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What Are Biological Weathering Lichens?

Lichens may not look like much, but they are actually first-class pioneers, capable of living where no plant can survive. They also can weather, or wear down, some of the hardest substances on Earth biologically, or physically, as well as chemically. Over the course of hundreds or thousands of years, lichens literally can help turn seemingly unchangeable rock into soil.

Lichen Biology
- Lichens appear in many colors.
  
  Lichens are actually two types of organisms, microscopic fungi and algae, working symbiotically, or together. The fungus keeps the algae moist, and the algae makes food for both organisms. This partnership allows lichens to live in some of the most inhospitable places on Earth, such as barren rock.
  
  The many types of lichen range in color from the palest green to bright orange to black. Most of the lichens normally seen are epilithic, meaning they reside on the surface of the rock. The endolithic lichens make their way into rock, while chasmolithic lichens can survive in cracks within rock.
Physical Weathering
- With microscopic rootlike filaments, lichens can physically weather rocks. Parts of lichens called hyphae and rhizines find their way between the grains of rock. Once there, the lichens' repeated swelling and shrinking increases the size of the gaps, allowing more lichens to enter and continue the process. Eventually, the rock forms cracks along its surface and/or sheds part of its top layer.
Chemical Weathering
- Lichens make chemicals that react with other substances to eat away at rock slowly. One of the biggest weathering agents is oxalic acid, which occurs naturally in lichens. When oxalic acid reacts with the environment, it leaches calcium carbonate from rocks and replaces it with a weaker substance.
  
  Lichens help to erode rocks in other ways. Water from the sky and carbon dioxide from the lichens mix with a common rock compound called calcium bicarbonate to form calcium carbonate, a liquid that is easily washed away by rain. In the same way, lichens can help turn rock-hard magnesium bicarbonate into the highly erodible magnesium bicarbonate.
Effects
- In some cases, lichen growing inside a rock can weather the rock 200 to 300 times faster than geological processes such as wind and water. During decades and centuries, lichens can turn barren boulders into chunks of rock with deep fissures and a layer of organic matter. This allows small plants such as mosses to gain a foothold. These plants further weather rock so soil and, eventually, a new ecosystem can form.
  
  Lichens can bring unwanted weathering to statues, tombstones, stone buildings and other elements that humans would rather remain intact and unvarnished.

Nature