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USGS CMG InfoBank: Desert Varnish

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Comment: 16:42 - 18:16 (01:34)
Source: Annenberg/CPB Resources - Earth Revealed - 22. Wind, Dust and Deserts
Keywords: "Richard Hazlett", desert, "desert varnish", "manganese oxide", clay, weathering, evaporation, precipitation, "acid rain", "chemical weathering", rainfall, microbe, "biological activity"

Our transcription: Many outcrops, as well as desert stones, are also covered with "desert varnish," a thin shiny coating on the rocks that increases with age.
The varnish is composed of dark manganese oxide and clays.
One explanation for the formation of this feature involves weathering, evaporation, and precipitation.
This varnish forms over rock surfaces throughout the desert as the result of acid weathering, chemical weathering during periods of rainfall or heavy moisture, winter moisture.
Obviously, this water can't travel very far before it evaporates because of the dry desert conditions.
So it precipitates a residue of dissolved mineral constituents as a thin film across the rocky surface over which it's flowing -- hence the buildup in time of these manganese oxides.
Wind-blown clay grains adhering to rock surfaces may assist varnish formation by soaking up moisture from adjoining soil.
Microbes could also play a role in producing varnish through complex biological processes.
Desert varnish can be 2,000 years in the making providing a writing surface for rock inscriptions from ancient cultures.

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