Desert varnish is the thin dark coating found on exposed rock surfaces in arid regions. Desert varnish is composed of clay minerals, oxides and hydroxides of manganese and iron, along with other trace elements. The predominante elements are Manganese and Iron. Now let's see what desert varnish looks like.
Desert varnish on sandstone walls of Canyon de Chelly National Monument in Arizona
Canyon walls are ancient sandstone formed when this region was a desert much like the Sahara Desert is today. So far I have only recognized desert varnish on sandstone canyon walls. I suppose it is also on other hard rock surfaces I just do not recognize it there.
The dark streaks on this sandstone wall are desert varnish.
Desert Varnish on sandstone cliffs Canyon de Chelly National Monument
Note the desert varnish on the sandstone wall is the dark streaks comprised of oxides of iron and magnesium.
Desert varnish on sandstone walls deep in Canyon de Chelly National Monument
Desert varnish on sandstone cliffs of Canyon de Chelly
Impressive sandstone walls with dark streaks of desert varnish.
Desert varnish on tall sandstone cliff in Canyon de Chelly National Monument
This canyon wall might be 400-feet tall.
The dark streaks are desert varnish on this sandstone cliff.
Desert varnish on sandstone cliff in Canyon de Chelly National Monument
Desert varnish on sandstone cliffs of Canyon de Chelly National Monument, Arizona
This is a good example of desert varnish (the dark shiny surface on that sandstone is a mixture of manganese and iron oxide).
The color of desert varnish depends on the relative amounts of manganese and iron in it. Manganese-rich varnishes are black, while manganese-poor thus, iron-rich varnishes are red to orange; those intermediate in composition are usually a shade of brown.
Desert varnish surfaces tend to be shiny when the varnish is smooth and rich in manganese.
I would say that this desert varnish is "rich" in manganese.
Desert varnish on sandstone cliff in Canyon de Chelly, Arizona
The dark cap rock has a lot of iron in it. Desert varnish has stained the sandstone wall.
Desert varnish is composed of clays and other extremely small particles cemented to rock surfaces by manganese emplaced and oxidized by bacteria living there. It is produced by the physiological activities of these microscopic bacteria which are able to take manganese out of the environment, then oxidize and emplace it onto rock surfaces.
These unique, manganese-oxidizing bacteria tend to thrive in deserts and appear to fill an environmental niche unfit for faster growing organisms which feed only on organic materials. Some note that while the area has to be arid dew is an important ingredient in the process.
Sources for desert varnish components appear to come from outside the host rock rather than being leached from it. The most likely source is from dust and surface runoff. Streaks of black varnish often occur where water cascades over cliffs.
Desert Varnish on sandstone in Sinks Canyon near Lander, Wyoming
Sinks Canyon exposes over 400-million years of geologic history. Originally formed by ice age glaciers the Middle Pogo Agie River is continuing the erosion process that has shaped and is shaping Sinks Canyon.
The uppermost formation exposed in Sinks Canyon is Tensleep Sandstone. That massive, light colored, wall at the top is the Tensleep Sandstone.
The dark streaks on the face of the Tensleep Sandstone is "desert varnish" created by oxides of iron and magnesium.
Desert Varnish on Tensleep Sandsone wall of Sinks Canyon near Lander, Wyoming
View of Sinks Canyon wall comprised of hard Tensleep Sandstone on top and the softer Amsden Formation on the tree covered slope.
Desert Varnish on Tensleep Sandstone forming the caprock of Sinks Canyon near Lander, Wyoming
Thousands of years are required to form a coat of manganese-rich desert varnish so it is usually found on hard rock surfaces such as this sandstone canyon wall, because it would disappear on easily eroded surfaces.
Mike & Joyce Hendrix
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