 by Ned Rozell October 25, 2003
Obsidian is volcanic magma that cooled too fast to mineralize; instead it turned into glass that's black as a raven. This rapid volcanic cooling--going from molten rock to glass in just a few hours--leaves behind a field of obsidian fragments. People in the southwest call the smallest of these fragments "Apache tears." Larger bulbs of obsidian were the real prizes. Prehistoric people broke them apart and shaped them into tools with a sharp cutting edge. Fields of obsidian were a major discovery for ancient people, as they are today for archeologists. For thirty years, John Cook, an archeologist retired from the Bureau of Land Management, has found obsidian tools and weapons throughout Alaska. A few years ago, Cook stumbled upon a field of obsidian on the Indian River near Hughes. By analyzing the rock's chemical composition, researchers found that ancient people who lived from 10,000 years to 2,000 years ago traded Indian River obsidian throughout Alaska. Archeologists still find the rock at sites as far north as Barrow, as far east as the Yukon Territory, and as far south as Homer. While the Indian River obsidian field was a major source of tools and weapons for ancient Alaskans, there are other places in Alaska, such as along the upper Tanana River, where the obsidian tools had a different chemical signature. Scientists bombarded rock samples with x-rays until atoms within the rock revealed themselves. The researchers then found the upper Tanana obsidian had a different fingerprint than the obsidian at the Indian River field. Cook was on a quest to find the source of the upper Tanana obsidian so he could learn more about the people who used it. At the Indian River site, for example, Cook saw dozens of flaking stations, spots where early Alaskans chipped tools from rock. Paul Layer, head of the geology and geophysics department at the University of Alaska Fairbanks, knew a laser beam could help Cook find the origin of the upper Tanana obsidian samples. Layer is also director of the Geophysical Institute's geochronology lab, where researchers can find the age of a rock by melting a tiny crystal with a laser beam until the rock releases argon gas. A vacuum sucks the argon through an instrument called a mass spectrometer, which allows scientists to tell when a rock formed or when forces within the earth reheated it. By using the laser, Layer determined the obsidian at the upper Tanana archeological sites formed just after a volcano erupted 20 million years ago. Because the obsidian fragments from the Indian River field are 40 million years old, Layer's work proved the Indian River site isn't the source of the upper Tanana artifacts. After reading scientific papers about the dates of eruptions in the Wrangell-St. Elias mountains, Layer learned that the eruptions get older the farther east one goes in the mountains. Mt. Sanford and Mt. Drum have all erupted in recent times, for example, while mountains closer to the Canada border haven't erupted in millions of years. When he read about a spot where eruptions happened 20 million years ago--near the headwaters of the Nabesna and White rivers--Layer told Cook. Cook searched the area with
a USGS geologist and other archeologists from BLM and the National
Park Service. Near Ptarmigan Lake, they found a motherlode of
obsidian nodules. Cook picked up some samples and sent them to
Layer, who confirmed their age and chemical composition to be
the same as the obsidian from the upper Tanana archeological
sites.
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