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, whereas 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: first order - geomorphic processes control the stability of bedrock surfaces on which coatings form; second order - coatings originating in rock fissures occur on subaerial surfaces when erosion of the overlying rock occurs; third order - the habitability of surfaces for fast-growing lithobionts such as lichens determines whether slowly accreting coatings occur; fourth order - the raw ingredients must have a transport pathway to the rock surface, and of course, they must be present; fifth order - physical, geochemical, or biological barriers to transport then result in the accretion of the coating.