Physical and Chemical Weathering

Physical and Chemical Weathering

Middle School

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Learning Objectives

• Define and differentiate between mechanical and chemical weathering.

• Describe different types of mechanical weathering, like frost and root wedging.

• Identify the main agents of chemical weathering like water, oxygen, and acid.

• Explain how mechanical weathering can increase the rate of chemical weathering.

Key Vocabulary

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Weathering

The mechanical and chemical processes that change objects and break down rock on Earth's surface over time.

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Mechanical Weathering

The process where rocks are physically broken into smaller pieces without changing their chemical composition.

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Chemical Weathering

The process that breaks down rock through chemical reactions, creating new compounds and changing its composition.

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Abrasion

The grinding of rock by other rock particles carried by wind, water, ice, or gravity.

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Oxidation

The process where oxygen reacts with another element, such as iron to form rust.

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Hydrolysis

The process where the chemical bonds of a mineral are broken down by reaction with water.

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What is Weathering?

• Weathering is the process that breaks down rocks on the Earth's surface.

• This process creates smaller pieces of rock called sediment, shaping the landscape.

• Mechanical weathering physically breaks rocks without changing their chemical composition.

• Chemical weathering breaks down rocks by changing their chemical composition.

Multiple Choice

What is the primary difference between mechanical and chemical weathering?

• Mechanical weathering changes the rock's composition, while chemical weathering only changes its size.

• Mechanical weathering breaks rock into smaller pieces, while chemical weathering changes the minerals into new substances.

• Mechanical weathering is caused by water, while chemical weathering is caused by ice.

• Mechanical weathering only happens in mountains, while chemical weathering only happens in caves.

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Types of Mechanical Weathering

Frost Wedging

• Water seeps into small cracks or pores in rocks.

• When the water freezes into ice, it expands and acts like a wedge.

• Repeated cycles of freezing and thawing cause the rock to break apart over time.

Root Wedging

• Plant roots grow into existing cracks and fissures within rocks.

• As the roots grow thicker and longer, they exert significant pressure on the rock.

• This constant pressure from the growing roots eventually splits the rock apart.

Multiple Choice

A rock splits open because water got into a crack, froze, and expanded. What type of mechanical weathering is this?

• Abrasion

• Root Wedging

• Frost Wedging

• Exfoliation

Agents of Chemical Weathering

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Water (Hydrolysis)

• ​Water molecules can pull apart the ions found in minerals.

• ​​This chemical reaction with water is a process called hydrolysis.

• ​It changes the mineral’s composition, making the rock much weaker.

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Oxygen (Oxidation)

• ​Oxidation is a reaction where an element combines with oxygen.

• ​​For example, iron in rocks reacts with oxygen to form rust.

• ​This common process weakens the rock, making it easier to break.

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Carbonic Acid (Carbonation)

• ​Carbon dioxide (CO₂) in the air dissolves into rainwater.

• ​​This combination forms a weak acid which is called carbonic acid.

• ​It can dissolve certain rocks like limestone, which helps form caves.

Multiple Choice

What forms when oxygen reacts with iron in rocks, causing the rock to weaken?

• Carbonic acid

• Rust

• Sediment

• Clay

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How Weathering Processes Work Together

• Mechanical weathering breaks rocks down, increasing their total surface area.

• More exposed surface area allows chemical reactions to happen much faster.

• For example, a broken cube of rock has more surfaces for water to attack.

• Decaying organic matter produces CO₂, which helps form weak carbonic acid.

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Multiple Choice

How does mechanical weathering affect the rate of chemical weathering?

• It slows it down by creating a protective layer.

• It has no effect on chemical weathering.

• It speeds it up by increasing the rock's surface area.

• It changes the rock's color, which stops chemical reactions.

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Common Misconceptions

Misconception	Correction
Weathering and erosion are the same thing.	Weathering breaks down rock, while erosion moves the broken pieces.
Weathering only happens very slowly.	While often slow, it can happen quickly, like a rock splitting overnight.
Only water causes weathering.	Ice, wind, temperature changes, and living organisms also cause weathering.
Mechanical weathering changes the type of rock.	It only changes the rock's size and shape, not its chemical composition.

Multiple Choice

What is the process called when water reacts with minerals and weakens rock by changing its composition?

• Erosion

• Hydrolysis

• Mechanical weathering

• Condensation

Multiple Choice

A statue made of copper has turned green over many years. This is an example of which process?

• Frost wedging, as water froze in the statue's cracks.

• Abrasion, as wind and sand wore away the surface.

• Oxidation, a type of chemical weathering where the copper reacted with oxygen.

• Root wedging, as plants grew on the statue.

Multiple Choice

Imagine two identical pieces of limestone. One is left as a whole block, and the other is crushed into a fine powder. If both are exposed to acid rain, what would you predict will happen?

• The whole block will dissolve faster because it is a single, solid piece.

• Both will dissolve at the exact same rate.

• The crushed powder will dissolve faster because it has a greater total surface area for the acid to react with.

• Neither will dissolve because limestone is not affected by acid rain.

Multiple Choice

Which of the following scenarios is the best example of how biological activity causes mechanical weathering?

• A chemical reaction between plant decay and minerals in a rock.

• A large tree root growing into a crack in a boulder and splitting it apart.

• The formation of carbonic acid from CO₂ produced by bacteria.

• Iron in a rock rusting after being exposed to oxygen and water.

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Summary

• Mechanical weathering physically breaks rock into smaller pieces.

• Chemical weathering changes a rock's composition through chemical reactions.

• Mechanical weathering speeds up chemical weathering by increasing surface area.

• Both types of weathering work together to shape Earth's landscape.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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• 3

• 4