Panels of Desert Varnish
Desert varnish forms several patterns. In some places, entire canyon walls are colored deep reddish brown or black. In other places, desert varnish forms vertical stripes alternating between black and red and tan in grand displays, hundreds of feet tall and many hundreds of feet along cliff. On more complex surfaces, such as the edge of a hole in the roof of an alcove, desert varnish follows the intricate pattern of water streaming over rock.
Desert varnish is most common on walls facing north or east and less common on walls facing south and west, suggesting that temperature has an influence. Patterns of desert varnish clearly suggest that water seeping down walls and into alcoves is a major determinant for the formation of varnish. Varnish forms more commonly on north-facing walls because water quickly evaporates from walls in warm, direct sunlight.
Desert varnish is a thin veneer of clay, minerals and microbes that forms on the surface of rock. It accumulates slowly, at a rate of 1 to 40 micrometers or less than the width of a human hair in 1,000 years. The minerals are oxidized forms of manganese and iron, while the microbes include a substantial diversity of bacteria and some fungi.
The relative proportions of manganese and iron produce the range of colors. A predominance of manganese oxide produces black varnish while a predominance of iron oxide makes the varnish red. Similar concentrations of manganese and iron oxides produce tans and browns.
Desert varnish forms when water, clay, microbes and minerals interact over long periods of time. Water seeping over rock encourages microbes and fungi to grow and allows windblown clay dust to adhere. Manganese and iron arrive in water-soluble form, but manganese and iron oxidizing bacteria and fungi change the minerals to insoluble forms, cementing the clay and minerals to the rock surface and concentrating the minerals. Black varnish has 50 times the concentration of manganese in nearby soils.
Labyrinth Canyon Wilderness, Utah
Panels of Desert Varnish
Desert varnish forms several patterns. In some places, entire canyon walls are colored deep reddish brown or black. In other places, desert varnish forms vertical stripes alternating between black and red and tan in grand displays, hundreds of feet tall and many hundreds of feet along cliff. On more complex surfaces, such as the edge of a hole in the roof of an alcove, desert varnish follows the intricate pattern of water streaming over rock.
Desert varnish is most common on walls facing north or east and less common on walls facing south and west, suggesting that temperature has an influence. Patterns of desert varnish clearly suggest that water seeping down walls and into alcoves is a major determinant for the formation of varnish. Varnish forms more commonly on north-facing walls because water quickly evaporates from walls in warm, direct sunlight.
Desert varnish is a thin veneer of clay, minerals and microbes that forms on the surface of rock. It accumulates slowly, at a rate of 1 to 40 micrometers or less than the width of a human hair in 1,000 years. The minerals are oxidized forms of manganese and iron, while the microbes include a substantial diversity of bacteria and some fungi.
The relative proportions of manganese and iron produce the range of colors. A predominance of manganese oxide produces black varnish while a predominance of iron oxide makes the varnish red. Similar concentrations of manganese and iron oxides produce tans and browns.
Desert varnish forms when water, clay, microbes and minerals interact over long periods of time. Water seeping over rock encourages microbes and fungi to grow and allows windblown clay dust to adhere. Manganese and iron arrive in water-soluble form, but manganese and iron oxidizing bacteria and fungi change the minerals to insoluble forms, cementing the clay and minerals to the rock surface and concentrating the minerals. Black varnish has 50 times the concentration of manganese in nearby soils.
Labyrinth Canyon Wilderness, Utah