Weathering of rocks induced by lichen colonization - a review

The evidence presented by numerous investigations of the interface between lichens and their rock substrates strongly suggests that the weathering of minerals can be accelerated by the growth of at least some lichen species. The effects of lichens on their mineral substrates can be attributed to bot h physical and chemical processes. The physical effects are reflected by th e mechanical disruption of rocks caused by hyphal penetration, expansion an d contraction of lichen thallus, swelling action of the organic and inorgan ic salts originating from lichen activity, Lichens also have significant im pact in the chemical weathering of rocks by the excretion of various organi c acids, particularly oxalic acid, which can effectively dissolve minerals and chelate metallic cations, As a result of the weathering induced by lich ens, many rock-forming minerals exhibit extensive surface corrosion. The pr ecipitation of poorly ordered iron oxides and amorphous alumino-silica gels , the neoformation of crystalline metal oxalates and secondary clay mineral s have been frequently identified in a variety of rocks colonized by lichen s in nature. For a better understanding of the impacts of lichens on enviro nments, further work on the comprehensive involvement of the lichen effects on weathering of natural rocks, deterioration of building stones and stone work, and formation of primitive soils should be carried out. (C) 2000 Else vier Science B.V, All rights reserved.