Radiometric uranium-238, which decays to lead-206, and

Radiometric dating is used to estimate the age of rocks and other objects based on the fixed decay rate of radioactive isotopes. Learn about half-life and how it is used in different dating methods, such as uranium-lead dating and radiocarbon dating, in this video lesson.

Radiometric Dating

The aging process in human beings is easy to see. As we age, our hair turns gray, our skin wrinkles and our gait slows. However, rocks and other objects in nature do not give off such obvious clues about how long they have been around.

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So, we rely on radiometric dating to calculate their ages. Radiometric dating, or radioactive dating as it is sometimes called, is a method used to date rocks and other objects based on the known decay rate of radioactive isotopes. Different methods of radiometric dating can be used to estimate the age of a variety of natural and even man-made materials.

Radioactive Decay

The methods work because radioactive elements are unstable, and they are always trying to move to a more stable state. So, they do this by giving off radiation.

This process by which an unstable atomic nucleus loses energy by releasing radiation is called radioactive decay. The thing that makes this decay process so valuable for determining the age of an object is that each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life. So, if you know the radioactive isotope found in a substance and the isotope’s half-life, you can calculate the age of the substance.

Half-Life

So, what exactly is this thing called a half-life? Well, a simple explanation is that it is the time required for a quantity to fall to half of its starting value. So, you might say that the ‘full-life’ of a radioactive isotope ends when it has given off all of its radiation and reaches a point of being non-radioactive. When the isotope is halfway to that point, it has reached its half-life.

Uranium-Lead Dating

There are different methods of radiometric dating that will vary due to the type of material that is being dated. For example, uranium-lead dating can be used to find the age of a uranium-containing mineral. It works because we know the fixed radioactive decay rates of uranium-238, which decays to lead-206, and for uranium-235, which decays to lead-207. So, we start out with two isotopes of uranium that are unstable and radioactive. They release radiation until they eventually become stable isotopes of lead.

These two uranium isotopes decay at different rates. In other words, they have different half-lives. The half-life of the uranium-238 to lead-206 is 4.47 billion years. The uranium-235 to lead-207 decay series is marked by a half-life of 704 million years. These differing rates of decay help make uranium-lead dating one of the most reliable methods of radiometric dating because they provide two different decay clocks. This provides a built-in cross-check to more accurately determine the age of the sample.

Potassium-Argon and Rubidium-Strontium Dating

Uranium is not the only isotope that can be used to date rocks; we do see additional methods of radiometric dating based on the decay of different isotopes. For example, with potassium-argon dating, we can tell the age of materials that contain potassium because we know that potassium-40 decays into argon-40 with a half-life of 1.3 billion years. With rubidium-strontium dating, we see that rubidium-87 decays into strontium-87 with a half-life of 50 billion years.

By anyone’s standards, 50 billion years is a long time. In fact, this form of dating has been used to date the age of rocks brought back to Earth from the moon.

Radiocarbon Dating

So, we see there are a number of different methods for dating rocks and other non-living things, but what if our sample is organic in nature? For example, how do we know that the Iceman, whose frozen body was chipped out of glacial ice in 1991, is 5,300 years old?Well, we know this because samples of his bones and hair and even his grass boots and leather belongings were subjected to radiocarbon dating. Radiocarbon dating, also known as carbon-14 dating or simply carbon dating, is a method used to determine the age of organic material by measuring the radioactivity of its carbon content. So, radiocarbon dating can be used to find the age of things that were once alive, like the Iceman. And this would also include things like trees and plants, which give us paper and cloth. So, radiocarbon dating is also useful for determining the age of relics, such the Dead Sea Scrolls and the Shroud of Turin.

With radiocarbon dating, the amount of the radioactive isotope carbon-14 is measured. Compared to some of the other radioactive isotopes we have discussed, carbon-14’s half-life of 5,730 years is considerably shorter, as it decays into nitrogen-14. Carbon-14 is continually being created in the atmosphere due to the action of cosmic rays on nitrogen in the air.Carbon-14 combines with oxygen to create carbon dioxide.

Because plants use carbon dioxide for photosynthesis, this isotope ends up inside the plant, and because animals eat plants, they get some as well. When a plant or an animal dies, it stops taking in carbon-14. The existing carbon-14 within the organism starts to decay back into nitrogen, and this starts our clock for radiocarbon dating. A scientist can take a sample of an organic material when it is discovered and evaluate the proportion of carbon-14 left in the relic to determine its age.

Lesson Summary

Let’s review. Radiometric dating is a method used to date rocks and other objects based on the known decay rate of radioactive isotopes. The decay rate is referring to radioactive decay, which is the process by which an unstable atomic nucleus loses energy by releasing radiation.

Each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life or, in other words, the time required for a quantity to fall to half of its starting value.There are different methods of radiometric dating. Uranium-lead dating can be used to find the age of a uranium-containing mineral. Uranium-238 decays to lead-206, and uranium-235 decays to lead-207. The two uranium isotopes decay at different rates, and this helps make uranium-lead dating one of the most reliable methods because it provides a built-in cross-check.Additional methods of radiometric dating, such as potassium-argon dating and rubidium-strontium dating, exist based on the decay of those isotopes.

Radiocarbon dating is a method used to determine the age of organic material by measuring the radioactivity of its carbon content. With radiocarbon dating, we see that carbon-14 decays to nitrogen-14 and has a half-life of 5,730 years.

Learning Outcomes

As a result of watching this video, you might be able to:

  • Compare radiometric dating, radioactive decay and half-life
  • Understand that uranium-lead dating is one of the most reliable radiometric dating methods
  • Relate the processes of potassium-argon and rubidium-strontium dating
  • Determine how radiocarbon dating works and recognize why it is important
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