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Absolute Time 1

© Geoff Habiger

In 1945 the United States detonated the first nuclear weapons. Although the events of 1945 ushered in the nuclear age, scientists have known about other properties of uranium, one of the elements used in nuclear reactions. In the 1890’s, Antoine Henri Becquerel made the observation that minerals that were composed of uranium, like uranite, would cause a covered photographic plate to darken. This was an effect previously only associated with x-rays.

From Becquerel’s observations it was further observed that uranium decays spontaneously and gives off energy in the form of particles and electromagnetic energy, or radiation. The radiation given off is in the form of gamma rays. Since the discovery that uranium gives off radiation, other discoveries have been made and today the geologist can use the radioactive decay to determine the age of certain rocks.

Let us first look at how geologists use radioactivity to determine the age of a rock. This involves the discussion of some basic nuclear physics, which I will overview in simple terms.

Every element is made of protons, electrons, and neutrons. For a given element, the number of protons always remains the same, but in some elements the number of neutrons can vary. These variations of the element are known as isotopes. As an example let us look at the element uranium. Uranium has two naturally occurring isotopes, U-235 and U-238. The numbers represent the mass of the two isotopes and are the sums of the protons and neutrons in the uranium atoms. The number of protons does not vary, uranium has 92 protons, and so the variation in mass is from the neutrons, 143 for U-235 and 146 for U-238.

Many isotopes, like the uranium isotopes, are unstable and over time will decay into isotopes with a different mass number, a different number of protons, or both. Through experiments, scientists know that the process of decay proceeds at a constant rate, which is different for each isotope. The time it takes for the isotope to decay so that half of the original isotope has been changed to the daughter isotope is termed the isotopes half-life.

It is this knowledge of the isotopes and their half-lives that allow geologists to determine the age of a rock. When a geologist takes a sample of rock to test, they must first decide which isotope to test for. (The chart shows several different isotopes and their half-lives.) Then, the rock is tested to determine how much of the original isotope is present and how much of the daughter isotope is present. Knowing these numbers, and the rate of decay, the geologist can calculate the age of the rock.

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