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- How many places on the outcrop do you see signs of biological weathering?
- How do you think the plant life on the rock will change over the years?
The outcrop at this location is experiencing several different types of weathering, including biological weathering. It is a good example of how multiple forces can affect and change an outcrop over millions of years, but also over shorter periods of time. You will be able to examine this outcrop and determine which forces are at work, where each of them is happening, how things might change in the future.
Weathering is a key part of the process of soil formation. Many people refer to any loose material on Earth’s surface as soil, but to geologists, soil is the material that includes organic matter, forms within the top few tens of centimetres of the surface, and is important for sustaining plant growth.
The types of weathering that take place has a major influence on soil composition and texture. Soil forms through the mechanical and chemical weathering of rocks and sediments, and the accumulation and decay of organic matter. The factors that affect the nature of soil and the rate of its formation include:
- Climate, especially average temperature and precipitation amounts, and the consequent types of vegetation
- The parent rock or sediment that was weathered to make the soil
- The slope of the surface where soil is accumulating
- How long soil has been forming at a location
Biological Weathering
Biological weathering is the disintegration of rocks as a result of the action by living organisms. Plant and animals have a significant effect on the rocks as they penetrate or burrow into the soil respectively. Biological weathering can work hand in hand with physical weathering by weakening rock or exposing it to the forces of physical or chemical weathering.
For instance, some plants and trees grow within the fractures in the rock formation. As they penetrate into the soil, and their roots get bigger, they exert pressure on rocks and make the cracks wider, and deeper that weaken and eventually disintegrate the rocks. Microscopic organisms can also produce organic chemicals that can contribute to the rock’s mineral weathering.
The outcrop at this location shows at least a couple of signs of weathering and erosion.
Here are the four main types of biological weathering:
Growing Plant Roots
It is common to see some roots growing within the face of a rock, contributing to biological weathering. The roots of plants and trees penetrate into the soil in search of nutrients and water. As the roots penetrate the soil, they go through cracks or joints in the rocks and as they grow they progressively crack the rock apart. Bigger growing roots can also exert pressure on the adjacent rocks. Some plant roots also emit organic acids that aid to dissolve the rock’s minerals.
Microbial Activity
Some plant microbial activity releases organic acidic compounds. These compounds can break down iron and aluminum minerals in the rocks. Microscopic organisms like algae, moss, lichens and bacteria are such kind of plants. They grow on the surface of the rocks and produce organic chemicals that are capable of breaking down the outer layer of the rock by altering the rock’s chemical composition. They release what are termed as acidifying molecules (organic acids and protons) and chelating compounds (siderophores and organic acids). The amount of biological activity that breaks down minerals depends on how much life is in that area.
As much as these compounds are produced through biological process, they arise out of bio-chemical reactions which accelerate chemical and physical weathering. Nonetheless, the process is categorized as biological weathering since it is biological in nature. Besides, these microscopic organisms also bring about moist chemical micro-environments which encourage the chemical and physical breakdown of the rock surfaces.
Burrowing Animals
Burrowing animals such as moles, squirrels and rabbits can speed up the development of fissures. Many animals such as the Piddock shells drill into rocks for protection either by releasing acids to dissolve the rocks or fragment away the rock grains. These animal activities can create fissures in the rocks and also eats away the rock’s minerals.
As this process continues, gaps and holes develop within the rock, further exposing the rocks to chemical, biological and physical weathering. Burrowing animals can as well move the broken rock pieces to the surface and so indirectly increasing the processes of rock weathering.
Human Activities
Human activities equally dig, crash, and widen the cracks and wind up fragmenting the rocks apart. These activities include mining, road construction, and housing developments.