Weathering and Erosion

Weathering and Erosion

Middle School

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Learning Objectives

• Define weathering, erosion, and deposition and how these processes interact with each other.

• Explain how energy from the sun and Earth's interior drives the cycling of materials.

• Describe how geoscience processes change Earth's surface at different time and spatial scales.

• Differentiate between physical and chemical weathering and their effects on different landforms.

• Identify the components of soil and describe the process of soil formation.

Key Vocabulary

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Weathering

Weathering is the natural process that breaks down large rocks into much smaller pieces.

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Erosion

Erosion involves the movement of sediment from one place to another by water, wind, or ice.

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Deposition

Deposition is the process where transported sediment, soil, and rocks are dropped in a new location.

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Sedimentation

The process of sediment being deposited in layers, which can eventually form new sedimentary rocks.

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Geoscience Processes

These are the many natural forces and events that continuously shape and change the Earth’s surface.

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Matter Cycling

The continuous movement and recycling of essential materials and elements throughout the Earth's various systems.

Key Vocabulary

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Ice Wedging

Physical weathering occurs when water freezes and expands in rock cracks, slowly breaking the rock apart.

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Chemical Weathering

The breakdown of rock through chemical reactions, which changes its composition and makes it much weaker.

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Humus

Dark, organic material in soil, formed from decomposed plants and animals, providing essential plant nutrients.

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Soil Horizon

A distinct layer of soil, parallel to the ground, with its own unique set of characteristics.

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Energy from the Sun

The sun's energy is responsible for driving the wind and water cycles, causing surface erosion.

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Earth's Internal Heat

Heat from within the Earth powers processes like melting rock, which leads to new rock formations.

Weathering, Erosion, and Deposition

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Weathering

• ​Weathering is the process that breaks down large rocks into smaller pieces.

• ​​These smaller pieces of rock are known as sediment.

• ​This process happens in place without moving the broken materials.

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Erosion

• ​Erosion is the process that transports sediment to a new location.

• ​​The primary agents that cause erosion are water, wind, and ice.

• ​Erosion moves weathered material away from its original place.

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Deposition

• ​Deposition is the process where transported sediment is dropped or settled.

• ​​This process builds up new landforms like deltas or sand dunes.

• ​It occurs when the agent of erosion loses its energy.

Multiple Choice

Which process is defined as the breakdown of large rocks into smaller pieces, known as sediment?

• Weathering

• Erosion

• Deposition

• Compaction

Multiple Choice

What is the primary difference between the processes of weathering and erosion?

• Weathering happens in place, while erosion involves movement.

• Weathering is caused by water, while erosion is caused by ice.

• Weathering builds new landforms, while erosion carves them out.

• Weathering creates sediment, while erosion creates large rocks.

Multiple Choice

A river carrying a large amount of sediment flows into the ocean. Which statement best explains what will happen to the sediment?

• The sediment will be deposited as the river loses energy, forming a delta.

• The river will speed up, causing more weathering to occur in the ocean.

• The sediment will be carried by wind from the ocean to form sand dunes.

• The rocks will be broken down into smaller pieces by the ocean waves.

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The Energy Driving Earth's Processes

Energy from the Sun

• ​Energy from the sun is the main driver for processes on the Earth’s surface.

• ​​It powers the water cycle and creates wind, which results in weather patterns.

• ​This energy causes the weathering, erosion, and sedimentation of land by rain and wind.

Energy from the Interior

• ​Heat from Earth's hot interior drives the processes that happen deep underground.

• ​​This energy melts rock to form magma and is responsible for tectonic plate movement.

• ​These actions push rock to the surface, where it is then weathered and eroded.

Multiple Choice

What are the two main sources of energy that drive the processes on Earth's surface and in its interior?

• Energy from the sun and heat from Earth's interior

• Energy from wind and energy from water

• Energy from tectonic plates and energy from magma

• Energy from weather patterns and energy from sedimentation

Multiple Choice

Which of the following processes is caused by heat from Earth's hot interior?

• The creation of wind

• The movement of tectonic plates

• The process of sedimentation

• The formation of weather patterns

Multiple Choice

Based on the relationship between Earth's two main energy sources, which statement best describes the complete process of rock transformation?

• Interior heat pushes rock to the surface, where solar energy drives weathering that breaks it down.

• Solar energy causes rocks to melt into magma, which is then pushed to the surface by interior heat.

• Wind and rain from solar energy push rocks deep underground, where they are melted by interior heat.

• Tectonic plate movement weathers and erodes rock, which is then moved by wind and rain.

