions
of years old.
Dr. Chaffin: The radioactive age—4 billion years—as shown on this bar graph plots way out to the right on a time line, and a biblical age based on a straight-forward interpretation of genealogies and other statements in scripture is on the left of the bar graph only several thousand years old.
In between these age estimates are results of other tests that were known in the 1800's: those based on erosion and on cooling rates. The cooling rates really only give an upper limit and they could point to a much smaller age. The erosion age is invalidated if there was a massive flood such as the Bible speaks of in the time of Noah. Such a Flood would deposit many more feet of sediments in many locations and would erode sediments in others, so Flood deposition would invalidate age estimates that ignore it. So looking at this graph, we see that the radioisotopes bar graph really doesn't support the two in the middle; it is off to the right. As far as the heat flow, Lord Kelvin published studies of the earth's thermal regime concluding that the earth's crust could not be much older than about 40 million years and could be much younger, including as young as several thousand years. The rate of cooling of the earth is extremely slow. Nearly 100 percent of any original heat present when the earth acquired its nearly spherical form at creation or whatever you want to postulate for the earth's origin, when the earth acquired its nearly spherical form, most of the internal heat that was present then must be present now if the uniform rates that we observe today were in operation throughout earth's history. However, it nevertheless, would only require a time on the order of thousands rather than millions of years for the modern thermal gradient present in the earth's crust to be established. In 1906, R. J. Strutt published studies of the amount of radioactive substances in rocks. In the process of giving up their energy, radioactive substances cause heating of the rocks. Lord Kelvin did not know about this source of heat when he published his studies. So evolutionists have often pointed to this as invalidating his studies. However, a 1978 study by two creationists, Harold Slusher and T. P. Gamwell, shows that the radioactive heating does not alter the order of the magnitude of Kelvin's calculations. The age of the earth indicated by the earth's thermal flux is much less than indicated by evolutionary models. [H. S. Slusher and T. P. Gamwell, 1978, The age of the earth: A study of the cooling of the earth under the influence of radioactive heat sources, Institute for Creation Research, Santee, CA.]
Studies indicate that accelerated radioactive decay may explain the heat flux. Modern data show conclusively that in each tectonic province, the heat flow Q in watts per square meter and the heat production H in watts per cubic meter are related by a simple linear equation: Q = Q0 + D H where D is a constant whose value is about 10 kilometers and Q0 is a constant for a given tectonic province. Evolutionary models which postulated a specific distribution of radioactive elements with depth failed to agree with observational data from deep bore holes—the 12-kilometer deep Kola project in Russia and the 9-kilometer deep KTB project in Germany. In a recently published creation book, Baumgardner proposed a biblically based model that explained this data. To quote Baumgardner, "A modest pulse of nuclear decay during the Genesis Flood has caused the near-surface radioactive heat production to dominate the surface heat flow." The details are in the book, Radioisotopes and the Age of the Earth [published by the Institute for Creation Research and Creation Research Society, pages 49-94].