Discover how scientists determine the age of fossils, rocks, and other geologic phenomena by using the known half-lives of isotopes within each specimen, a technique known as radioactive dating.

Radioactive Dating

Ever wonder how scientists concluded the age of the earth to be about 4.6 billion years old or how geologists determined the ages of caverns, rocks, volcanoes, the Himalayas, or even the age of Pompeii bread? Well, scientists are able to answer all of these wondrous questions and more by use of a process called radiometric, or radioactive, dating.

Radioactive dating enables geologists to record the history of the earth and its events, such as the dinosaur era, within what they call the geologic time scale. Radioactive dating uses the ratios of isotopes and their specific decay products to determine the ages of rocks, fossils, and other substances.

Radioactivity Defined

Elements occur naturally in the earth, and they can tell us a lot about its past. Carbon, uranium, and potassium are just a few examples of elements used in radioactive dating.

Each element is made up of atoms, and within each atom is a central particle called a nucleus. Within the nucleus, we find neutrons and protons; but for now, let’s just focus on the neutrons. These neutrons can become unstable, and when they do, they release energy and undergo decay. Scientists call this behavior radioactivity.

Radioactivity occurs when the nucleus contains an excess amount of neutrons. When an atom varies in the number of neutrons, the variation is called an isotope. Isotopes are unstable forms of elements. During radioactivity, the unstable isotope breaks down and changes into a different substance. A new, more stable isotope, called the decay, or daughter product, takes its place. The isotope doesn’t actually deteriorate; it just changes into something else.

The Half-Life

Isotopes decay at a constant rate known as the half-life. The half-life is the amount of time it takes for half of the atoms of a specific isotope to decay. Remember, isotopes are variations of elements with a different number of neutrons. The half-life is reliable in dating artifacts because it’s not affected by environmental or chemical factors; it does not change.

When scientists find a sample, they measure the amount of the original, or parent, isotope and compare it to the amount of the decay product formed. They then count the number of half-lives passed and compute the absolute age of the sample. Absolute age is just a fancy way of saying definitive or specific age as opposed to the relative age, which only refers to how old or young a substance is in comparison to something else.

To illustrate, let’s use the isotope uranium-238, which has a half-life of 4.5 billion years. This means that after approximately 4.5 billion years, half of an original sample containing this isotope will decay into its decay product, forming the new isotope, Pb 206 (lead 206). If another 4.5 billion years were to pass, then half of the remaining half of uranium-238 would also decay, leaving 25% uranium to 75% lead. If a scientist were to compute this, he or she would say two half-lives went by at a rate of 4.5 billion years per half-life; therefore, the sample is approximately 2 times 4.5 billion, or 9 billion years old. That’s a lot of years.

So you see, earth scientists are able to use the half-lives of isotopes to date materials back to thousands, millions, and even to billions of years old. The half-life is so predictable that it is also referred to as an atomic clock.

Radiocarbon Dating

Since all living things contain carbon, carbon-14 is a common radioisotope used primarily to date items that were once living. Carbon-14 has a half-life of approximately 5,730 years and produces the decay product nitrogen-14. Just as in the example with uranium, scientists are able to determine the age of a sample by using the ratios of the daughter product compared to the parent.

Also, when dating with carbon-14, scientists compare the amount of carbon-14 to carbon-12. These are both isotopes of the element carbon present in a constant ratio while an organism is living; however, once an organism dies, the ratio of carbon-14 decreases as the isotope deteriorates. Radiocarbon dating can only be used to date items back to as far as about 50,000 years old. Radiocarbon dating was used to identify a forged painting based upon the concentrations of carbon-14 detected on the canvas within the atmosphere at the time that the picture was painted.

Lesson Summary

So, to sum this all up, radioactive dating is the process scientists use to conclude the ages of substances dating back several to many years ago by using the isotopes of elements and their half-lives. An isotope is a variation of an element based upon the number of neutrons. The disintegration of the neutrons within the atom of the element’s nucleus is what scientists call radioactivity.

An isotope disintegrates at a constant rate called the half-life, or the time it takes for half the atoms of a sample to decay. The half-life can also be termed an atomic clock. By counting the number of half-lives and the percentages remaining of parent and daughter isotopes, scientists are able to determine what they call the absolute age of a discovery. Carbon-14 is a specific isotope used in dating materials that were once living. Other common isotopes used in radioactive dating are uranium, potassium, and iodine.