Dating obsidian arrowheads

The decision to use it as a relative or absolute dating method depends upon whether the environmental conditions (eg.

soil temperature and soil relative humidity) of the archaeological site are known. Obsidian hydration dating is based on the fact that a fresh surface is created on a piece of obsidian in the tool manufacturing, or flintknapping, process. When a piece of obsidian is fractured, atmospheric water is attracted to the surface and begins to diffuse into the glass.

A distinct diffusion front can be recognized by an abrupt change in refractive index at the inner edge of the hydration rind.

These fronts or rinds of hydration are more dense than the unhydrated inside, and the unhydrated zone has different optical properties.

Obsidian, or volcanic glass, is formed by the rapid cooling of silica-rich lava.

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.

cracking of a lava flow on cooling, manufacture of an obsidian artifact, or glacial abrasion of an obsidian pebble).

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.

Hydration Profile: Hydration begins after any event which exposes a fresh surface (e.g.

This results in 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. The thickness of the hydration rind can be identified in petrographic thin sections cut normal to the surface and observed under a microscope.


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