Desert varnish: nature’s dark rock lacquer

Transcript

On bare rock surfaces that stay stable for long periods, desert varnish builds a thin coating that can turn stone orange, red, brown, or black, and even give it a shiny luster. The coating is a mixture of clay, iron, and manganese oxides, with material arriving from windblown dust, flowing water, and microorganisms on the rock surface. Over thousands of years, these processes help form dark varnish, and people have long chipped through it to make petroglyphs that reveal the lighter rock beneath.