Friday Rock Blogging: Desert Varnish
If you kick a dark pebble in the middle of the desert, you will sometimes find that it is light underneath. What this means is that you have disturbed a pebble that has been sitting there untouched for thousands of years. During that time, it accumulated a thin coating of windblown gunk – mainly clay dust, and manganese and iron oxides – known as desert varnish.
Desert varnish has a complex internal structure; there are thin sections below the fold.
These entirely gratuitous micrographs are from a paper in the latest issue of Geology. Three scientists at Arizona State took a careful look at some sections of desert varnish picked up in the Sonoran desert, and find that most of the things that happen to large-scale sediments also happen to desert varnish: chemicals are dissolved at one place and precipitated at another, cracks heal, microbes might eat the delicious manganese.
This all makes it difficult to give a precise age for a varnished surface.
This is a false color image of the distribution of iron and manganese in a desert varnish sample. Red is iron, green is manganese, and darker areas are mostly silicates (clay). The segregation of iron and manganese indicates that something wacky is going on.
Laurence A.J. Garvie, Donald M. Burt, and Peter R. Buseck, 2008, Nanometer-scale complexity, growth, and diagenesis in desert varnish, Geology 36:3 pp.215-218. DOI: 10.1130/G24409A.1.