Obsidian Hydration Dating Calculator

Estimate the age of obsidian artifacts using hydration rim width and diffusion constants.

Last reviewed: June 2026
Built and maintained by · Last updated June 2026
microns
feet
µm²/yr
Estimated Age
2,500 years
Approximate creation date
Effective Temperature
16.71 °C
Approximate Calendar Date
474 BC
Important Disclaimer: This calculator is an educational model for obsidian hydration dating. Real artifact dating requires laboratory rim measurement, verified obsidian source chemistry, a locally appropriate hydration rate, temperature history, and archaeological context. Do not use this page as a legal, cultural-resource, or laboratory dating determination.

How the Obsidian Hydration Dating Calculator works

Obsidian hydration dating estimates how long a fresh obsidian surface has been exposed by measuring the hydration rim that forms as water diffuses into volcanic glass. In archaeological use, the rim is measured under a microscope on a prepared sample, then interpreted with a hydration rate for the obsidian source and site conditions.

This calculator uses a simplified relationship: age = rim width squared / hydration rate. The page treats the hydration rate as square micrometers per 1000 years, so it multiplies the result by 1000 to show years. It also estimates an effective temperature from elevation with a simple lapse-rate model, but the age calculation itself depends on the rim width and selected rate.

What the inputs mean

  • Hydration rim width: the measured thickness of the hydrated layer, in microns. Small measurement differences can produce large age differences because the rim width is squared.
  • Site altitude: used here only to estimate a rough effective temperature context. Real projects need better local temperature history.
  • Diffusion constant K: the hydration rate. It should match the obsidian source, region, and temperature assumptions, not a generic artifact type.

Worked example

A 5 micron rim with K = 10 gives 5 squared / 10 x 1000, or about 2,500 years. If the measured rim were 6 microns with the same K, the estimate would become 3,600 years. That sensitivity is why careful rim measurement and a defensible rate matter more than the calculator arithmetic.

What can make a result unreliable

Obsidian hydration dates are sensitive to glass composition, surface history, temperature, humidity, and the rate calibration used. Heating, reuse of an older artifact, erosion of the surface, contamination, or a rate from the wrong geochemical source can move the result far from the real archaeological age. The calculated calendar date should be treated as a model output unless it is backed by lab work and site context.

Sources and limitations

This page follows the general method described in USGS obsidian hydration dating references and USGS discussion of source composition and exposure-temperature history in obsidian hydration dating of volcanic events. It is an educational calculator, not a substitute for professional archaeological dating.

Frequently Asked Questions

Obsidian hydration dating measures the hydrated rim that forms after a fresh obsidian surface is exposed. Water diffuses into the volcanic glass over time, and the rim thickness can be related to age when the source, rate, and temperature history are known.
The calculator uses a simplified age = rim width squared / hydration rate relationship, then multiplies by 1000 because the page treats the selected rate as micrometers squared per 1000 years.
The page estimates an effective temperature from elevation with a simple lapse-rate model. That estimate is only a rough context value; real obsidian hydration work uses site-specific temperature history or validated effective hydration temperature methods.
No. Useful dating requires a measurable hydration rim, a known geochemical source, a suitable hydration rate for that source and site, and archaeological context. Weathering, heating, reuse, or an incorrect rate can make the estimate unreliable.