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The Cycling of Earth's Materials

• Earth is a closed system where matter is constantly cycled and reused.

• Surface processes like weathering and erosion break down rocks into sediment.

• Sediment layers are compacted by pressure, forming sedimentary rock through sedimentation.

• This rock can be melted or deformed, returning to the surface to restart the cycle.

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Multiple Choice

What is the main principle of the cycling of Earth's materials?

• Matter on Earth is constantly created and destroyed.

• Matter on Earth is limited and is continuously reused in a cycle.

• Earth's materials only exist in one form, like rock or sediment.

• The amount of matter on Earth is always increasing.

Multiple Choice

How do smaller pieces of rock, known as sediment, become sedimentary rock?

• The pieces are melted by intense heat from Earth's core.

• The pieces are broken down further by continuous weathering.

• Layers of sediment are pressed together by pressure over time.

• The pieces are rapidly cooled after a volcanic eruption.

Multiple Choice

What conclusion is best supported by the information about the cycling of Earth's materials?

• Once a rock is formed, it remains in that form forever.

• The materials that make up a rock today could have been part of a different rock in the past.

• All rocks on the surface are eventually destroyed and can never be reused.

• Sedimentary rocks are the final stage of the cycle and cannot change further.

How Rocks Break Apart: Physical Weathering

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Abrasion

• Rocks get worn down when they scrape against each other.

• Wind blasts sand against rocks, making them much smoother.

• Water makes rocks and sediment bump against one another.

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Ice Wedging

• Water seeps into small cracks and crevices in the rocks.

• When this water freezes, it expands with a powerful force.

• This process widens cracks and can break the rock apart.

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Plant Roots

• As plants grow, their roots can extend into rock cracks.

• The growing roots slowly expand, creating pressure on the rock.

• Over time, this pressure can split even very large boulders.

Multiple Choice

Which statement best defines physical weathering?

• Rocks are broken down into smaller pieces without changing their chemical makeup.

• Rocks are transformed into new types of rock by heat and pressure.

• Rocks are dissolved by acidic water and carried away.

• Rocks are moved from one place to another by erosion.

Multiple Choice

How are the processes of ice wedging and plant root growth similar in the way they break rocks?

• They both involve water seeping into the cracks of rocks.

• They both scrape away the outer surface of the rock.

• They both happen very suddenly and with great force.

• They both create an outward force from inside a rock's cracks.

Multiple Choice

A hiker finds a collection of very smooth, rounded rocks in a fast-flowing river. Based on the processes of physical weathering, what is the most likely reason the rocks are smooth?

• Abrasion from water causing rocks to scrape against each other.

• Ice wedging from water freezing and expanding in rock crevices.

• Plant roots growing into cracks and splitting the rocks apart.

• Abrasion from wind blasting the rocks with sand.

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Dissolving Rocks and Uneven Landscapes

Chemical Weathering

• Rainwater absorbs carbon dioxide (CO₂) from the air, which creates a weak carbonic acid.

• This weak acid is strong enough to slowly dissolve certain types of rocks, such as limestone.

• Over thousands of years, this process can form large underground caves and surface sinkholes.

Differential Erosion

• This process occurs because different types of rock erode away at very different rates.

• Softer or weaker rock wears away much more quickly than harder, more resistant rock does.

• This leaves behind dramatic landforms, like the layers of the Grand Canyon and Devils Tower.

Multiple Choice

What causes the chemical weathering that can dissolve certain rocks like limestone?

• A weak acid is formed when rainwater mixes with carbon dioxide.

• Rocks are broken down by the force of heavy rainfall.

• Softer rocks physically wear away harder rocks over time.

• Underground rivers carve out tunnels in all types of rock.

Multiple Choice

What is the main reason that differential erosion creates uneven landscapes?

• All rocks are dissolved by carbonic acid at the same speed.

• Different types of rock wear away at different rates.

• Only the softest rocks are affected by rainwater.

• Harder rocks protect softer rocks from all erosion.

Multiple Choice

If a large area contains both hard, resistant rock and soft limestone, what is the most likely result after thousands of years of rainfall?

• The limestone would dissolve and form caves, while the harder rock would remain.

• The harder rock would completely protect the limestone from any rainwater.

• Both types of rock would erode at the exact same rate, creating a flat plain.

• The harder rock would dissolve faster than the limestone, leaving it behind.

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Geoscience Processes: Time and Scale

Spatial Scale

• Geoscience processes change the Earth's surface across different distances, known as spatial scales.

