Growing up in fundamentalist Christianity, it was drilled into my head from an early age that radioactive dating is highly inaccurate. “These dating techniques are not precise. They are nothing more than guesses. Bible-believing Christians can be confidant that the earth is only 6,000-10,000 years old.”
Sorry, fundamentalist Christians, but this is simply ignorant nonsense.
Jerry Coyne, biologist and professor: Since about 1945 we have been able to measure the actual ages of some rocks—using radioactivity. Radioisotopes are atoms that contain an unstable combination of neutrons and protons, or excess energy in their nucleus. Certain radioisotopes are incorporated into igneous rocks [rocks formed from the solidification of molten rock material]. Radioisotopes gradually decay into other elements at a constant rate, usually expressed as the “half-life”—the time required for half of the isotope to disappear. If we know the half-life, how much of the radioisotope was there when the rock formed (something that geologists can accurately determine), and how much remains now, it’s relatively simple to estimate the age of the rock.
Different isotopes decay at different rates. Old rocks are often dated using uranium-235 (U-235), found in the common mineral zircon. U-235 has a half-life of around 700 million years. Carbon-14, with a half-life of 5,730 years, is used for much younger materials like wood, bone, or human artifacts such as the Dead Sea Scrolls. Several radioisotopes usually occur together, so the dates can be crossed-checked, and the ages invariably agree. The rocks that bear fossils, however, are not igneous but sedimentary, and can’t be dated directly. But we can obtain the age of fossils by bracketing the sedimentary layers with the dates of adjacent igneous layers that contain radioisotopes.
The claim by Young Earth Creationists that radioactive dating is unreliable —because the rates of decay might have changed over time or with the physical stresses experienced by the rocks—is specious. Since the different radioisotopes in a rock decay in different ways, they wouldn’t give consistent dates if the decay rates changed. Moreover, the half-lives of isotopes don’t change when scientists subject them to extreme temperatures and pressures in the laboratory. And when radiometric dates can be checked against dates from the historical record, as with the carbon-14 method, they invariably agree. It is the radiometric dating of meteorites that tells us that the earth and solar system are 4.6 billion years old. (The oldest earth rocks are a bit younger—4.3 billion years in samples from northern Canada—because older rocks have been destroyed by movements of the earth’s crust.)
And there are other ways to check the accuracy of radiometric dating. One of them uses biology, and involved an ingenious study of fossil corals. Radioisotope dating showed that these corals lived about 380 million years ago. Scientist John Wells of Cornell University discovered another method to determine the age of these corals. He made use of the fact that the friction produced by ocean tides gradually slows the earth’s rotation over time. Each day—one revolution of the earth—is a tiny bit longer than the last one. The length of the day increases by about two seconds every 100,000 years. Since the duration of a year—the time it takes the earth to circle the sun—doesn’t change over time, this means that the number of days per year must be decreasing over time.
From the known rate of slowing, Wells calculated that his corals were alive —380 million years ago if the radiometric dating was correct—each year would have contained about 396 days, each 22 hours long. If there were some way that the fossils themselves could tell us how long each day was when they were alive, we could check whether that length matched up with the 22 hours predicted from radiometric dating.
The coral fossils can do this!
As corals grow, they produce both daily and annual growth rings. In the fossil specimens, one can see how many daily rings separate each annual ring, telling us how many days were included in each year when the coral was alive. Counting rings in his coral fossils, Wells calculated that these fossils experienced about 400 days per year, which means that each day was 21.9 hours long. That is only a tiny deviation from the predicted 22 hours. This clever biological calibration gives us additional confidence in the accuracy of radiometric dating.
—Excerpts from Why Evolution is True by Jerry Coyne, professor of Evolution at the University of Chicago, chapter 2.
End of post.