What is weathering?

Question: I understand what weathering a storm is, but what does the term “weathering” mean in relation to soil?

The simplest way to think about weathering is that it means “change,” albeit a special kind of change.

The breaking of rock through the action of ice is a form of physical weathering. Photo: James Wheeler (www.flickr.com).

The breaking of rock through the action of ice is a form of physical weathering. Photo: James Wheeler (www.flickr.com).

Weathering is a process by which rocks and minerals are changed. This may be a physical or chemical process of change.

In physical weathering, particles of rocks and minerals are broken into smaller pieces as they bounce around in streams, get crushed and ground up, are split by ice or roots, or as the outside heats up and expands faster than the inside. But each piece of material still has all the same properties of the parent rock or mineral, only smaller.

In chemical weathering, the particles may become smaller, but other properties also change. The new particles are not like the parent; they become a different mineral. Water molecules may be added to, or lost from, the chemical structure. Electrons may be gained or lost. The mineral may dissolve or react with acid.

These chemical changes often result in changes in the physical properties of the mineral, such as color (like rusting iron), hardness (the mineral becomes softer), and density. How the mineral holds heat may also change.

As an example of the process, think about eating a cracker.

If you break it into two pieces, each piece is still a cracker, only smaller. This is like physical weathering.

But if you take a bite of the cracker and begin chewing it, two things happen. The first is physical weathering, as your teeth break the cracker into smaller pieces. The second is a chemical change, as the saliva in your mouth mixes with the cracker pieces and begins to digest them. Look in the mirror and stick out your tongue. The stuff in your mouth is no longer a cracker; its chemical and physical properties have been changed.

And so, physical weathering is about breaking things into smaller pieces, while chemical weathering is about changing minerals into different minerals, with resulting changes in physical properties.

–Answered by Clay Robinson, a.k.a., Dr. Dirt

Have a question for Soils Matter? Post it to the comments section below or email us at soils-matter@soils.org

 

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