Organic weathering, also called bioweathering or biological weathering, is the general name for biological processes of weathering that break down rocks. This includes the physical penetration and growth of roots and digging activities of animals (bioturbation), as well as the action of lichens and moss on various minerals.
How Organic Weathering Fits Into the Larger Geological Picture
Weathering is a process by which surface rock breaks down. Erosion is a process by which weathered rock is moved by natural forces such as wind, waves, water, and ice.
There are three types of weathering:
- Physical or mechanical weathering (for example, water gets into cracks in rock and then freezes, pushing against the rock from the inside);
- Chemical weathering (for example, oxygen interacts with iron in rocks, causing the iron to turn to rust and thus weakening the rock)
- Organic or biological weathering (for example, the roots of a tree grow into boulders in the soil and splitting the boulders apart over time)
While these different types of weathering can be described as different from one another, they also work together. For example, tree roots may split boulders more easily because the rocks have been weakened as a result of chemical or physical weathering.
Plant-Related Biological Weathering
Tree roots, because of their size, cause a significant amount of biological weathering. But even much smaller plant-related actions can weather rocks. For example:
Weeds pushing through road surfaces or cracks in boulders can expand gaps in the rock. These gaps fill with water. When the water freezes, the roads or boulders crack.
Lichen (fungi and algae living together in a symbiotic relationship) can cause a great deal of weathering. Chemicals produced by fungi can break down the minerals in rocks. Algae consume the minerals. As this process of breakdown and consumption continues, rocks start to develop holes. As described above, holes in rocks are vulnerable to physical weathering caused by the freeze/melt cycle.
Animal-Related Biological Weathering
Animal interactions with rock can cause significant weathering. As with plants, animals can set the stage for further physical and chemical weathering. For example:
- Tiny burrowing animals secrete acids or scrape their way into rock to create rocky burrows. This process weakens the rock and actually starts the weathering process.
- Larger animals leave feces or urine on rock. The chemicals in animal waste can corrode minerals in rock.
- Larger burrowing animals shift and move rock, creating spaces where water can accumulate and freeze.
Human-Related Biological Weathering
Human beings have a dramatic weathering effect. Even a simple path in the woods has an impact on the soil and rocks that make up the path. Major changes affected by humans include:
- Construction — moving, scoring, and smashing rock for construction of buildings and transportation systems
- Mining — massive projects involve stripping entire hillsides or making major changes to or removing rock from under the surface of the Earth
- Agriculture — in addition to moving rocks to make farming possible, human beings also change the composition of the soil through fertilization and application of herbicides.