Scientific Dates for Prehistoric Times

Radioactive clocks tell time in the millions of years, but how accurately do they tell it?

This article and the two following ones describe and evaluate the different means of radioactive dating used by geologists to measure the ages of rocks and the remains of once-living organisms. They have been prepared by a nuclear physicist of many years’ experience in both research and industry in the field of radioactivity.

“Sinkhole is rich archeological find. 10,000-year-old remains place humans in Ice Age Florida, scientists believe.”

“The oldest Stone Age hut in Japan has been unearthed near Osaka. Archaeologists date the hut to about 22,000 years ago.”

“About a million years ago a river flowed through eastern Corona (California), and mastodons, camels, horses and rabbits were among the prehistoric animals that frequented its banks.”

THE foregoing recent claims are typical of those announcing discoveries by archaeologists and paleontologists. The first thing people want to know about a new find is, How old is it? The scientist who talks to reporters is always ready to give an answer, whether it is based on evidence or it is merely a guess.

When you read such reports, does the question sometimes cross your mind, How do they know? How certain is it that humans lived in Florida 10,000 years ago and in Japan 22,000 years ago, or that mastodons and camels were roaming the California landscape a million years ago?

There are several different scientific methods of dating ancient remains. Some are more reliable than others, but none are as certain as ages based on historical records. But man’s historical records go back only 6,000 years at the most. When we go back beyond this time, the scientific dates are all we have.

Radioactive Dating

Of various methods for scientific dating, the most reliable are the radioactive clocks. They depend on the rates of radioactive decay processes. While other methods depend on aging processes that may go faster or slower under differing environmental conditions, such as changing temperature, radioactive decay rates have been shown to be unaffected by the extremes of external conditions.

The Uranium-Lead Clock

We can illustrate the method with the first radioactive clock devised, the one based on the decay of uranium to lead. Radioactive decay goes strictly according to a law of statistical probability. The amount of uranium decaying in a unit of time is always proportional to the amount left. This results in a curve like that in the drawing (page 19), which shows the amount left after any given time. The time it takes for half the uranium to decay is called its half-life. One half of the remaining half will decay in the next half-life, leaving one quarter of the original amount. After three half-lives, one eighth will be left, and so on. The half-life of uranium is 4.5 billion years.

Since the uranium is transformed into lead, the amount of lead is increasing all the time. The amount accumulated up to any given time is shown by the broken curve. The lead curve is the complement of the uranium curve, so that the total number of lead atoms and uranium atoms is always the same, equal to the number we started with.

Now suppose we have a rock containing uranium but no lead, and we seal it up tight so that nothing can get into or out of the rock. Then, some time later we open it up and measure the amounts of both elements. We can tell from that how long the rock has been sealed. For example, if we find equal amounts of lead and uranium, we know that one half-life, that is, 4.5 billion years, has passed. If we find that just 1 percent of the uranium has decayed to lead, we can use the mathematical formula for the curve to figure that 65 million years have elapsed.

Note that we do not have to know how much uranium was in the rock to start with because all we have to measure is the proportion of lead to uranium at the end of the period​—which is just as well because none of us were around to measure anything at the beginning of the experiment.

Now you may be thinking that these are immense periods of time we are talking about, millions and billions of years. What is the possible use of a clock that runs so slowly? Well, we learn that the earth itself has been in existence for a few billion years, and there are rocks in a few places that appear to have been there for a good part of that time. So geologists find such a clock quite useful in studying the history of the earth.

How Certain Are They?

We must admit that the dating process isn’t quite as simple as we have described it. We mentioned that the rock has to be free from lead at the beginning. This is usually not the case; there is some lead to start with. This gives the rock what is called a built-in age, something more than zero. Also, we assumed that the uranium was tightly sealed in the rock so that nothing could get in or out. Sometimes this may be true but not always. Over long periods of time, some of the lead or the uranium might seep out into groundwater. Or more uranium or lead might get in, especially if it is a sedimentary rock. For this reason, the uranium-lead clock works best on igneous rocks.

Other complications arise from the fact that another element, thorium, which may be in the mineral, is also radioactive and slowly disintegrates into lead. Besides that, uranium has a second isotope​—the same chemically but different in mass—​that decays at a different rate, also forming lead. Each of these ends up in a different isotope of lead, so we need not only a chemist with his test tubes but also a physicist with a special instrument to sort out the various isotopes, leads of different mass.

Without going into detail on these problems, we can understand that the geologists using the uranium-lead clock have to look out for a number of pitfalls if they are to get a reasonably trustworthy answer. They are glad to have other radiometric methods to verify their age measurements. Two others have been developed that can often be used on the same rock.