• These scales can be microscopic, like a raindrop dissolving minerals in a rock.

• They can also be massive, such as the uplift of entire mountain ranges.

Time Scale

• Time scales for Earth's changes can be gradual, happening over millions of years.

• They can also be catastrophic, changing a landscape in a matter of minutes.

• Landforms that seem stable are actually changing over very long geologic time scales.

Multiple Choice

What do the terms "time scale" and "spatial scale" describe in the context of geoscience processes?

• The different speeds and distances over which Earth's surface changes.

• The tools used to measure mountains and oceans.

• The age of the Earth and the size of its continents.

• The temperature and pressure deep inside the Earth.

Multiple Choice

What is the main difference between a gradual and a catastrophic change to the Earth's surface?

• The amount of time the change takes to occur.

• The size of the area that the change affects.

• Whether the change happens on land or in the water.

• The type of rock that is involved in the change.

Multiple Choice

The uplift of a mountain range happens over millions of years, while a single raindrop can dissolve minerals in a rock. What do these examples together imply about landforms?

• Only massive, catastrophic events like mountain uplift can truly change the Earth's surface.

• Landforms like mountains are stable and do not really change over geologic time.

• Both large-scale, long-term processes and small-scale processes contribute to shaping landforms.

• The uplift of a mountain range is a catastrophic event that happens in minutes.

What Is Soil and How Does It Form?

• Soil is Earth's top layer, a mix of minerals, humus, air, and water.

• Humus, from decomposed life, offers nutrients for plants and helps soil retain water.

• Soil forms slowly from weathered rock with the help of living organisms.

Multiple Choice

What is soil primarily composed of?

• A mixture of minerals, humus, air, and water

• Only sand, silt, and clay

• Only decomposed plants and animals

• Only weathered rock and water

Multiple Choice

What is the primary function of humus in soil?

• It breaks down large rocks into small particles.

• It provides nutrients and helps retain water.

• It creates air pockets for plant roots.

• It helps warm the soil for seed germination.

Multiple Choice

If an area has plenty of weathered rock but very few living organisms, what can be concluded about the soil that forms there?

• The soil would lack humus and be poor in nutrients.

• The soil would be unable to hold any water.

• The soil would have no mineral particles.

• The soil would form much more quickly.

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Classifying Soil: Types and Profiles

Soil Types

• Sandy soil feels gritty and drains water quickly, while clay soil is slick and holds water.

• Silty soil feels smooth to the touch and is very rich in essential nutrients.

• Loam is a balanced mixture of sand, silt, and clay, making it ideal for farming.

Soil Profile

• The O Horizon is the top organic layer, and the A Horizon is dark and rich topsoil.

• The B Horizon, or subsoil, is lighter in color because it contains less humus.

• The C Horizon is weathered parent material that rests on top of the solid bedrock.

Multiple Choice

What is the primary reason loam is considered the ideal soil for farming?

• It is a balanced mixture of sand, silt, and clay.

• It is composed of gritty particles that drain very quickly.

• It is made of slick material that holds a lot of water.

• It is the top organic layer made of decaying leaves.

Multiple Choice

Why is the B Horizon, or subsoil, typically lighter in color than the A Horizon?

• The B Horizon contains less humus than the A Horizon.

• The B Horizon is made of weathered parent material.

• The B Horizon is the top organic layer of soil.

• The B Horizon rests directly on top of solid bedrock.

Multiple Choice

A farmer's land has a thick, dark A Horizon, but the soil itself is composed mostly of clay. Why might this land be unsuitable for crops that need good drainage?

• The clay will hold too much water, preventing it from draining.

• The soil will feel too gritty and drain too quickly for roots.

• The dark A Horizon indicates that the soil lacks nutrients.

• The soil is mostly weathered parent material from the C Horizon.

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Common Misconceptions

Misconception	Correction
Weathering and erosion are the same thing.	Weathering breaks down rock, while erosion is the process of moving it.
Earth's surface is static and unchanging.	The surface is constantly changing, but often too slowly for us to notice.
Soil is just dirt.	Soil is a complex system of rock, air, water, and nutrient-rich humus.
All rocks wear away at the same speed.	Harder rocks wear away much more slowly than softer rocks.

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Summary

• Weathering, erosion, and deposition constantly reshape the Earth's surface.

• The sun's energy and Earth's internal heat drive these changes.

• These changes occur at all scales, from microscopic to mountain-building.

• Soil is a vital, layered mixture of minerals and organic matter.

Poll

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

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