Geography
Why Desert Varnish Forms on Stable Rock Surfaces
Quick fact
Desert varnish is so thin—often thinner than a human hair—that it takes thousands to tens of thousands of years to form a barely visible coating, and it only thrives on rock surfaces that have been stable for millennia.
Why this is interesting
You've seen those shiny, dark patches on desert rocks, like someone painted them with a thin layer of bronze. But how does a rock get a patina that looks metallic without any human help?