OK, varnished furniture, varnished wooden floors, but varnished rocks? Strange, but many desert rocks are indeed varnished with a paper-thin layer of dark material on their surfaces. A layer that, in some cases, is thousands of years old. Chip an exposed rock—if the rock beneath is notably lighter, you're seeing rock varnish.
What it is was long a mystery. That the mineral manganese was largely
responsible for the blackish coloration was apparent, and oxides of iron are always
present. The original thought was that minerals within the rock were dissolved and
deposited on the surface. One problem—varnish occurs on rocks otherwise lacking
manganese or iron. Today, part of the puzzle is solved. About 70% of the varnish is
clay, with manganese and iron for the rest—except for bacteria that seem to be very
much involved. Apparently airborne particles are trapped by the bacteria that live
under and within the varnish, using manganese as a substitute for oxygen--and protected
from the ultra-violet rays of the sun by the manganese oxides: the original sun
screen!
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.