Archaeology/Anthropology: Obsidian Dating

ARCHAEOLOGY/ANTHROPOLOGY: OBSIDIAN DATING

Stratigraphy and Seriation

Archaeologists and anthropologists have always had difficulty trying to determine how old something is. At first, they used a technique called stratigraphy, based on the assumption that older things tend to be buried under newer things. A technique called seriation is a little more sophisticated. It assumes things get more complex over time. Radiocarbon dating was developed in the 1940s, but its use is limited to material that has been alive at some time.

Thermoluminescence

Two new techniques to date old, nonliving material were developed in 1960: thermoluminescence and obsidian dating. Thermoluminescence can be used accurately to date rocks and pottery as far back as one thousand years. All rocks and pottery contain small amounts of radioactive elements. Over time the radioactive elements decay, giving off electrons in the process. These electrons get trapped within solid material. When a piece of rock or pot is heated to about 350 degrees Celsius, the trapped electrons are freed, and the material gives off a tiny bit of light. By detecting how much light is given off, the age of the material under study can be determined. This technique was developed by George C. Kennedy and Leon Knopoff at the University of California Institute of Geophysics in Los Angeles.

How Obsidian Ages

Obsidian dating was based on work by geologists and archaeologists working with the Smithsonian Institute, including Irving Friedman, Robert L. Smith, Clifford Evans, Betty J. Maggers, and Donovan L. Clark. Obsidian is volcanic glass. It is usually black and is found all over the world. People pick it up and make tools out of it by scraping its surface to form a desired shape. Once this is done, as water gets on the surface, it gradually seeps down through the material. As a result, old obsidian has an outer rind which contains water. The inner parts are dry. The thickness of this rind can be accurately measured under a microscope. The thicker the rind, the longer it has been since a person made the obsidian into a tool.

Weather and the Reliability of Obsidian Dating

Obsidian dating is not foolproof. Weather conditions can affect how quickly water seeps into the obsidian. If one obsidian tool were from the Arctic and another from the tropics, different tables would be used to determine their ages even if their rinds were equally thick. Also, different types of obsidian have different rates at which water diffuses into them. Obsidian dating is inexpensive, quick, and accurate up to about a million years old.

Sources:

"Heating Rocks Gives Age," Science News Letter, 77 (16 January 1960): 35;

"Volcanic Glass Moisture Helps Date Old Objects," Science News Letter, 77 (12 March 1960): 169.