Rock-shield Lichen - Xanthoparmelia sp.

Rocks can be weathered by the growth of foliose lichens by physical and chemical processes. First, they can chemically weather minerals in the rocks, such as calcite, by producing organic acids. The acids react with the minerals in the rock, dissolving them, and weakening the rock. Second, lichens can physically weather rocks by penetrating the cracks in the rock with their rhizoids. The expansion and contraction of the roots causes smaller cracks to expand.