Obsidian Hydration - An Inexpensive, but Problematic Dating Technique

Obsidian Hydration: A Cheap Way to Date Stone Tool Making -- Except...

Obsidian Outcrop at the San Andreas Fault, California
Obsidian outcrop near the San Andreas fault on Red Hill, a Salton Butte volcano near Calipatria, California. David McNew / Getty Images News / Getty Images

Obsidian hydration dating (or OHD) is a scientific dating technique, which uses the understanding of the geochemical nature of the volcanic glass (a silicate) called obsidian to provide both relative and absolute dates on artifacts. Obsidian outcrops all over the world, and was preferentially used by stone tool makers because it is very easy to work with, it is very sharp when broken, and it comes in a variety of vivid colors, black, orange, red, green and clear.

How and Why Obsidian Hydration Dating Works

Obsidian contains water trapped in it during its formation. In its natural state, it has a thick rind formed by the diffusion of the water into the atmosphere when it first cooled--the technical term is "hydrated layer". When a fresh surface of obsidian is exposed to the atmosphere, as when it is broken to make a stone tool, more water is released and the rind begins to grow again. That new rind is visible and can be measured under high-power magnification (40-80x).

Prehistoric rinds can vary from less than 1 micron (µm) to more than 50 µm, depending on the length of time of exposure. By measuring the thickness you can easily determine if one artifact is older than another (relative age). If you can determine the rate at which water diffuses into the glass for that particular chunk of obsidian (that's the tricky part), you can use OHD to determine the absolute age of objects.

The relationship is disarmingly simple: Age = DX2, where Age is in years, D is a constant and X is the hydration rind thickness in microns.

The Tricky Part

It's nearly a sure bet that everybody who ever made stone tools and knew about obsidian and where to find it, used it. Making stone tools out of obsidian breaks the rind and starts the obsidian clock counting.

The measurement of rind growth since the break can be done with a piece of equipment that probably already exists in most laboratories. It does sound perfect doesn't it?

The problem is, the constant (that sneaky D up there) has to combine at least three other factors that are known to affect the rate of rind growth: temperature, water vapor pressure and glass chemistry.

Temperature fluctuates daily, seasonally and over longer time scales in every region on the planet. Archaeologists recognize this and started creating an Effective Hydration Temperature (EHT) model to track and account for the effects of temperature on hydration, as a function of annual mean temperature, annual temperature range and diurnal temperature range. Sometimes scholars add in a depth correction factor to account for the temperature of buried artifacts, assuming the underground conditions are significantly different than surface ones--but the effects haven't been researched too much as of yet.

Water Vapor and Chemistry

The effects of variation in water vapor pressure in the climate where an obsidian artifact has been found have not been studied as intensively as the effects of temperature. In general, water vapor varies with elevation, so you can typically assume that water vapor is constant within a site or region.

But OHD is troublesome in regions like the Andes mountains of South America, where people brought their obsidian artifacts across enormous ranges in altitudes, from the sea level coastal regions to the 4,000 meters (12,000 foot) high mountains and higher.

Even more difficult to account for is differential glass chemistry in obsidians. Some obsidians hydrate faster than others, even within the exact same depositional environment. You can source obsidian (that is, identify the natural outcrop where a piece of obsidian was found), and so you can correct for that variation by measuring the rates in the source and using those to create source-specific hydration curves. But, since the amount of water within obsidian can vary even within obsidian nodules from a single source, that content can significantly affect age estimates.

Obsidian History

Obsidian's measurable rate of rind growth has been recognized since the 1960s. In 1966, geologists Irving Friedman, Robert L. Smith and William D. Long published the first study, the results of an experimental hydration of obsidian from the Valles Mountains of New Mexico.

Since that time, significant advancement in the recognized impacts of water vapor, temperature and glass chemistry has been undertaken, identifying and accounting for much of the variation, creating higher resolution techniques to measure the rind and define the diffusion profile, and invent and improved new models for EFH and studies on the mechanism of diffusion. Despite its limitations, obsidian hydration dates are far less expensive than radiocarbon, and it is a standard dating practice in many regions of the world today.

Sources

This article is a part of the About.com guide to the Scientific Dating Methods, and the Dictionary of Archaeology.

Eerkens JW, Vaughn KJ, Carpenter TR, Conlee CA, Linares Grados M, and Schreiber K. 2008. Obsidian hydration dating on the South Coast of Peru. Journal of Archaeological Science 35(8):2231-2239.

Friedman I, Smith RL, and Long WD. 1966. The hydration of natural glass and the formation of perlite. Geological Society of American Bulletin 77(323-328).

Liritzis I, Diakostamatiou M, Stevenson C, Novak S, and Abdelrehim I. 2004. Dating of hydrated obsidian surfaces by SIMS-SS. Journal of Radioanalytical and Nuclear Chemistry 261(1):51–60.

Liritzis I, and Laskaris N. 2011. Fifty years of obsidian hydration dating in archaeology. Journal of Non-Crystalline Solids 357(10):2011-2023.

Michels JW, Tsong IST, and Nelson CM. 1983. Obsidian Dating and East African Archeology. Science 219(4583):361-366.

Nakazawa Y. 2015 The significance of obsidian hydration dating in assessing the integrity of Holocene midden, Hokkaido, northern Japan. Quaternary International in press.

Ridings R. 1996. Where in the world does obsidian hydration dating work? American Antiquity 61(1):136-148.

Rogers AK, and Duke D. 2014. Unreliability of the induced obsidian hydration method with abbreviated hot-soak protocols. Journal of Archaeological Science 52:428-435.

Stevenson CM, and Novak SW. 2011. Obsidian hydration dating by infrared spectroscopy: method and calibration. Journal of Archaeological Science 38(7):1716-1726.

Tripcevich N, Eerkens JW, and Carpenter TR. 2012. Obsidian hydration at high elevation: Archaic quarrying at the Chivay source, southern Peru. Journal of Archaeological Science 39(5):1360-1367.