7.1.2 Chemical Weathering

Chemical Weathering

Open Ended

What are some of the agents of physical weathering?

In other words, what causes it?

​What are the two types of weathering?

Physical/Mechanical vs. Chemical Weathering

Physical/mechanical weathering is the physical breakdown of rock into sediment.

Chemical weathering is the chemical breakdown of rock material through a chemical change.

• Atoms are rearranged

• They form different compounds

• Chemical weathering involves a chemical change (new substance) to a rock or mineral

Have you ever seen an old stone statue or monument? Maybe you have seen one in a cemetery or a park.

• The color of the stone or metal has probably changed a lot over time.

• Even the shape or surface texture of the object may have changed.

Gradual changes in a rock's shape, texture, and color over time are caused by several factors. These include erosion, mechanical (physical) weathering, and chemical weathering.

Chemical weathering is the breakdown of rock material due to chemical reactions.

• When this occurs, the atoms are rearranged and form different compounds. Rocks that undergo chemical weathering change in chemical composition.

• The rocks that have been chemically weathered are different from those in the parent rock.

Chemical weathering can occur when rocks come into contact with substances, such as water, air, and chemicals.

Water dissolved gypsum from nearby mountains, and the dissolved ions washed into the shallow lakes. When the water evaporated, the gypsum crystallized again, forming the white sand at White Sands National Park in New Mexico.

When oxygen in the air reacts with iron found in rocks, oxidation occurs, causing the rocks' composition to change. This leads to a reddish color in the rock.

Chemicals in the air can mix with water, forming acidic rain, which can dissolve certain types of rocks.

Multiple Select

Which of the following are examples of chemical weathering?

• animals burrowing

• acid rain dissolving rock

• iron in rock reacting with oxygen

Chemical weathering can occur through the following processes:

• Hydrolysis

• Oxidation

• Carbonation

• Dissolution

Hydrolysis

Usually, the new material is weaker. This is how many clay minerals form. For example, feldspar, a mineral commonly found in granite, will turn to clay if it mixes with water.

Hydrolysis is a process where water chemically bonds with minerals in a rock and produces a new substance.

Hydrolysis is an important type of chemical weathering and contributes to soil formation.

Oxidation

Oxidation happens when oxygen chemically combines with other substances.

Rusting is an example of oxidation.

Oxidation is most common in rocks and minerals that contain a lot of iron. This is because iron easily reacts with oxygen. These rocks often have a reddish appearance.

Oxidation and Copper

Copper also reacts with oxygen in the atmosphere, giving material a greenish patina.

Patina is a type of film that covers a surface.

It is the copper in a bronze statue that makes it turn green.

Dissolution & Carbonation

​When rainwater mixes with carbon dioxide, it forms carbonic acid (a weak acid). Carbonic acid can break down and dissolve minerals within rock. This type of dissolution is referred to as carbonation.

Other substances can form acid rain, which can also dissolve rock, like sulfur dioxide and nitrogen oxides.

​Cave Formation in Pennsylvania

Pennsylvania has a lot of limestone.

Remember, when carbon dioxide mixes with rainwater, it creates carbonic acid.

This seeps into the ground and between cracks in the limestone. It dissolves the limestone.

This creates caverns underground as it is washed away.

​what is one effect of chemical weathering on underground features in Pennsylvania?

A. It causes granite to form from clay deposits.

B. It creates acidic water that reacts with limestone, forming caverns underground.


C. It gives the Grand Canyon its colorful appearance.


D. It mixes carbon dioxide with rainwater to create limestone.

Labelling

Type question here including an

Drag labels to their correct position on the image

oxidation

carbonation

hydrolysis

Rate of Chemical Weathering

Chemical weathering happens at different speeds based on several factors.

• Temperature

• Rainfall

• Vegetation

• Rock type

Factors can build on each other, causing chemical weathering to speed up even more.

Multiple Select

Which of the following factors affects the rate of chemical weathering?

• Rock type

• Rainfall

• Temperature

• Vegetations