The Potassium-Argon Clock

The one that has been most widely used is the potassium-argon clock. Potassium is a more common element than uranium​—potassium chloride is sold in grocery stores as a substitute for common salt. It consists mostly of two isotopes with masses 39 and 41, but a third isotope, of mass 40, is weakly radioactive. One of the products of its decay is argon, an inert gas that makes up about 1 percent of the atmosphere. The potassium of mass 40 has a half-life of 1.4 billion years, which makes it suitable in measuring a range of ages from tens of millions up to billions of years.

In contrast with uranium, potassium is widespread in the earth’s crust. It is a constituent of many minerals in the most common rocks, both igneous and sedimentary. Required conditions for the potassium-argon clock to work are the same as explained above: The potassium must be free of argon when the clock is started, that is, when the mineral is formed. And the system must remain sealed for the duration, allowing no potassium or argon to escape or enter.

How well does the clock work in practice? Sometimes very well but at other times poorly. It sometimes gives ages greatly different from those of the uranium-lead clock. Usually, these are smaller; such results are attributed to loss of argon. But in other rocks, the potassium and uranium ages agree very closely.

A most newsworthy use of the potassium-argon clock was in dating a rock that was brought back from the moon by the astronauts of Apollo 15. Using a chip from this rock, scientists measured the potassium and argon and determined the age of the rock to be 3.3 billion years.

The Rubidium-Strontium Clock

Another radioactive clock for minerals has been developed more recently. It is based on the decay of rubidium into strontium. Rubidium decays incredibly slowly. Its half-life is 50 billion years! So little of it has decayed in even the oldest rocks that meticulous measurements are necessary to distinguish the added strontium-87 from the primordial strontium. There may be a hundred times more strontium than rubidium in the mineral, and even in a billion years, only a little more than 1 percent of the rubidium decays. In spite of these difficulties, the minute amount of strontium produced by decay has been measured in a few cases. This clock is valuable for checking the ages found by other methods.

An exciting example of the use of this method was on a meteorite that astronomers believe might be like the rocks that theoretically fell together to form the planets, a remnant of the primordial material from which the solar system was made. The indicated age, 4.6 billion years, was consistent with this view.

An outstanding success of the rubidium-strontium clock was in dating the same moon rock described above. Five different minerals in the rock were tested, and they joined in indicating an age of 3.3 billion years, the same as the potassium-argon age.a

In some cases the comparative ages obtained by these three geological clocks are in close agreement and give confidence that the ages in such cases are very likely correct. It should be emphasized, however, that such cases show what kind of agreement is possible​—but only under ideal conditions. And conditions are usually not ideal. Far longer lists could be given of comparisons that clash with one another.

Paleontologists Try to Date the Fossils

Paleontologists have attempted to copy the geologists’ success in dating rocks only a few million years old. Some of their fossils, they believe, might fall in that age range. Alas, the potassium-argon clock does not work so well for them! Of course, fossils are not found in igneous rocks but only in sediments, and for these radiometric dating is usually not trustworthy.

An illustration of this is when fossils have been buried in a thick fall of volcanic ash that has later been consolidated to form a tuff. This is actually a sedimentary stratum, but it is made of igneous matter that solidified in the air. If it can be dated, it will serve to give the age of the fossil enclosed in it.

Such a case was found in the Olduvai Gorge in Tanzania, where fossils of apelike animals attracted special attention because their finders claimed they were linked to humans. First measurements of argon in the volcanic tuff in which the fossils were found showed an age of 1.75 million years. But later measurements at another qualified laboratory gave results a half million years younger. Most disappointing to evolutionists was the finding that the ages of other layers of tuff, above and below, were not consistent. Sometimes the upper layer had more argon than the one below it. But this is all wrong, geologically speaking​—the upper layer had to be deposited after the lower and should have less argon.

The conclusion was that “inherited argon” was spoiling the measurements. Not all the argon previously formed had been boiled out of the molten rock. The clock had not been set to zero. If only one tenth of 1 percent of the argon previously produced by the potassium was left in the rock when it melted in the volcano, the clock would be started with a built-in age of nearly a million years. As one expert put it: “Some of the dates must be wrong, and if some are wrong maybe all of them are wrong.”

Notwithstanding expert opinions that these dates may be quite meaningless, the original age of 1.75 million years for the Olduvai fossils continues to be quoted in popular magazines committed to evolution. They give the lay reader no warning that such ages are really no more than guesses.

[Footnotes]

[Blurb on page 18]

Geologists using the uranium-lead clock have to look out for a number of pitfalls

[Blurb on page 20]

They give no warning that such ages are no more than guesses

[Graph on page 19]

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The decrease in uranium is directly proportionate to the increase in lead

100%

50%

25%

12.5%

Half-lives 1 2 3

lead (argon)

(potassium) uranium

[Diagram on page 18]

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Uranium

Lead

How much uranium (or lead) did this rock originally have?

How much uranium (or lead) leached into the rock later?

How much lead derived from the decay of thorium?