• Scientific Dates for Prehistoric Times
    Awake!—1986 | September 22

    • 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.

  • Scientific Dates for Prehistoric Times
    Awake!—1986 | September 22

    • 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.”