I am leaving for The Amazing Meeting this morning (I speak at 11:00 on Sunday, if you’re attending), but I had dinner with Bill Nye last night (who is a keynote speaker at TAM). We got to talking about his victory over Ken Ham in the debate last February 4. The week before the debate, both the NCSE and Michael Shermer and I had helped coach him on what to expect and how to approach the event. During our session at Shermer’s house, Bill was especially interested in having me explain how radiometric dating worked, and how to handle creationist lies about it. In the debate, Bill did the smart thing and give example after example of clear-cut things that demonstrated the earth was older than 6000 years: tree rings and the oldest trees, ice cores that have 680,000 continuous seasonal bands recorded in them, objects that are hundreds to millions of light-years away, etc. However, he was stumped when Ham dragged out a long-debunked creationist trope about a volcanic lava in Australia dated at 45 m.y. which surrounded trees which were radiocarbon dated at 40,000 years old. In such a short coaching session, it was not possible to explain to him about every creationist distortion of radiometric dating. Luckily, the example slipped past the attention of the audience quickly, and made no difference in the long run (nor did any specific point—as I said before, the debate was about perception and winning “hearts and minds”).
The basic principle of radiometric dating is simple and very well understood. Certain isotopes of elements, such as potassium-40, rubidium-87, uranium-238, and uranium-235, spontaneously break down or “decay” into atoms of different “daughter” elements (argon-40, strontium-87, lead-206, and lead-207, respectively, for the “parent” material just listed) by emitting nuclear radiation (alpha and beta particles and gamma radiation) plus heat. The rate of this radioactive decay is well known for all these elements, and has been checked and double-checked in the laboratory hundreds of times. Geologists obtain a fresh sample of the rock, break it down into its component mineral crystals, and then measure the ratio of parent atoms to daughter atoms within the mineral. That ratio is a direct mathematical function of the age of the crystal. This site gives an excellent summary.
Like any other technique in science, there are limitations and pitfalls that have to be avoided. Because dating is a measure of the time since a crystal cooled and locked in the radioactive parent atoms, it only works in rocks that cool down from a molten state, or igneous rocks (such as granites or lava rocks). Creationists brag about how we cannot directly date the crystals in a sandstone or any other sedimentary rock (the crystals are recycled from older rocks, and have no bearing on the age of the sediment). But geologists long ago circumvented this problem by finding hundreds of places all over earth where datable volcanic lava flows or ash falls are interbedded with fossiliferous sediment, or where intruding granitic magma bodies cut across the sedimentary rocks and provide a minimum age. From settings such as these, the numerical ages of the geological time scale are derived, and their precision is now so well resolved that we know of the age of most events that are millions of years old to the nearest 100,000 years.
If the crystal structure has somehow leaked some of its parent or daughter atoms, or allowed new atoms in to contaminate the crystal, the parent/daughter ratio is disturbed and the date is meaningless. But geologists are always on the lookout for this problem, running dozens of samples to determine whether their age is reliable, and cross-checking their dates against other sources of determining the age. The newest techniques and machinery are so precise that a skilled geochronologist can spot the error in almost any date, and quickly reject those that don’t meet very high standards. Creationists will mention a specific date that proved to be wrong as evidence that the entire field of geochronology is unreliable, when in fact it was the geologists themselves who spotted the erroneous date, and quickly rejected it. As Dalrymple (2004) points out, it’s as if we had a variety of watches and clocks, a few of which don’t keep accurate time. But that fact doesn’t mean we completely ignore clocks and watches altogether, as creationists are doing by rejecting ALL radiometric dating out of hand. We simply keep checking them against each other to determine which ones are reliable and which ones are not.
In another commonly repeated claim, creationists mock geologists over the example of a living clam that gave a radiocarbon date of many thousands of years old. But this is a well-understood anomaly. Normal radiocarbon dating works when nitrogen-14 from the atmosphere is transformed by cosmic radiation into carbon-14, which is then incorporated into living tissues. When the organism dies, the carbon-14 begins to decay, making the bone or shell or wood (or anything bearing carbon) datable, as long as it is less than 40,000 years old (since the radiocarbon decay rate is relatively fast). These peculiar clams live in water covering ancient limestone that releases radioactively dead carbon into the water. That ancient carbon (instead of the normal carbon derived from the atmosphere) is then incorporated into the mollusk shells, where it throws the ratio off. Radiocarbon specialists have long been aware of this minor problem, and never rely on dates where this kind of contamination could be an issue.
Going back to the point raised in the debate, why did the Australian anomaly occur? The thing to remember is that each radioactive “clock” ticks at a different rate, but all of them keep good time (if they are used properly). Radiocarbon has a very short half-life of 5370 years, so it decays extremely fast. By 40,000 years (some labs can push it to 80,000 years now), it is radiocarbon dead—no more decay is occurring, and you cannot use it to measure anything any more. Thus, radiocarbon is primarily used by scientists who work on really young events of the last Ice Age: archeologists and those who work on the last glacial cycle. A real scientist would never even consider using it for anything older, and any fool who does so shows that he has no clue what he’s doing. Other isotopic systems are useful in different age ranges. U-Pb (both isotopic pairs) and Rb-Sr decays over billions of years, so it is only used on the oldest earth rocks, plus moon rocks and meteorites. K-Ar is the system used by most geologists, since it can date rocks as young as 1 m.y., and as old as the oldest rocks we have, so its useful age range covers the vast majority of common geologic settings. To go back to our clock analogy, radiocarbon is like a clock that ticks really fast and runs down quickly. U-Pb and Rb-Sr are like a big grandfather clock that ticks very slowly but doesn’t run down except over a very long time.
As this site nicely explains, the Australian trees in lava scenario only demonstrates the complete incompetence of creationists. They used K-Ar to date the lava at 45 m.y., which means that no real geologist would waste their time dating the radiocarbon-dead trees. But the idiot creationist ran the radiocarbon on them anyway, and sure enough, he got dates around 40,000—which only means the sample is radiocarbon dead and older than 40,000, not that the age of sample is 40,000 years. It’s like looking at the time at a fast-running clock that has stopped and comparing it to the slow grandfather clock. The creationist says that since they are different, NO clocks can be trusted, when in fact you were foolishly looking at a clock that has run down and stopped.
Creationists don’t give scientists any credit for being skeptical and self-critical about their own data. But anyone who deals with geochronology knows that the dates are subject to constant scrutiny by multiple labs, and anything that is fishy is quickly challenged and rejected. The result in an extremely robust set of data, where multiple independent radioactive atomic systems (for example, potassium-argon, uranium-lead, and rubidium-strontium) are used on the same samples, so if any one of them is giving problems, it clearly can be thrown out. The creationists point to one or two examples of supposedly unreliable dates, but when three or more independent dating methods are run in different competing labs on the same rock and give the same answer, there is no chance that this is accident! After nearly a century of analyses, with thousands of dates checked and rechecked like this, geologists are as confident about the reliability of radiometric dating as they are about any other field of science. The earth is about 4.6 billion years old; this is as much a fact as the observation that it is round!
Dalrymple, Brent. 1991. The Age of the Earth. Stanford University Press, Stanford, California.
Dalrymple, Brent. 2004. Ancient Earth, Ancient Skies: The Age of Earth and Its Cosmic Surroundings. Stanford University Press, Stanford, California.