Obsidian Flake
Copyright: Melanie Magdalena

Volcanic glass is known as obsidian. Humans in the past often used obsidian as a raw material when making chipped stone tools like arrowheads and spear points. Obsidian is formed by the rapid cooling of silica rich lava. Its precise chemical composition often varies from one outcrop to another, but it always contains seventy percent silica by weight.

Geologists Irving Friedman and Robert Smith, investigated obsidian's potential as a time marker. They introduced the obsidian hydration dating method to the archaeological community in 1960. It may be used in two ways: as a relative dating method to determine if one artifact is older or younger than another, or as an absolute dating method where a calendar date (AD/BC) is produced. The decision to use it as a relative or absolute dating method depends upon whether the environmental conditions of the archaeological site are known.

How does obsidian hydration dating work? Obsidian contains about 0.2 percent water. When a piece of obsidian is chipped and exposed to atmospheric water, it attracts the water to the surface and begins to diffuse into the glass. The result is the formation of a water rich hydration rind that increases in depth with time. The hydration process continues until the fresh obsidian surface contains about 3.5 percent water, this is the saturation point. The thickness of the hydration rind can be identified in petrographic thin sections cut normal to the surface and observed under a microscope. Friedman and Smith reasoned that the degree of hydration observed on an obsidian artifact could tell archaeologists how long it had been since that surface was created by a flintknapper.

There are some limitations to dating using obsidian hydration. Hydration forms at different rates on different obsidians. The rate of hydration can differ throughout the world. Soil temperature and relative soil humidity need to be well estimated. It is best to use a sample with known age and similar composition to the sample being dated.

Another limitation is that an artifact, for example, a tool that was used to skin a deer is used and then discarded. Several hundred years later, a second individual finds the tool and sharpens it and uses the refurbished tool for a completely different use - like to shave bark of a tree. When the archaeologists finds it in a site and sends it to be dated, instead of dating the surface that was used to shave bark, the surface that was used to skin deer is dated, giving an erroneous date for the site it was in.

References:

"Obsidian Hydration Dating." Accessed on 25 January 2012.

About the Author

I'm Jose Pierre and I like learning about all aspects of culture, both ancient and modern. I enjoy learning how they communicated, expressed themselves, and their technology.

Dating Techniques by Jose-Estrada and Melanie Magdalena

There are many dating techniques used in the scientific world. Archaeologists, geologists, and paleontologists use dating techniques to determine the age and time frames for civilizations, geologic patterns, and organisms. There are two main types: relative and absolute. Relative dating is abstract while absolute dating requires lab tests... (and money!)

RELATIVE DATING
In simple English, relative dating is using geologic record to find out just how old a rock or artifact is by using the surrounding context. These are not calendar dates, basically what is closer to the surface is younger than what is under the surface. Archaeologists use several techniques to develop a chronological sequence to order styles, types, and assemblages. Geologists also use different relative dating techniques to identify different rock layers; paleontologists date fossils using geologic record.

Relative Dating Techniques:

• The Law of Superposition or Steno’s law is the principle that each bed of sedimentary rock is older than the layers above and younger than the layers below. This concept of rock layers is also known as stratigraphy. An example of stratigraphy is the Arenosa shelter in Texas.
• Index Fossil Concept is the idea that similar fossils are of similar age or limited to a time span.
• Time-Markers are artifacts that are proven to be from a particular period of time.
• Seriation is a change of style, usually in artifacts. as new technologies arise they replace the older over time. Seriation does not tell us the age of a site, it only tells us if a site is older or younger than another based on the style of artifacts.

Arenosa Shelter, Lower Pecos Canyonland

Index fossils and seriation

ABSOLUTE DATING

Absolute dating “provides calendar reference to dates” (Shafer 2007). There are multiple techniques for absolute dating, some can be used for multiple types of samples while others are specialized.

Absolute Dating Techniques:

• Dendrochronology, or tree ring dating, uses the annual growth rings in trees to assign a calender age to ancient wood. Tree-ring dating was developed by astronomer A. E. Douglass based on the principle that moisture varies from year to year - this moisture ratio is preserved in trees making trees an effective climate change map. One ring is developed each year, the color intensity (light/dark) can distinguish if it was formed in the spring/summer (light ring), or in the summer/fall (dark ring). By combining dendrochronology samples with radiocarbon dates, tree rings have revealed atmospheric changes and the changes in Carbon-14 quantities. A mathematical formula is applies to radiocarbon dates to make up for these fluxes.
• Radiocarbon Dating is only good for organic remains that are younger than about 45,000 years old. Willard Libby developed this technique based on the fact every living thing absorbs the radioactive Carbon 14 (C-14) isotope. C-14 is produced in the atmosphere by cosmic rays and every living thing breathes in Carbon from the atmosphere. When an organism is alive it maintains equilibrium of ^14C. When it dies its C-14 count begins to decay. The amount of C-14 halves every 5,730 years (half-life) after the death of the organism. Carbon-14 is commonly measured in two ways:

1. Conventional Radiocarbon Dating uses a Geiger counter and measures the beta particle emission from a sample. The slower the emission rate, the older the sample. This method is destructive and requires a twenty-five gram sample at the very least.
2. Accelerator Mass Spectrometry (AMS) is a method of Radiocarbon dating that directly counts the proportion of carbon isotopes in samples of one gram or less. Compared to conventional radiocarbon dating, AMS requires less material and is also non-destructive (meaning the sample can be kept after the dating test).

• Trapped Charge Dating is based on the fact electrons become trapped in mineral crystal lattices due to background radiation. The total amount of radiation the specimen received is divided by the annual dose of radiation. There are three sub-dating techniques for trapped charge dating:

1. Thermoluminescence (TL) is used to date ceramics and burned stone artifacts. TL measures the total radiation dose of a artifact by heating the specimen to 500 degrees centigrade. the trapped electrons in quarts or feldspar crystals move back to their orbits, releasing energy in the form of light. the amount of light released gives the needed measurement of total radiation dose, which is then divided by the annual dose of radiation.
2. Optically Simulated Luminescence (OSL) is used to date dirt by determining last time it was exposed to sunlight. It is measured by passing a light of particular wavelength over the sample, the trapped electrons go into orbit again emitting their own light; the intensity of the light reveals the total radiation measurement. soil samples must be collected carefully and cannot be exposed to sunlight.
3. Electron Spin Resonance (ESR) is used to date tooth enamel. tooth enamel contains hydroxyapatite, which does not trap charges when formed. Once the tooth is deposited in the ground, it starts to a accumulate background radiation. The tooth is exposed to electron magnetic radiation and the amount that it absorbs is proportional to it total radiation dose.  

• Argon-Argon Dating is used to date volcanic ashes to when they they were erupted that are between 500,000  and several million years old. Argon-Argon dating measures the ratio of argon-39 and argon-40 in volcanic ashes. This dating technique only requires a small sample.

Tree rings

An example of Radiocarbon dating

So now the mystery of dating techniques has been solved. Different types of dating are used for different situations and different types of finds. Every scientist has their own favorite, but now you can go out and make rough estimates for your finds!

References:
Kelly, Robert L. and David Hurst Thomas. (2010) Archaeology (5th ed.). Belmont, CA: Wadsworth/Cengage Learning.
Shafer, Harry J. (2007) Archaeology 101. Texas Archaeological Academy.