Weathering, Erosion, and Deposition

Weathering, Erosion, and Deposition

Middle School

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Learning Objectives

• Define weathering, erosion, and deposition, and explain their differences.

• Compare and contrast the processes of mechanical and chemical weathering.

• Identify the main agents of erosion and the factors that affect its rate.

• Explain how deposition builds new landforms and how weathering reshapes Earth’s surface.

Key Vocabulary

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Weathering

The breakdown of rocks into smaller pieces, called sediments, by physical or chemical processes on Earth's surface.

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Erosion

The process of moving weathered rock and soil from one place to another by wind or water.

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Deposition

When eroded sediments are dropped or settled in a new place, often building up new landforms.

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Mechanical Weathering

The physical process of breaking rocks into smaller pieces without changing the rock's chemical composition.

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Chemical Weathering

The process that breaks down rocks using chemical reactions, changing the rock's composition to form new substances.

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Differential Weathering

The process where softer, less resistant rocks wear away faster than harder, more resistant rocks nearby.

Weathering, Erosion, and Deposition

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Weathering

• ​Weathering is the first step, acting as a destructive force.

• ​​It breaks down large rocks into much smaller pieces called sediments.

• ​These sediments can range from large boulders to fine clay.

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Erosion

• ​Erosion is the second step, acting as a transportation system.

• ​​It moves weathered sediments from their original location to new ones.

• ​This is caused by agents like wind, water, and ice.

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Deposition

• ​Deposition is the final, constructive step that builds new land.

• ​​It happens when eroded materials are laid down or are settled.

• ​This process is responsible for creating many different landforms.

Multiple Choice

Which sequence correctly describes the roles of weathering, erosion, and deposition in reshaping the Earth's surface?

• Breaking down rocks, transporting the pieces, and then depositing them to build new land.

• Building new landforms, transporting them, and then breaking them down with weathering.

• Transporting materials, breaking them down, and then depositing them in a new location.

• Breaking down rocks, building new landforms, and then transporting the pieces.

Mechanical Weathering: The Physical Break-Up

• Physical forces break rocks into smaller pieces.

• Water freezing in cracks can split rocks apart.

• Growing plant roots can also wedge rocks open.

Multiple Choice

A rock in a cold climate breaks apart after water enters its cracks, freezes, and expands. What type of mechanical weathering is this?

• Frost/Ice Wedging

• Temperature Changes

• Abrasion

• Plant Roots

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Chemical Weathering: Changing a Rock's Identity

Dissolving

• Certain minerals found in rocks can easily be dissolved by water over time.

• Slightly acidic rainwater can dissolve rocks like limestone, creating unique underground formations.

• Over millions of years, this process carves out enormous caves and passageways.

Oxidation

• Oxygen present in the air or water reacts with iron-rich minerals in rocks.

• This chemical reaction is the same process that we commonly know as rusting.

• This forms new substances called iron oxides, making the rock weak and crumbly.

Multiple Choice

What process of chemical weathering occurs when oxygen reacts with iron-rich minerals in a rock, causing it to rust and crumble?

• Oxidation

• Frost Wedging

• Dissolving

• Abrasion

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Factors Influencing Weathering

• Local climate and topography, the shape of the land, influence weathering rates.

• Softer, less resistant rocks weather and erode much faster than harder rocks.

• Differential weathering leaves resistant rock behind, creating irregularly shaped landscapes.

• Human pollution can create acid rain, which accelerates chemical weathering.

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Multiple Choice

What is it called when softer rock weathers away faster than harder rock, creating an irregularly shaped landscape?

• Topographical Weathering

• Climate-based Weathering

• Acid Rain Effect

• Differential Weathering

Erosion: Transporting Earth's Materials

• Erosion removes and transports weathered material from one place to another.

• Water is the most powerful agent of erosion, especially when flowing fast.

• Rocks become smoother and rounder the farther they are transported.

Multiple Choice

You find a large rock with very sharp, angular edges. What can you infer about its journey?

• It was primarily shaped by chemical weathering, not erosion.

• It has traveled only a short distance from its original location.

• It has traveled a long distance and is very smooth.

• It was eroded by a fast-flowing stream over a long distance.

Deposition: Building New Land

• Deposition occurs when erosion agents like wind or water drop sediments.

• A river slowing down can deposit sediment to form a delta.

• This process also creates beaches, sand dunes, and barrier islands.

Multiple Choice

Why is deposition considered a constructive process?

• Because it moves sediments from one place to another.

• Because it builds up land and creates new landforms like deltas and beaches.

• Because it breaks down rocks into smaller pieces.

• Because it happens when agents of erosion gain energy.

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A Dynamic Cycle: Building Up and Tearing Down

• Earth's landscapes are always changing due to constructive and destructive forces.

• Plate tectonics are constructive forces that build up mountains and other landscapes.

• Weathering and erosion are destructive forces that wear down rocks and landscapes.

• The Appalachians were once taller but were worn down by weathering and erosion.

Multiple Choice

According to the information, what is the primary reason the Appalachian Mountains are much shorter today than they were in the past?

• Constructive forces from plate tectonics made them smaller.

• Deposition built up the land around them, making them appear shorter.

• Millions of years of weathering and erosion have worn them down.

• They were formed as small hills and have not changed.

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Common Misconceptions

Misconception	Correction
Weathering and erosion are the same thing.	Weathering breaks rock down, while erosion moves the broken pieces.
Erosion is always a fast, dramatic event.	Most erosion is a slow and continuous process.
Human actions do not affect weathering.	Human pollution creates acid rain, which accelerates chemical weathering.
Smooth, rounded rocks are newly broken.	Smooth, rounded rocks indicate they have traveled a long distance.

Multiple Choice

What is the primary difference between how mechanical and chemical weathering alter a rock?

• Mechanical weathering breaks the rock physically, while chemical weathering changes the rock’s chemical composition.

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

• Mechanical weathering happens in hot climates, while chemical weathering happens in cold climates.

• Mechanical weathering changes the rock’s chemical makeup, while chemical weathering does not.

Multiple Choice

How does the process of erosion by a river influence the physical characteristics of a rock fragment over time?

• The rock becomes smaller, smoother, and more rounded as it travels a longer distance.

• Erosion has no effect on the physical appearance of a rock fragment.

• The rock's chemical composition changes, but its shape does not.

• The rock becomes larger and more angular the longer it travels.

Multiple Choice

Imagine a coastal area where a river, carrying a heavy load of sediment, enters the calm waters of the ocean. What is the most likely outcome at the river's mouth over time?

• The river will carve a deep canyon into the ocean floor due to increased erosional energy.

• Mechanical weathering will increase, creating large, sharp-edged boulders.

• The river will begin to flow faster, causing more erosion of the coastline.

• The river will slow down, causing deposition and the formation of a delta.

Multiple Choice

How do plate tectonics, weathering, and erosion work together to shape mountain ranges?

• Plate tectonics is a destructive force, while weathering and erosion are constructive forces that build mountains.

• Plate tectonics builds mountains, and weathering/erosion are also constructive forces that make the mountains taller.

• Plate tectonics and weathering/erosion are both destructive forces that work together to flatten landscapes.

• Plate tectonics is a constructive force that builds mountains, while weathering and erosion are destructive forces that wear them down.

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Summary

• Weathering breaks rocks, erosion moves sediment, and deposition builds new land.

• Weathering is mechanical or chemical; erosion is caused by water, wind, ice, and gravity.

• Deposition is a constructive process that creates landforms like deltas and beaches.

• Earth’s surface is constantly reshaped by constructive and destructive forces.

Poll

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