As such the chemical composition of the varnish is different from that of the rocks on which they form and thus the components, especially manganese, of the varnish must be derived from air-borne dust and rainwater.
Rock varnish “grows” on rock surfaces rather than simply staining the rock surface. There is still much to learn about rock varnish but it is clear that rock vanish comes in many different forms differentiated usually by their chemical makeup.
You might start to wonder just how old those stains are on those rocks.
Rock varnish is much more than just chemicals that leached out of material above and ran down than surface of a rock leaving a stain.
Dark layers in varnish are rich in Mn and Ba, but poor in Si and Al.
Orange and yellow layers are poor in Mn and Ba, rich in Si and Al.
The 87Sr/86Sr analyses of fines and clasts in South Mountain debris flows of different ages reveal that desert dust supplies the fines.At least some forms of varnish appear to contain an organic component as a result of bacteria that live on or near the surface of the rock.What the exact role of the bacteria plays in the growth of rock varnish is still not clear.Two major precipitation events in the summer of 2014 generated 35 debris flows in the same study area of South Mountain—providing support for the importance of probability analysis as a key step in a hazards analysis in warm desert settings.Two distinct mechanisms generated the 2014 debris flows: intense precipitation on steep slopes in the first storm; and a firehose effect whereby runoff from the second storm was funneled rapidly by cleaned-out debris-flow chutes to remobilize Pleistocene debris-flow deposits.