Weathering, Erosion, and Deposition

Introduction

Earth's surface is always changing. Mountains crack, cliffs retreat, rivers shift sideways, beaches grow and shrink, and glaciers carry rock across valleys. Three connected processes explain much of this change: weathering, erosion, and deposition. Weathering breaks rock down at or near Earth's surface. Erosion removes and transports weathered material. Deposition happens when transported material settles or is dropped.

You can remember the sequence as break down → move → settle. The three processes often overlap in the same landscape, and the same grain of sediment may be weathered, moved, deposited, picked up again, and moved farther many times.

This aiMOOC is designed for Grades 7–8. You will learn to identify the processes from evidence, explain why they happen, compare the work of water, wind, ice, and gravity, and use models to predict how landscapes change.

Learning Goals

By the end of the course, you should be able to explain the difference between weathering, erosion, and deposition; distinguish physical from chemical weathering; identify major agents of erosion; connect changes in energy to sediment transport and deposition; recognize landforms made by erosion and deposition; interpret photographs and simple models; and propose ways to reduce harmful soil erosion while recognizing that erosion is also a natural Earth process.

The Three Processes at a Glance

Weathering, erosion, and deposition form a connected system. A rock does not have to pass through the three stages only once. Sediment can be stored temporarily on a riverbank or beach, then eroded again during a storm and deposited somewhere else.

Process	What happens?	Does the material move to a new place?	Typical evidence
Weathering	Rock or minerals break down or change at Earth's surface.	Not necessarily	Cracks, rounded edges, dissolved surfaces, rust-colored minerals, loose fragments
Erosion	Rock, soil, or sediment is removed and transported.	Yes	Channels, exposed roots, retreating cliffs, scoured banks, transported sediment
Deposition	Transported sediment settles or accumulates.	Yes, after transport	Sand bars, deltas, dunes, beaches, floodplain layers, moraines

A useful question is: What happened to the material? If it broke down where it was, think weathering. If it was carried away, think erosion. If it accumulated in a new location, think deposition.

Weathering: Breaking Rock Down

Weathering changes rock at or near Earth's surface. It creates smaller particles and altered minerals that can later become sediment. Weathering does not require the material to travel to a different place.

Physical Weathering

Physical weathering, also called mechanical weathering, breaks rock into smaller pieces without changing the basic chemical composition of the minerals. Smaller pieces have more surface area exposed to air and water, so physical weathering can make chemical weathering happen faster.

One important process is frost wedging or freeze–thaw weathering. Liquid water enters a crack. If the water freezes, it expands and can push the crack wider. Repeated freezing and thawing can loosen fragments.

Other physical weathering processes include heating and cooling, the growth of salt crystals in pores and cracks, abrasion when particles scrape rock surfaces, and pressure release that allows rock to fracture. Plant roots can also widen existing cracks as they grow.

Chemical Weathering

Chemical weathering changes minerals through chemical reactions. Water is especially important because it can dissolve substances and carry dissolved chemicals.

Slightly acidic water can dissolve minerals in limestone. Oxygen can react with iron-bearing minerals in a process called oxidation, producing iron oxides that often have reddish or brown colors. Other reactions, including hydrolysis, can turn some minerals into clay minerals.

Physical and chemical weathering can work together. A crack created by physical weathering gives water more access to fresh mineral surfaces, while chemical weathering can weaken rock and make it easier to break.

Living Things and Weathering

Living organisms can contribute to both physical and chemical weathering. Roots may force cracks wider. Lichens and microorganisms can release substances that help alter minerals. Burrowing animals can expose fresh material to air and water. These effects connect the biosphere with the geosphere.

What Controls Weathering Rates?

Weathering rates depend on several interacting factors. Climate matters because temperature and water affect many physical and chemical reactions. Rock type matters because minerals differ in hardness and chemical stability. Surface area matters because smaller fragments expose more material. Time matters because longer exposure usually allows more change. Cracks, slope position, soil cover, vegetation, and local water chemistry can also influence what happens.

When you compare two rocks, avoid saying that one factor always controls the result. In real landscapes, several variables usually act at the same time.

Erosion: Moving Material

Erosion is the removal and transport of rock, soil, and sediment. The main transporting agents are moving water, wind, moving ice, and gravity. Erosion can happen slowly, grain by grain, or rapidly during floods, landslides, storms, and rockfalls.

A transporting agent must supply enough energy or force to move particles. In general, larger or heavier particles require more energy to move than smaller particles. When the transporting agent loses energy, deposition becomes more likely.

Water Erosion

Running water is a major agent of erosion. Raindrops can loosen soil particles. Water flowing across the ground can carry those particles downslope. Concentrated runoff can form rills and gullies, while streams and rivers erode beds and banks.

