Scientists use relative and

*radiometric dating* techniques to determine a fossil’s age.

Without relative and radiometric dating techniques, no one would know whether dinosaurs lived 5,000 or 150,000,000 years ago. Geologists, paleontologists and anthropologists use these techniques to establish the age of fossils. By learning how relative and radiometric dating methods work, you can better understand how science uncovers the history of our world.

## Relative Dating

**Relative dating** methods came before radiometric, and are less precise. According to the 2009 edition of “Essentials of Physical Anthropology,” relative **dating methods** rely on the comparison of two or more things to establish a relative age. For example, if two fossils are found at the same site, but at different depths, scientists assume that the fossil found at the greater depth is older. These methods do not give fossils an exact age. Instead, they establish a relative timeline.

## Stratigraphic Dating

One of the main methods of relative dating, stratigraphic dating uses the depth at which something is found to determine its relative age. Stratigraphic dating is based on the tendency of lower strata (layers) of rock to be older than higher strata. The deeper something is found, the older it generally is. Stratigraphic dating can be problematic, however. Volcanic activity, earthquakes and the unpredictability of rock formation and accumulation can make the chronology established by this method inaccurate.

## Fluorine Analysis

Bone can also be dated through fluorine analysis, another popular relative dating technique. Groundwater typically contains fluorine, which accumulates within bones in the earth as they fossilize. Comparing the fluorine content of two or more fossilized bones found at the same site can therefore show which are older and which are more recent. Fluorine analysis is limited by the fact that it can only be used to ascertain the relative ages of bones from the same site.

## Radiometric Dating

Radiometric dating was developed later than relative dating, and requires more advanced technology. Radiometric dating methods rely on the principles of radioactive decay, and work by measuring the decay of certain radioactive isotopes within fossils and other samples. Over time, these isotopes decay and form other elements. This happens at predictable rates, which makes radiometric dating a more accurate and reliable indicator of age than relative dating.

## K/Ar Dating

One of the major radiometric dating techniques used today measures the decay of potassium-40. Using established knowledge of potassium-40’s half-life, which is the rate at which it decays and changes into argon-40, scientists can date rock material that is millions of years old. In fact, K/Ar dating has been used to date the age of the earth itself. K/Ar dating only works on inorganic material, however. It cannot date bone.

## Carbon-14 Dating

To date organic as well as inorganic material, scientists often use the carbon-14, or radiocarbon, dating technique. Like K/Ar dating, radiocarbon dating relies on the measurement of the decay of one isotope into another. Radiocarbon dating uses the decay of carbon-14 to determine age. Carbon-14 decays at a significantly faster rate than potassium-40, which makes radiocarbon dating more appropriate for later material, such as Neanderthal and human fossils. Its accuracy drops significantly on samples more than 40,000 years old.

## Maximizing Accuracy

Relative and *radiometric dating techniques* all have their disadvantages. The degrees of error in each method can make establishing chronologies tricky. In most cases, geologists, paleontologists and anthropologists cross-correlate the results of multiple dating methods for more accurate and tested conclusions. This is an important point to keep in mind, since the results of just one dating technique will almost always be less accurate than a conclusion based on multiple dating techniques.