New research shows that the black coating on sandstone known as rock varnish or desert varnish can form much more rapidly than previously thought.
The abstract of a paper in Geology1 states:
Rock varnish is a thin dark coating best known from deserts, and is believed to grow extremely slowly. Varnish samples from near Socorro, New Mexico (United States), contain as much as 3.7% PbO, derived from nearby smelters operating from A.D. 1870 to 1931. Additional varnish, measuring as much as 4 μm beyond the Pb-rich layer, indicates continued growth from 1931 to 2003. Comparison with other varnish confirms that the Pb is not an artifact. Based on Pb layer thickness, and the period of smelter operation, these very young rock varnishes yield growth rates of 28–639 μm/k.y., substantially higher than previously documented fastest rates of 40 μm/k.y. These rates imply that the average 1–2 μm/k.y. rate for older varnish is not the active growth rate. Rather, it is a long-term value including periods of nondeposition, erosion, and active growth. Therefore, models of rock varnish formation should be reevaluated with consideration of much faster maximum growth rates.
The new maximum growth rate is nearly 16 times the old estimate.
1. Spilde, Melim, Northup and Boston, “Anthropogenic lead as a tracer of rock varnish growth: Implications for rates of formation,” Geology, published online January 4, 2013, doi: 10.1130/G33514.1 v. 41 no. 2 p. 263–266.
Interpretive signs about desert varnish appear in national parks throughout desert parks in the United States and probably elsewhere. How many of them are going to be updated as a result of this revelation? Probably few. They will continue to tell unwary visitors that it’s a slow, slow, slow process. As this paper shows, not necessarily. Significant buildup could occur in just a few thousand years. Even so, does any scientist possess the wherewithal and know-how to understand all the variables? That this paper shows up in 2013 after decades of research on desert varnish should cause perceptive readers to see, once again, that human “scientific knowledge” is limited and subject to change without warning.