Threshold of Weathering Needed for Fire to Erode Rock Art: Case Study of a Hohokam Petroglyph, Central Arizona, USA

A Sonoran Desert petroglyph panel experienced an intense wildfire event in July 2021 that eroded the entire surface, removing the Hohokam-style rock art. Field observations during sampling in 1995 indicated that the panel: (1) was coated with a heavy rock varnish, (2) had a 'fresh' visual appearance, and (3) had some granite-derived sand (angular grus) at the panel's base. Micron-scale back-scatter and nanoscale transmission electron microscopy of pre-fire samples revealed a minimal amount of decay (granite grussification): mainly minor grain-to-grain separation; minor internal dissolution; and a little feldspar grain cracking. Our basic finding is that even this minimal amount of grussification was enough to set the stage for the wildfire to erode the entire panel. Pre-fire micron-scale cracking may have enabled the fire's steep thermal gradient to spall the surface. Panel erosion was likely enhanced by pre-existing grain-to-grain porosity to facilitate further fire-induced granular disintegration. Pre-fire nanoscale dissolution within mineral grains, formed along crystal defects, provided a weakness that then led to grain cracking of quartz and other granitic minerals. The implication for the conservation of rock art on granitic panels is worrisome, but clear and simple: condition assessments need to indicate whether any granitic sand occurs at a panel's base. Given that many places experiencing climate change are also experiencing drought and enhanced risk from wildfire, the appropriate management recommendation would then be to remove all vegetation near the panel on a regular basis.