5 Chemical Weathering Examples and How They Occur

When it comes to chemical weathering, it’s all about chemistry. By looking at the term “chemical weathering,” you can see that a chemical reaction causes something to break down or “weather.” That “something” is rocks and minerals.

In chemical weathering, rocks and minerals are reacting to acids, oxygen, carbon and water. That’s why no two rocks ever look exactly the same. It’s also the reason that we have those awesome looking caves and unique rock formations all over the world.

While chemical weathering creates nifty formations, the way it breaks down rocks also causes fractures in ancient structures like the Great Sphinx of Egypt. It also causes the surface to break down on gravestones.

To truly understand chemical weathering and how it works, you need to look at examples of the different types of chemical weathering. Not all reactions that lead to chemical weathering happen or work quite the same way.

You also have to remember that chemical weathering types can work separately, but they often work together to create landforms and break down minerals. Check out how carbonation, oxidation, hydration, hydrolysis and acidification work.

When you think of carbonation, think carbon! Carbonic acid is the culprit when it comes to the carbonation type of chemical weathering. As rain goes through the air and into the ground, it grabs carbon dioxide, creating carbonic acid. This weak acid reacts with the calcium carbonate in stones when it seeps into the cracks.

A stone that is particularly susceptible to carbonation is limestone, which is made of mostly calcium carbonate. Carbonation of limestone creates unique structures like the South China Karst. Karsts are all over the globe and include unique caves, streams, sinkholes and unusual rock formations.

Oxygen causes oxidation. You can think about oxidation like rust on a car. Rust forms when the iron or steel in your car reacts with the oxygen in the air to form iron oxide. The resulting red substance can be quite brittle, so much so that you could literally poke a hole in it with your finger. Rocks with iron can go through the same process, too.

Minerals with high iron content are affected by oxidation including pyroxene and amphibole. The oxidation gives these rocks a reddish look, very similar to the patina on a car.

This isn’t the hydration used in your body, but it’s similar. Hydration is a type of chemical weathering where water reacts chemically with the rock, modifying its chemical structure.

One example of mineral hydration is when H₂O (water) is added to CaSO₄ (calcium sulfate) to create CaSO₄+₂H₂O (calcium sulfate dihydrate). It changes from anhydrite to gypsum.

The addition of the water to the anhydrite chemically reacts to create a totally new compound in gypsum. Hydration has led, in part, to the gypsum sand dunes at White Sands National Monument.

Water can add to a material to make a new material, or it can dissolve a material to change it. In hydrolysis, the acid in the water works to dissolve minerals within specific rocks.

Examples of hydrolysis in action include turning feldspar into clay and making sodium minerals into saltwater solutions. The flare slopes of Australia are partly created by hydrolysis.

You’ve probably heard of acid rain. However, most people don’t know what it is or how it contributes to chemical weathering. Acid rain is water with sulfuric and nitric acids from the burning of coal and fossil fuels, along with volcano eruptions.

The acids create a reaction when they hit stone, causing the surface to wear and the composition to soften. Acidification can also be caused by organisms like lichens, which are created from algae and fungi.

One well-known case of rapid weathering and blackening of stone is the weathering on the 1,000-year-old Leshan Giant Buddha in China. The 232-foot-tall Buddha required six months of repair for weathering after only 12 years due, in part, to acidification.

Chemical weathering isn’t the only type of weathering that affects the land. It has at least a couple of counterparts.

• Physical weathering is when rocks change without a chemical component, like landforms that are caused by natural Earth movements.
• Biological weathering occurs when rocks are weakened by plants and animals, like when plant roots grow through rocks.

Physical and biological weathering work in conjunction with chemical weathering to break down and erode land.

Chemical weathering is when chemicals in rain and moving water react with rocks and minerals to change or weaken them in some way. Chemical weathering always causes some type of chemical reaction within the rock or mineral itself. Expand your knowledge of the breaking down of rocks and soil through examples of erosion.