Obsidian Hydration - An Inexpensive, but Problematic Dating Technique

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.

Fast Facts: Obsidian Hydration Dating

  • Obsidian Hydration Dating (OHD) is a scientific dating technique using the unique geochemical nature of volcanic glasses. 
  • The method relies on the measured and predictable growth of a rind that forms on the glass when first exposed to the atmosphere. 
  • Issues are that rind growth is dependent on three factors: ambient temperature, water vapor pressure, and the chemistry of the volcanic glass itself. 
  • Recent improvements in measurement and analytical advances in water absorption promise to resolve some of the issues. 

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 absorbed 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 one can easily determine if a particular artifact is older than another (relative age). If the rate at which water diffuses into the glass for that particular chunk of obsidian is known (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.

Defining the Constant

Obsidian from Montgomery Pass, Nevada
Obsidian, natural volcanic glass exhibiting rind, Montgomery Pass, Mineral County, Nevada. John Cancalosi / Oxford Scientific / Getty Images

It's nearly a sure bet that everybody who ever made stone tools and knew about obsidian and where to find it, used it: as a glass, it breaks in predictable ways and creates supremely sharp edges. Making stone tools out of raw 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.

The local 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 changes in altitudes, from the sea level coastal regions to the 4,000-meter (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.

Water Structure Research

Methodology to adjust the calibrations for the variability in climate is an emergent technology in the 21st century. New methods critically evaluate the depth profiles of hydrogen on the hydrated surfaces using secondary ion mass spectrometry (SIMS) or Fourier transform infrared spectroscopy. The internal structure of the water content in obsidian has been identified as a highly influential variable which controls the rate of water diffusion at ambient temperature. It has also been found that such structures, like water content, vary within the recognized quarry sources.  

Coupled with a more precise measuring methodology, the technique has the potential to increase the reliability of OHD, and provide a window into the evaluation of local climatic conditions, in particular paleo-temperature regimes. 

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 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.

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