Biological weathering is the process in which plants, animals, and bacteria break down rocks into smaller pieces. This weathering can be done, for example, through tree roots growing in cracks in rocks and eventually breaking the rock apart.
As time goes on, things break down and change. Throughout Earth’s history, mountains have come and gone, continents merge and drift away, and landscapes change as they experience different climates and weather events. One process that contributes to this cycle of change is weathering.
What is weathering?
Weathering is the breakdown of rocks because of physical, chemical, or biological processes. Physical weathering is simply physical forces breaking down rocks, which can include temperature changes, causing expansion and contraction of rocks, winds that slowly erode rocks over time, and even water that gets between the crevices of rocks and breaks them apart when it freezes and expands. Chemical weathering is when rocks break down because of different chemical reactions, such as the absorption or dehydration of rocks that cause substantial changes to its structure.
Of interest here is biological weathering, which we see every day in cities, towns, and even rural areas. Biological weathering is simply the disintegration of rocks because of animals, plants, and microbes. Many growing plants exert pressure on rocks as they grow because of their roots, which starts out small and gets in between the cracks or any exposed parts of the rock. As they grow, they cause damage to the rock, weathering it.
“A river cuts through rock not because of its power, but because of its persistence.” — Jim Watkins
While this process is physical, it involves a biological factor and is grouped with biological weathering. There are many animals that break apart rocks, or consume rocks, for a variety of purposes, most of which ends up changing the rock in some way. Microbes generally weather rocks by releasing chemicals that change the composition of the rocks. Again, while this is a chemical process, it does involve a biological factor and is grouped accordingly. Many organisms engage in weathering because it serves important purposes for their survival and continued well being. Biological weathering also plays important roles in changing the environment as the examples will show.
Humans are one of the most obvious animals for biological weathering when you consider much of the Earth we have consumed, modified, and removed for our own uses and needs. We have quarries and mines all over the world with the sole purpose of digging through rock and earth to acquire fuel, jewelry, and other items of interest. From making jewelry to making countertops, we use rocks and minerals for a variety of purposes in our lives. During the stone age, which lasted from 3.3 million years ago to about 8700 BCE to 2000 BCE, we used stones for everything from cooking to building and even hunting.
It played a crucial role in our development and survival as a species and a community. This period also marked the beginning of humans sculpting the world beyond just weathering rocks and stone. Another human biological weathering comes from the amount of pollution that we release into the air. Through the burning of fossil fuels and natural gases, we have released sulfur dioxide, nitrogen oxide, and other chemicals into the atmosphere. When these chemicals are exposed to the sun and water in the sky they become acid rain, which falls onto the Earth and weather rocks and other materials they come into contact with. The more acidic the rain, which is based on the where pollution is worse, the more weathering will occur.
Many burrowing animals, like earthworms, break apart rocks and soil as they tunnel through them. Over time, the weathering breaks the rocks down into smaller particles until they become soil. As earthworms burrow through soil and loose particles of rocks, they create tunnels that aerate the soil to improve soil health and drainage. They also consume organic and inorganic materials to release nutrient-rich waste, which further helps to improve the soil’s health.
Biological weathering caused by plants is mainly a mechanical process that results in the displacement of rocks as the plants grow. They will find any opening or crevice and encroach on it. We see it all around city blocks as the roots of trees lift up concrete and pavement. We see it when buildings and homes are abandoned. Nature grows over these areas and consumes it over time. It breaks down the buildings as it continues to grow and displace its surroundings, which is made of many things including rocks. Plants also contribute to biological weathering via chemicals as they can release acids to break down rocks.
Microbial organisms like lichen, which is made up of algae and fungi, live on the surface of rocks. They form a symbiotic relationship that supports the survival of each other and allows them to thrive and spread. As a result of this, they release acidic compounds that get into the rocks and weather it. The more the colony is able to grow, the more acid that is produced. This can be a limiting factor for the lichen community because if they weather too much rock, then they risk losing their home and destabilizing their symbiotic relationship. Lichen also grows on other plant matter, like trees, and breaks those down as well.
All forms of weathering do not act independently of each other. They all function together and accelerate the weathering of rocks. Plants are still growing as it rains, which can be acidic, and as it freezes it can result in more cracks and openings for roots to grow into, which further creates more cracks for water and ice to form. Wind, water, and Lichen act together to weather rocks near oceans and other bodies of water.
Many biological weathering factors can engage in different kinds of weathering, like physical or chemical, and this further complicates the simultaneous occurrences of weathering we see in the world. Understanding the changes and the relationship between the different types of weathering, allows us to understand how much the Earth has changed and how it might continue to change as we, and everything else, continue to weather away at the rocks.