How does desert varnish form

The sample was prepared by fracturing a thin slab of a varnish-coated rock. The exposed surface filling most of the frame is a fracture through the varnish layer. A portion of the underlying rock surface can be seen in the lower left portion of the image. Note how the varnish smoothly goes around the rounded mineral grain in the bottom center-right side of the image. This is a fungal dendritic growth from sandstone from Meteor Crater, Arizona which also looks like a manganese deposit to the casual observer.

Material on related forms of manganese oxide in nature can be found in the manganese dendrite page. Witch Creek Fire. Santa Rosa Plateau. Joshua Tree. Left: The sun-baked boulders of the Alabama Hills in Owens Valley, California are coated with a black layer of clay and manganese oxide precipitated by colonies of remarkable bacteria living on the rock surface.

Right: The varnish-coated rock has broken away revealing the lighter granitic core. The sun-baked boulders in the Colorado Desert of southern California are coated with a reddish-brown layer of clay and iron oxide precipitated by colonies of bacteria living on the rock surface.

Native Americans once used the varnish surface to carve their elaborate petroglyphs. In the Anza-Borrego Desert of southern California, the sun-baked boulders are devoid of lichens. Instead, the rocks are coated with "desert varnish" a reddish layer of clay and iron oxide precipitated by remarkable bacteria. This large boulder has split apart revealing the lighter granodiorite beneath. Dorn, R. Microbial origin of desert varnish. Science , , — Farr, T.

Rock coatings in Hawaii. Geological Society of America Bulletin , 95 , — Hungate, B. Canadian Journal of Microbiology , 33 , — Jones, C. Characteristics and origin of rock varnish from the hyperarid coastal deserts of Northern Peru. Quaternary Research , 35 , — Kuhlman, K. Diversity of microorganisms within rock varnish in the Whipple Mountains, California. Applied and Environmental Microbiology , 72 , — Liu, T. How fast does rock varnish grow?

Geology , 28 , — Murchie, S. Old desert varnish-like coatings and young breccias at the mars pathfinder landing site. LPSC , 35 , Abstract Perry, R.

Chemistry and Structure of Desert Varnish Desert varnish: evidence for cyclic deposition of manganese. Nature , , — Mars primordial crust: unique sites for investigating proto-biologic properties. Origin of Life and Evolution of Biospheres , 36 5—6 , —