The Differences Between Chemical & Mechanical Weathering

By Meg Michelle
Weathering, damage, rocks
Rock image by Leopold from Fotolia.com

Weathering is a term used to describe the slow degradation of rocks and minerals. Many processes may cause weathering, from chemical to mechanical. Over time, this type of weathering can cause a complete destruction of rocks. Knowing the differences between types of weathering can help specialists protect rocks that are part of important land formations.

Differences Between Mechanical and Chemical Weathering

During chemical weathering, the substance of the rock physically changes. Certain elements or compounds, such as water or oxygen, can cause chemical reactions in a rock, making it softer or dissolving the rocks altogether. In contrast, mechanical weathering causes a breakdown of rocks without a change in chemical composition. Also known as physical weathering, this type of weathering is commonly caused by the movement of the Earth or the freezing and thawing of water.

Examples of Chemical Weathering

Chemical weathering occurs when chemical reactions take place inside a rock's structure, altering the composition of the rock itself. Acid rain is one cause of such weathering. Gases such as carbon dioxide and oxygen can cause chemical weathering. Oxidation occurs when oxygen attaches to certain substances in the rock, changing the composition of the rock. Carbonation is a similar process that occurs with carbon dioxide, in which rocks actually dissolve in rain water. Plants can also cause chemical weathering. Mosses and other plants growing on and around a rock can release acids that can cause chemical weathering.

Water is another factor that can cause chemical weathering. Hydration is one form of water weathering. During hydration, a water molecule will attach to other molecules in a substance, such as clay, causing it to swell. Hydrolysis is another form of water weathering. In hydrolysis, water molecules moving through the rock may attach to certain elements, physically changing the makeup of the rock's structure, making it softer and more susceptible to mechanical weathering.

Examples of Mechanical Weathering

Water is also responsible for mechanical weathering. When water freezes, it expands. If the water is inside a rock, that expansion can cause cracks or even cause bits of the rock to fall off completely. Temperature can cause a similar problem. As temperatures grow warmer, the rocks themselves expand.

Movement of the Earth can also cause mechanical weathering. The crust of the Earth is in constant motion. The same rubbing and collision of rocks that can lead to earthquakes can also cause weathering in rocks.

About the Author

An avid lover of science and health, Meg Michelle began writing professionally about science and fitness in 2007. She holds a bachelor’s degree in physics from Creighton University and master’s degree in science writing from Johns Hopkins. Her work has appeared in publications such as EARTH Magazine.