Radioactive elements are unstable; they breakdown spontaneously into more stable atoms over time, a process known as radioactive decay. Radioactive decay occurs at a constant rate, specific to each radioactive isotope. Since the 1950s, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks.

Radiometric clocks are "set" when each rock forms. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools off. It's the resetting process that gives us the ability to date rocks that formed at different times in earth history.


Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials. Geologists have used this method to date rocks as much as 4 billion years old. It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40) ,decays to the gas Argon as Argon-40 (Ar-40). By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.

Ar 40 is used for several reasons. First of all, Argon is inert. It does not chemically react with other elements at all. So Argon does not attach itself to the rock or any minerals in the rock. Secondly, Argon is usually a gas. These features are thought to allow any naturally occurring Argon from contaminating our measurements of the Argon 40 that is being produced from the radioactive decay of K 40.
When volcanic material flows over the land, the naturally occurring Argon gas is driven off by the excess heat. When the rock is molten hot, it is more liquid in texture, allowing the Argon gas to escape. If all the gas is driven off, then there should be no Argon left in the rock.


Once the rock cools and hardens, it is considered to be a closed system, because any new Ar 40 that is produced by the breakdown of K40 is trapped inside the rock crystal and cannot get out. So the scientist assumes that he or she is able to measure only that Ar 40 which is produced from K 40 since the rock has cooled. All the other Ar 40 was forced out of the rock by the heat.

By forcing out the naturally occurring Ar 40, the clock of the dating mechanism is reset or set to zero. Later, when we start discussing the K-Ar dating technique from a Creationary perspective, we will see that this reseting of the clock is a major issue. The clock might not always be reset by the heat in the Rock. There are other factors which might not allow the Argon to coming out of the rock as well.

The Potassium-Argon process does not date the age of the rock. What it does, is to tell you how long ago the rock was reset, or set to zero. In addition, some rocks may have been reheated so that the clock was partially reset or fully reset at a later date. So if there are multiple heatings of the rock, the K-Ar dating process may give the researcher a number that is not what the researcher expects to find.

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(Credit: Mike Brown)

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Carbon-14 is a method used for young (less than 50,000 year old) sedimentary rocks. This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things. When living things die, they stop taking in carbon-14, and the radioactive clock is "set"! Any dead material incorporated with sedimentary deposits is a possible candidate for carbon-14 dating.

Radiometric dating has been used to determine the ages of the Earth, Moon, meteorites, ages of fossils, including early man, timing of glaciations, ages of mineral deposits, recurrence rates of earthquakes and volcanic eruptions, the history of reversals of Earth’s magnetic field, and many of other geological events and processes.

Because the radioactive half-life of a given radioisotope is not affected by temperature, physical or chemical state, or any other influence of the environment outside the nucleus save direct particle interactions with the nucleus, then radioactive samples continue to decay at a predictable rate. If determinations or reasonable estimates of the original composition of a radioactive sample can be made, then the amounts of the radioisotopes present can provide a measurement of the time elapsed.

Perhaps the two most widely known uses of carbon-14 dating was for the Turin Shroud (claimed by many to be the shroud used to wrap the body of Jesus) and also some ancient scrolls found in a cave near the Dead Sea.

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The bones of early Homo sapiens, known as Omo I, from Ethiopia's Kibish rock formation.They were found in 1967 and were thought to be 130,000 years old. Later, 160,000-year-old bones of our species were found elsewhere. Now, a new study and use of modern technology determined that Omo I lived about 195,000 years ago -- the oldest known bones of the human species. (Credit: John Fleagle, Stony Brook University)