Terrestrial Rock Coatings

Fourteen different types of coatings cover rock surfaces in every terrestrial weathering environment, altering the appearance of the underlying landform. Some accretions interdigitate, while others blend together, creating a great number of variations. Rock coatings are important in geomorphology because coatings: alter weathering rates; play a role in case hardening surfaces; offer clues to understanding environmental change; and can provide chronometric insight into the exposure of the underlying rock surface. Following a landscape geochemistry paradigm, five general hierarchies of control explain the occurrence of different types of rock coatings: 1st order—geomorphic processes control the stability of bedrock surfaces on which coatings form; 2nd order—coatings originating in rock fissures are found on subaerial surfaces when erosion of the overlying rock occurs; 3rd order—the habitability of surfaces for fast-growing lithobionts such as lichens determines whether slowly accreting coatings occur; 4th order—the raw ingredients must have a transport pathway to the rock surface, and of course, they must be present; 5th order—physical, geochemical or biological barriers to transport then result in the accretion of the coating.