No volcanoes in Alberta, so where does the Obsidian come from?

When working close to an obsidian source (i.e. volcanoes), archaeologist will regularely find obsidian tools and debitage. However, in the boreal forests of northern Alberta, obsidian is a rare find indeed. So to find any evidence of it at all is pretty significant. Volcanic glass, or obsidian, is one of the sharpest naturally occurring materials on earth. It is sometimes used by surgeons who require a cutting edge that is much thinner and sharper than a stainless steel scalpel. Not only is obsidian extremely sharp, but it is also relatively easy to flint knap and is used by many beginners trying to learn this ancient skill. These features made obsidian a highly valued knapping material.

Pre-contact peoples fashioned various tool types from it, such as knives, projectile points, or composite tools. One method of knapping obsidian was blade core technology. This method involves preparing an obsidian core so that a series of small blades can be removed. This method minimizes the amount of material that is wasted and allowed people to quickly reproduce blades that were a similar size and shape. These blades were then hafted to a bone or stick to create a cutting utensil. To create larger tools, obsidian would have been knapped using normal methods as well.

Obsidian blade and core
Photo credit: B. Bernard

Last summer, Tim and I were surveying some forestry cutblocks at the base of the southern slopes of the Marten Hills, east Slave Lake. During the screening process, I had targeted a series of small knolls that overlooked a muskeg. These knolls were close to the headwaters of the Fawcett River, where a natural pass would have allowed people to easily traverse through the hills. Tim and I walked to the top of the highest knoll and started digging test pits.

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Brian at the GiPk-3 hill top

We were completely shocked when my first test contained an obsidian blade. At this point we were extremely excited, and were sure that we had just found a crazy site. However, even though we dug 40+ test pits, we never found another artifact. The site turned out to be an isolated find. Nuts!

obsidian blade

So if there are no volcanoes in Alberta, where did this obsidian blade come from? Luckily, advances in technology allowed us to quickly solve this question. During his Master’s degree, Tim obtained a license to operate a pXFR analyzer (portable X-Ray Fluorescence), so Tree Time Services Inc. rented one of these devices. Other obsidian artifacts recovered by Tree Time services were all found to come from south of the U.S. border. However, the results of Tim’s analysis suggests that the blade was made of obsidian from Mt. Edziza, British Columbia.

pXRF
Results of pXRF, showing range for various volcanic sources.

That means that this piece of stone had been transported/traded by Pre-contact peoples over a distance of 1700 km! At 10 km travelling distance in a day (on foot), that is 170 days, or almost 6 months. Mt. Edziza obsidian has never been found in the Lesser Slave Lake region, however is is somewhat common in the Peace Region in northwest Alberta. GiPk-3 represents one of the furthest known Mt Edziza obsidian finds from its source. Although Tim and I were disappointed that we didn’t find more artifacts, this single find provided evidence for trade networks that spanned vast distances long before European contact.

Edziza to GiPk-3
Image Credit: Todd Kristensen

Sourcing with pXRF (portable X-Ray Fluorescence)

“Sourcing” is the study of associating artifacts with their geologic origin in order to infer human transport of materials. This field of research has revealed networks of trade and exchange among indigenous peoples in pre-contact times. But how do researchers figure out the actual source?

One method is with Portable X-Ray Fluorescence (pXRF) analysis. These instruments are used by geologists, archaeologists, and other specialists to gather chemical data of materials in a non-destructive manner. When the results are compared to known sample locations, archaeologists can infer how far the material an artifact was made from has traveled from its geological source. Take obsidian, for example. Obsidian has been found at archaeological sites thousands of kilometers from any volcano. Since there are several volcanoes in North America, identifying which artifact comes from which source affects the interpretation of trade networks.

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A pXRF analyzer

The analyzers themselves are a very intricate piece of technology that use radiation to gather chemical data of samples. The two main components are the X-Ray tube and the detector. Radiation is emitted down the X-Ray tube towards the sample object and interacts with its surface atoms. This interaction consists of radiation knocking an electron from the atom out of place, creating a vacancy in the electron cloud. This vacancy is filled by other electrons in the atom. The electrons shed energy in the form of X-Rays as they fill that vacancy, which is emitted out and collected by the detector.

The spacing between the orbital shells of electrons in the atom is unique to that element. An atom of potassium for instance, has different spacing between the electron shells than gold or iron. So, when an electron fills the vacancy and emits X-Rays, the emitted X-Rays are equivalent to the distance that is unique to that element. The detector collects these x-rays which are diagnostic of the elements present in the sample. Internal software then calibrates the readings into proportions of all elements in the sample.

How XRF works
Simplified diagram of an XRF. Left, an X-Ray tube emits radiation onto a sample, which interacts with the surface. The emitted X-Ray data is collected by a detector. Right, The radiation ejects an inner electron from the atom. This vacancy is filled by a 2nd (a) or 3rd (b) layer electron. This movement of the electrons emits X-Rays.

pXRF analyzers are useful tools for archaeologists since we study culturally significant objects, and do not want to alter or destroy the artifacts we wish to analyze. They are also relatively easy to use. Although users require a certification, they are safe when operated correctly, and the data can be downloaded and interpreted quickly.

A recent study of 750 obsidian artifacts has showed that indigenous trading networks spanned thousands of kilometers. Archaeological sites in Alberta contain obsidian which originated from north, central, and coastal British Columbia, Wyoming, Idaho, and Oregon, spanning a trade network almost 2,000 km across.