Age dating desert varnish
So, it was once assumed to be an absolute dating method.
The thickness of varnish has been used to ‘date’ geomorphological features, such as glacial moraines, debris flows, and overturned boulders.
Upper Left: A vertical face at Canyon de Chelly, Arizona, is streaked with heavy metal skins, iron films, lithobiont coatings, oxalate crusts, rock varnish, and silica glaze.
Upper Right: Lava flows in the arid regions of Mauna Loa show a distinct color change within decades as a direct result of accumulation of silica glaze.
They calculated the time to form desert varnish in a wide variety of arid and semi-arid locations in the south-west United States.
On desert varnishes of various thicknesses, they arrived at dates of 1.5 to 250 thousand years with formation rates varying from 0.6 to 40 microns per thousand years.
The relationship between ventral rubification, rock size, and age shown in this study suggest that stable land surface environments (i.e., stable landforms and large surface rocks) create conditions needed for strongly expressed varnish.
Liu and Broecker concluded that desert varnish can be a relative age indicator.
Desert varnish plays an important role in archeology.
Many petroglyphs are created by chipping through a dark coat of desert varnish to expose a lighter colored underlying rock.
The researchers also ‘discovered’ that rock varnish accumulation rates vary greatly from sample to sample even at one site, which would reinforce the fact that desert varnish is an unreliable dating method.
Recently, desert varnish has been discovered to form much more rapidly than the slow rates given by Liu and Broecker, as well as many others.