Rivers do not erode equally everywhere. Flow speed, water volume, channel shape, slope, rock type, vegetation, and sediment load all matter. In many meanders, faster flow near the outside of a bend increases erosion, while slower flow near the inside encourages deposition.

Over long periods, river erosion can deepen valleys and canyons. The landscape records both the resistance of the rock and the history of flowing water.

Wind Erosion

Wind can lift and carry small particles, especially where the ground is dry, loose, and sparsely vegetated. Fine dust can travel far, while sand usually moves closer to the ground by bouncing, rolling, or short jumps.

Wind can remove loose particles from a surface and can cause abrasion when wind-driven grains strike exposed rock. Vegetation often reduces wind erosion by slowing air near the ground and holding soil with roots.

Ice Erosion

Glaciers are masses of ice that move slowly under gravity. As a glacier moves, rock fragments frozen into or dragged beneath the ice can scrape bedrock. Glaciers can also loosen and carry rock material. These processes can widen valleys, polish surfaces, and transport sediment far from its original source.

Gravity and Mass Movement

Gravity constantly pulls material downhill. Mass movement or mass wasting includes rockfalls, landslides, slumps, debris flows, and very slow soil creep. Water often affects slope stability because it can add weight, reduce friction, or change the strength of sediment and rock.

Gravity-driven movement connects erosion and deposition: material leaves one place and commonly accumulates farther downslope.

Coastal Erosion

Waves and currents continually reshape shorelines. Breaking waves can loosen and transport sediment. Storms can rapidly remove beach material or undercut cliffs, while calmer conditions may return sediment to a beach.

A coastline is therefore not a fixed boundary. It is a dynamic zone where erosion, transport, and deposition can shift from one place or season to another.

Deposition: Building New Landforms

Deposition occurs when transported sediment settles or is dropped. This often happens when water, wind, or ice loses the ability to carry its sediment load. Deposition is not simply the end of the story: deposited material may later be eroded again.

Particle size affects settling. In many situations, larger or heavier particles are deposited when transport energy first decreases, while finer particles remain in motion longer. The exact pattern also depends on particle shape, water depth, turbulence, wind, obstacles, and other local conditions.

River Deposition

When a river leaves a steep valley and spreads onto flatter ground, its flow may slow and deposit sediment. Repeated deposition can build a fan-shaped landform called an alluvial fan.

Rivers can also deposit sediment on the insides of meanders, on floodplains during floods, and at their mouths. Where a sediment-rich river enters quieter water such as a lake or sea, a delta may develop if sediment accumulation is greater than the amount removed by waves and currents.

Deltas change over time because river channels shift and because currents, waves, storms, sea level, and human engineering can redistribute sediment.

Wind Deposition and Dunes

Wind deposition happens when wind speed drops or when an obstacle traps moving particles. Sand grains can accumulate into dunes. Dune shape depends on wind direction, sediment supply, vegetation, moisture, and other surface conditions.

Dunes are not necessarily permanent. Their surfaces can erode in one place while sand is deposited in another, causing the dune to migrate or change shape.

Glacial Deposition

Glaciers carry sediment ranging from fine particles to large boulders. When ice melts or loses its ability to transport material, sediment is deposited. A ridge or accumulation of glacial debris is called a moraine.

Glacial deposits can be poorly sorted because ice can transport many particle sizes together. Meltwater from glaciers can also sort sediment as flowing water carries and deposits particles.

Coastal Deposition

Beaches, sand bars, spits, and some barrier landforms can grow where waves and currents deposit sediment. Coastal erosion and deposition are linked: sand removed from one part of a coast may be moved and deposited somewhere else.

Human structures such as groins, jetties, seawalls, and dams can change sediment movement. A structure that causes sand to accumulate in one place can reduce the sediment supply somewhere else, so coastal decisions often involve trade-offs.

From Sediment to Sedimentary Rock

Weathering, erosion, transport, and deposition are major parts of the rock cycle. Weathering creates fragments or dissolved material. Erosion moves sediment. Deposition collects it in basins, river valleys, lakes, deserts, glaciers, and oceans.

If layers of sediment are buried, pressure can compact them. Minerals carried in groundwater can cement particles together. Over time, these processes can form sedimentary rock. Later uplift and renewed weathering may expose that rock again, continuing the cycle.

Reading a Landscape Like a Scientist

A landform is evidence, not a label. Scientists ask what observations support an explanation and whether another process could produce similar evidence.

Use these questions when studying a photograph, map, model, or field site:

Observation: What do you actually see, without explaining it yet?
Sediment: What particle sizes, shapes, or layers are present?
Transport: What agent could have moved the material?
Energy: Where would water, wind, ice, or gravity gain or lose transport ability?
Landform: Which features suggest erosion, deposition, or both?
Evidence: What additional observation would strengthen or challenge your explanation?

