Obsidian Tells Many Stories


by Linda Lennon

The rocks in Yellowstone National Park tell the story of Yellowstone’s geologic past. Archeologists are also able to use some types of rock in their studies of the earliest inhabitants of North America. The Obsidian Cliffs, located seven miles north of Norris Junction, are a rock formation that is able to speak to two groups of "historians," geologists and archeologists.

The geological story of the Obsidian Cliffs is one of a volcanic eruption, approximately 180,000 years ago. This eruption occurred quite a while after the catastrophic eruption and subsequent collapse of the Caldera. The composition of obsidian is approximately the same as granite, but its appearance is much different than that of granite. This is because in granite, the melted rock cools very slowly, and crystals of quartz, feldspar, and biotite are able to grow to a size that is visible to the naked eye. In obsidian, there are no visible crystals; in fact, there are no crystals at all.
Obsidian Cliff in Yellowstone Park.

The explanation for this is that the lava cools so quickly that mineral crystals cannot grow. The water content of the lava also plays a part. Dry lava solidifies more rapidly than lava which has a high water content. Either way, we have a rock with no crystals. Such a substance is called isotropic, which means that it has the same molecular structure in all directions. Glass is another isotropic substance. This quality allows obsidian to fracture in a very predictable, curved manner when it is impacted.Early Native Americans were able to fashion extremely sharp objects by flaking the edges of obsidian until a suitable cutting or hunting tool was formed.
Obsidian is volcanic glass.
Archeologists are also interested in these ancient obsidian artifacts. Certain tribes used recognizable knapping patterns. Wherever these identifiable obsidian tools are found, the tribe was present. Ancient tribal migration patterns can be established in this manner. By using geochemical evidence found in samples of obsidian, the source of the specific tool can be ascertained. In this way, obsidian from Yellowstone National Park has been traced to the Hopewell Mounds, east of the Mississippi River, and to Guatemala and Mexico.
  Making spear points from obsidian.

The time of tool manufacture can now be estimated using hydration dating. Obsidian, when fractured, begins to adsorb atmospheric water. This water accumulates at a known rate, as a hydration rim, which can be seen and measured. This method, known as hydration dating, has given archeologists a new tool to assist them in their investigations of the early inhabitants of North and Central America. By dating the origin of the obsidian artifact, a chronological framework can be assigned to the movement of the tribe. The rocks of the Obsidian Cliffs are bilingual. They can teach us about not only Earth’s processes, but also about the migration, trading, hunting, and rituals of the earliest inhabitants of North America. 
Hafting an obsidian point to a spear.  


Continue to the next page, Yellowstone Volcano Observatory.


Byram, Scott. 1995. Methodological Notes on the Use of Obsidian Hydration Data, in Current Archaeological Happenings in Oregon 20 (4): 6-14 
Cannon, Kenneth P., and Hughes, Richard E., 1993, Obsidian Source Characterization of Paleoindian Projectile Points from Yellowstone National Park, Wyoming. Current Research in the Pleistocene 10: 54-56 
Cannon, Kenneth P., and Hughes, Richard E., 1997, Provenance Analysis of Obsidian Paleoindian Projectile Points from Yellowstone National Park, Wyoming. Current Research in the Pleistocene 14: 101-104 (1997) 
Perkins, Dexter, 1998, Minerology, published by Prentice-Hall in Upper Saddle River, New Jersey

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