The Effects of Fire on Rock Art: Microscopic Evidence Reveals the Importance of Weathering Rinds

This paper presents results of the first study of pre-fire and post-fire samples collected from rock engravings and adjacent sandstone joint faces. A 2001 wildfire at Whoopup Canyon, Wyoming, stimulated a comparison of 1991 and 2003 samples. Optical microscopy of ultra-thin sections, backscattered electron microscopy, x-ray (energy dispersive and wavelength dispersive) analysis of cross sections, and high-resolution transmission electron microscopy reveal that fires create some thermal fractures that enhance panel erosion, but most of the fire-induced erosion occurs along weathering rinds that form long before petroglyph manufacturing. In addition, rock varnish on top of petroglyphs experiences spalling, and fire ash with a clear potassium spike strongly adheres to rock varnish on petroglyphs and spalled sandstone. In the past, site managers assumed minimal damage away from massive spalls and other macrodamage on fire affected petroglyphs, an assumption no longer tenable. Since it is difficult to protect rock art after a fire starts, mitigation efforts can include identification of areas of intense weathering-rind development as locales most susceptible to erosion, and clearing trees and shrubs near rock art by hand.