For example, an undercut riverbank suggests active erosion, while a nearby gravel bar suggests deposition. Together, the two features show that the river is moving sediment through the landscape.

A Systems View

Weathering, erosion, and deposition connect Earth's major systems. The atmosphere supplies wind and influences temperature and precipitation. The hydrosphere moves water through rivers, waves, rain, and groundwater. The cryosphere stores and moves frozen water. The biosphere affects soil, roots, and vegetation. The geosphere provides the rock, sediment, slope, and landforms being changed.

A change in one part of the system can affect the others. Removing vegetation, for example, can expose soil to raindrop impact and runoff. A wildfire can temporarily increase the amount of loose sediment available for transport. A dam can trap river sediment, reducing the amount that continues downstream. These examples show why Earth-surface processes are best understood as interacting systems rather than isolated events.

Human Impacts and Erosion Management

Erosion is natural, but human activities can increase or redirect it. Removing plant cover, disturbing soil, building roads on steep slopes, changing river channels, and altering coastal sediment pathways can increase erosion in some settings.

Management does not mean stopping every natural process. The goal is often to reduce harmful soil loss or protect infrastructure while allowing rivers, beaches, and ecosystems to function. Depending on the site, strategies may include keeping soil covered with vegetation, stabilizing disturbed ground, slowing runoff, using contour planting, protecting riparian vegetation, restoring wetlands, or changing where structures are built.

Good solutions are based on evidence. A method that works on a farm field may not work on a beach, and a structure that protects one location may change deposition or erosion somewhere else.

Interactive Tasks

Quiz: Test Your Knowledge

Which statement best describes weathering? (The breakdown or alteration of rock at or near Earth's surface) (!The movement of sediment from one location to another) (!The settling of transported sediment in a new location) (!The melting of rock below Earth's surface)

Which process includes the transport of sediment? (Erosion) (!Weathering) (!Crystallization) (!Compaction)

When is deposition most likely to occur? (When the transporting agent loses enough energy for sediment to settle) (!When rock melts into magma) (!When a mineral becomes harder) (!When Earth's core cools rapidly)

Which example is physical weathering? (Water freezes in a rock crack and widens it) (!Oxygen reacts with iron-bearing minerals) (!Acidic water dissolves limestone) (!Minerals react with water to form clay)

Which example is chemical weathering? (Oxygen reacts with iron-bearing minerals) (!A boulder splits after repeated freezing) (!Sand grains scrape a cliff) (!A rock breaks during a rockfall)

In a typical river meander, where is deposition most likely? (On the inside of the bend where flow is slower) (!On the outside of the bend where flow is faster) (!Only at the river source) (!Only beneath a waterfall)

Why can an alluvial fan form where a stream leaves a steep valley? (The stream loses transport energy and deposits sediment) (!The stream suddenly becomes salt water) (!The valley floor melts into magma) (!The sediment turns directly into ice)

What landform is commonly built by wind deposition? (Sand dune) (!Volcanic crater) (!Fault scarp) (!Lava plateau)

What is a moraine? (An accumulation of sediment deposited by a glacier) (!A crack produced only by an earthquake) (!A deep ocean trench) (!A layer of molten rock)

How can vegetation often reduce soil erosion? (Roots hold soil while stems and leaves slow water and wind near the surface) (!Roots turn all sediment into bedrock) (!Leaves prevent all rainfall from reaching the ground) (!Plants stop gravity from acting on slopes)

Memory Game

Weathering	Breakdown or alteration of rock at Earth's surface
Erosion	Removal and transport of Earth material
Deposition	Settling or accumulation of transported material
Frost wedging	Expansion of freezing water that widens rock cracks
Oxidation	Reaction of oxygen with minerals
Abrasion	Scraping and wearing by moving particles
Delta	Sediment-built landform near a river mouth
Moraine	Accumulation of debris left by moving or melting ice

Drag and Drop

Match the correct terms.	Topic
Mechanical weathering	Breaks rock without changing its basic mineral composition
Chemical weathering	Changes minerals through chemical reactions
Erosion	Carries rock, soil, or sediment away from a location
Deposition	Drops transported material when carrying ability decreases
Sediment	Loose particles that can be transported and deposited

...

Crossword Puzzle

Weathering	What process breaks down or alters rock at Earth's surface?
Erosion	What process removes and transports Earth material?
Deposition	What process causes transported material to settle?
Sediment	What is the loose material that can be moved and deposited?
Oxidation	What reaction involving oxygen can chemically weather minerals?
Moraine	What glacial landform is made from deposited debris?

