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===Biotic weathering processes=== Biotic processes often intersect with both physical and chemical weathering regimes, as the organisms that interact with rocks can mechanically or chemically alter them.{{cn |date= November 2024}} [[Lichen]] frequently grow on the surface of, or within, rocks. Particularly during the initial colonization process, the lichen often inserts its [[hypha]]e into small [[fracture]]s or mineral [[Cleavage (crystal)| cleavage planes]] that exist in the host rock.<ref name=":2">{{Cite journal|last1=Jie|first1=Chen|last2=Blume|first2=Hans-Peter|date=October 2002|title=Rock-weathering by lichens in Antarctic: patterns and mechanisms|url=http://link.springer.com/10.1007/BF02844595|journal=[[Journal of Geographical Sciences]]|volume=12|issue=4|pages=387β396|doi=10.1007/BF02844595|s2cid=128666735|issn=1009-637X|url-access=subscription}}</ref> As the lichen grows, the hyphae expand and force the fractures to widen. This increases the potential of fragmentation, possibly leading to rockfalls. During the growth of the lichen [[thallus]], small fragments of the host rock can be incorporated into the biological structure and weaken the rock.{{cn |date= November 2024}} [[File:Cliff and forested scree, Paces Lake, Nova Scotia.jpg|thumb|A tall cliff on the eastern shore of Paces Lake, Nova Scotia, with scree at its base. As the rate of erosion is quite slow, the scree has become partially forested.]] [[Freeze-thaw action]] of the entire lichen body due to microclimatic changes in moisture content can alternately cause thermal contraction and expansion,<ref name=":2" /> which also stresses the host rock. Lichen also produce a number of [[organic acid]]s as metabolic byproducts.<ref name=":2" /> These often react with the host rock, dissolving minerals, and breaking down the substrate into unconsolidated sediments.{{cn |date= November 2024}}
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