Geologic Dating
Geologists are always interested in the exact age of the rock and mineral samples which they are working with. While there are several different methods of determining a rock's age (such as examining the layer it is found in), the most reliable measure currently available to geologists is measuring the radioactive decay in isotopes found in a rock. This is referred to as "dating" a rock by the half-life of its isotopes. However, this process can be much more difficult than most people think it is.
Isotopes and Radioactive Decay
An isotope is an atom of a chemical element whose nucleus has the same atomic number but which has a different atomic weight. What this means is that an isotope can be thought of as a different version of a standard element on the periodic table. Due to its difference though, isotopes are atomically unstable, and prone to change into other, more stable elements. This process is referred to as radioactive decay. The rate at which radioactive decay occurs is that particular isotope's half-life. Just what all of that means is simple--once you get past the scientific terminology.
Measurement
The half-life of an isotope is how long it will take for half of the atoms present to decay and form another, more stable element. For instance, if there are 100 atoms of an isotope with a half-life of 20 years, then in 20 years there will be 50 atoms of it left. In another 20 years, there will be 25 atoms of it, and so on. This is an important process for geologists. If they find an isotope in a rock, along with all the atoms of the element the isotope decays into, then they can figure out how much of the original isotope has decayed. If they also know the rate of that decay, or the half-life, then they can determine how long this process took, and thus figure out the age of the rock.
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