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A Journey to a New Land

Eagle and Raven

Measuring Time

One of the most important things an archaeologist has to do is to figure out the age of an artifact or site. There are several ways to do this, and the archaeologist must decide which method is most practical and appropriate in each case.

Relative Dating Methods

Archaeological site layers

Relative dating methods do not tell archaeologists exactly how old things are, but only how old things are relative to each other. Archaeologists work on the principle that objects at the bottom of an undisturbed archaeological site were put there before objects that are above them, so objects found in the lower levels of a site are usually older than objects found in higher levels. This method can give archaeologists an indication of the age of the artifacts in all excavations where the deposits are layered (or stratified) and have not been disturbed. In some excavations, it is the only method the archaeologist can use.

 

Absolute Dating Methods

Absolute dating methods can give an estimate of the real calendar age of an artifact or site. There are several absolute dating methods that archaeologists can use, including radiocarbon dating and potassium argon dating.

Radiocarbon Dating

A 9,500 year-old wooden stick

This is the most common dating method used in archaeology. It is based on the fact that during life all plants and animals absorb a type of carbon known as 14carbon. After death, the 14carbon starts to slowly decay. Scientists know the rate at which 14carbon decays, so they can measure the amount left in an object and then calculate the amount of time that has passed since death. Radiocarbon dating can only be used on organic materials such as wood or bone, and is limited to objects less than about 50,000 years old.

A 9,500 year-old wooden stick wrapped in cordage from the Kilgii Gwaay archaeological site:

 

Potassium Argon Dating

Archaeological site

Potassium argon dating is an absolute dating technique that can be used on volcanic rocks. Radioactive potassium (40potassium) decays into argon over time, so the age of certain rocks or minerals can be discovered by measuring the amount of argon they contain. This method is useful for archaeologists working in areas where volcanic eruptions have left layers of ash above and below an archaeological deposit. The volcanic layers can be dated, and the archaeological material will date to the period between those two volcanic eruptions.

It is important to remember that this method give the age of the mineral, not the artifact. So we can't pick up an artifact that's made from volcanic rock and get a potassium argon date on the artifact. We wouldn''t be dating the artifact – we'd be dating the rock and the rock might have formed millions of years earlier.