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A Journey to a New Land

Orcas
Secondary Level Resources

Measuring Time

Any scientific study of the past, including the geologic past, relies on the use of dating methods to determine the age of sites, landforms, sediments or geologic events. A variety of dating methods are available, and the scientist must decide which method will provide the most accurate results in each case.

Relative Dating Methods

Archaeological site layers

Relative dating methods can not establish exactly how old things are, but only how old things are relative to other things. Relative dating is based on the law of superposition which states that material found at the bottom of a sedimentary sequence is older than the material above it. Relative dating methods were the first dating techniques to be developed, and are still widely used.

 

Absolute Dating Methods

Absolute dating methods can give an estimate of age in calendar years. There are several absolute dating methods that scientists can use, including radiocarbon dating and potassium argon dating.

Radiocarbon Dating

A 9,500 year-old wooden stick

The radiocarbon dating method was developed in the late 1940's by Dr. Willard F. Libby, who later won a Nobel Prize in chemistry for this work. After its introductionto the scientific community in 1950, it quickly became the method most frequently used to determine the age of organic materials.

This 9,300 year old wooden wedge was found at the Kilgii Gwaay archaeological site.

Radiocarbon dating is based on the fact that a radioactive isotope of carbon(14carbon) that is present in the earth's atmosphere is absorbed into the tissues of all organisms while they are alive. After death, the 14carbon starts to slowly decay at a known rate. The length of time that has passed since death can be calculated by measuring the amount of 14carbon that remains in the organism.

Radiocarbon dating can only be used on organic materials such as bone, wood,charcoal, and shell, and has a limit of about 60,000 - 70,000 years.

 

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 scientists working in areas where volcanic eruptions have occurred, and left layers of ash which can be dated using the potassium argon method.

 
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QuickTime videos:

Dr. Michael Wilson

Dr. Michael Wilson
Geology Department
Douglas College

Dr. Erle Nelson

Dr. Erle Nelson
Department of Archaeology
Simon Fraser University

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