LearningApps

Cloze Text

Weathering

or alters rock at Earth's surface. Erosion

weathered material from one place to another. Deposition happens when sediment

after transport. Freeze–thaw action is a form of

weathering. Reactions that alter minerals are called

weathering. Moving water, wind, ice, and gravity can act as agents of

. A river may deposit sediment when its flow

. Wind deposition can build a

. Debris deposited by a glacier can form a

. Together, these processes continually reshape Earth's

.

Open-Ended Tasks

Easy

Process Photo Hunt: Find or take three safe photographs showing possible weathering, erosion, or deposition, label each process, and write one observation that supports every label.
Sediment Storyboard: Create a six-panel comic that follows one grain of sand from weathering through erosion to deposition and shows which agent moves it.
Rock Surface Sketch: Draw a close-up of a weathered rock or building stone and annotate cracks, color changes, loose fragments, or other evidence without claiming a cause that you cannot observe.
Local Knowledge Interview: Interview an adult about a nearby place where a riverbank, slope, field, trail, or beach has changed, then separate the person's observations from possible scientific explanations.

Standard

Stream Table Investigation: Build a small stream model with sand or soil in a tray, change one variable such as slope or water flow, and record where erosion and deposition increase.
Schoolyard Erosion Map: Survey a safe outdoor area, map signs of runoff or soil movement, identify likely causes, and suggest one practical improvement supported by evidence.
Particle Transport Test: Compare how flowing water moves two or more particle sizes, keep the test conditions as constant as possible, and explain how your results relate to transport energy.
Meander Explanation Video: Produce a two-minute video or narrated animation that explains why erosion and deposition can occur on different sides of a river bend.

Advanced

Erosion Control Design: Design and test two methods for reducing sediment loss in a model slope, collect comparable data, and justify which design works better and why.
Landscape Change Comparison: Compare two maps or aerial images of the same river, delta, coast, or glacier from different times, identify evidence of change, and distinguish observations from interpretations.
Field Evidence Report: Visit a safe local landform with permission, document at least five pieces of evidence, infer the roles of weathering, erosion, and deposition, and discuss uncertainty in your explanation.
Earth Surface Documentary: Create a five-minute documentary or multimedia presentation connecting one real landscape to weathering, transport, deposition, the rock cycle, and one human decision about erosion management.

Learning Assessment

Process Diagnosis: Given an unfamiliar landscape photograph, identify at least two observations and use them to argue whether weathering, erosion, deposition, or a combination of processes is occurring.
Variable Reasoning: Predict how increasing slope or water flow would affect sediment transport in a model, then explain which variables you would need to keep constant for a fair test.
River Bend Transfer: Use the relationship between flow speed, erosion, and deposition to predict how a meander could shift sideways over time and explain what evidence would support your prediction.
Coastal Trade-Off: Evaluate a proposal to build a structure that traps beach sand, explaining one possible benefit and one possible downstream effect on erosion or deposition.
Glacier Evidence: Compare a photograph of a moraine with a photograph of a river delta and explain how different transporting agents can produce different deposited landforms.
Systems Explanation: Explain how a change in vegetation after fire or land clearing could affect runoff, erosion, sediment transport, and deposition farther downslope.

Evidence of Learning

Area	Evidence
Knowledge	You accurately distinguish weathering, erosion, transport, and deposition; identify physical and chemical weathering; and describe the roles of water, wind, ice, and gravity.
Scientific skills	You make careful observations, classify evidence, control variables in simple models, record results, compare patterns, and explain uncertainty.
Products	Your maps, diagrams, photographs, experiment records, reports, models, and videos communicate a clear process-based explanation.
Reasoning	You connect energy, particle movement, slope, water flow, rock properties, vegetation, and landform change rather than relying on memorized labels alone.
Transfer	You can apply the same ideas to an unfamiliar river, coast, desert, glacier, schoolyard, construction site, or agricultural landscape and propose evidence-based erosion management.

OERs on the Topic

The following resources provide reliable further reading and freely accessible educational material:

National Park Service: Weathering and Erosion: Clear explanations of physical and chemical weathering and erosion.
National Park Service: Teaching Resources—Erosion: Classroom resources and field-based learning ideas.
U.S. Geological Survey: What are sedimentary rocks?: Connects weathering, transport, deposition, compaction, and cementation.
National Geographic Education: Weathering: Background reading on weathering agents and landscape change.
National Geographic Education: Erosion: Background reading on erosion and deposition.

For connected encyclopedia reading, explore Erosion, Sediment, deposition, Alluvial fan, Delta, Dune, and Moraine.

Linked Learning Areas

Weathering, Erosion, and Deposition

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