Weathering, Erosion & Deposition · Grade 9 Science

Weathering, Erosion & Deposition

Learning goals

• I can distinguish physical (mechanical) weathering from chemical weathering.
• I can explain how erosion transports sediment and what causes deposition.
• I can connect weathering, erosion, and deposition to landforms shaped by New York's glacial history.

Readings for this unit

• What happens to rain after it reaches the ground?

Weathering is the slow breaking-down of rock right where it sits — nothing is carried away yet, the rock is just being taken apart in place. It comes in two flavors, and the difference between them is whether the rock's actual chemistry changes. Physical (or mechanical) weathering changes only a rock's size and shape, not what it is made of.

The most powerful physical weathering agent in a place like New York is frost wedging: water trickles into a crack, freezes, and — because water expands by about 9 percent when it turns to ice — pushes outward with surprising force, prying the crack a little wider with every freeze. Repeat that through hundreds of winters and it can split a boulder clean apart. Plant roots wedge into cracks and widen them the same way as they grow (root wedging), and rock tumbling against rock in a river or a rockslide grinds down surfaces through abrasion. All of it makes rock smaller without changing the mineral it is made of.

Chemical weathering, by contrast, actually rearranges what the rock is made of. Rainwater is naturally a little acidic, and over long spans of time it slowly dissolves limestone — the very same process that hollows out caves underground. Oxygen in air and water attacks iron-bearing minerals and rusts them (oxidation), which is why some rocks and soils are stained a red-brown color, and water can dissolve certain minerals away entirely. Warm, wet climates speed chemical weathering up; cold or dry ones slow it down.

Once rock has been broken into loose sediment, erosion is the step that picks it up and carries it somewhere new. The movers are gravity (landslides and slow downhill creep), running water (rivers, rain, crashing waves), wind, and ice (glaciers grinding forward). It helps to think of erosion as the moving truck of the rock cycle: it doesn't make any new sediment, it just transports what weathering already loosened, usually downhill and eventually toward the sea.

Every moving agent has a limited budget of energy, and the moment it slows down and runs low, it can no longer hold onto its load — so it drops it. That dropping is deposition. A rushing river that suddenly widens into a calm lake loses its energy and dumps its sediment, often piling up a fan-shaped delta; slowing wind lets its sand settle into dunes; a melting glacier simply lowers its cargo of rock to the ground as the ice disappears. Together, this trio — weathering, erosion, and deposition — has been quietly sculpting the land around you for ages. The very hills, ridges, and lake-dotted valleys of Westchester County and the Hudson Valley were carved out and then piled up by glaciers that advanced and retreated across the region during the last Ice Age.

Weathering breaks or changes rock where it is; erosion transports the products; deposition occurs when transport capacity falls. One place can experience all three over time. Label the process based on what happens to the material, not just the landscape's appearance.

Water, wind, ice and gravity sort and move particles differently. A slowing stream commonly drops coarser sediment first, but particle density and shape also matter. Compare observations with stated flow conditions rather than assuming every deposit follows one exact size rule.

Vegetation, slope, rainfall and land use affect erosion. Roots can stabilize soil, while removing cover can increase runoff. Test a proposed intervention on equal slopes and soil amounts, collect repeated runoff measurements and report whether a model tray represents a full watershed.

Build your understanding

Plan three teaching sessions: core explanation and Parts 1–2; Parts 3–4 with diagrams and reasoning checks; then the whole-unit application, worked examples, independent practice, and exit check. Read one part at a time, explain it without notes, and revisit any gap. Allow about 40–50 minutes per session; adjust after checking understanding.

Part 1 · Distinguish changes in size from composition

Physical weathering changes size or shape without requiring a new mineral identity; chemical weathering changes minerals through reactions. Splitting a cube can expose surfaces previously inside it. A 2 cm cube has volume 8 cm³ and surface area 24 cm². Eight separated 1 cm cubes retain total volume 8 cm³ but expose 48 cm². Extra available area can increase contact with water or reactive gases, affecting reaction rate. This link depends on exposure and conditions: tightly packed fragments do not expose all modeled faces equally.

Read the visual model

Compare equal total volumes and count all six faces of each separated cube. The small cubes are not a different amount of material.

Why can physical breakdown accelerate chemical change without itself being a chemical reaction?

Visual reasoning — after your attempt

It can expose more reactive surface while retaining the mineral’s identity during the breakup. Subsequent reactions may then proceed over a larger available area.

Physical weathering changes size or shape; chemical weathering alters minerals through reactions. Both can operate together. Cracks increase exposed surface area, which can affect chemical reaction rates. Temperature, water, mineral type and exposure time affect rates; not every rock responds identically.

Breaking one cube into smaller cubes increases total exposed area without changing total material volume. The extra area can expose more mineral to water, connecting a physical change to a chemical process.

Pause and explain

Why can physical breakdown accelerate chemical weathering?

Check your reasoning after answering

It exposes more surface and pathways for reactions without itself requiring a composition change.

Part 2 · Explain transport through energy and material properties

Bed load moves along the bed by rolling, sliding or short hops; suspended load is carried within the water; dissolved load consists of chemical species in solution. A grain’s size, density and shape, the flow and interactions with other grains affect its transport and settling. Increasing flow can enable different transport, but a single universal speed threshold is not implied. Identify the pathway in the evidence: clear water can still carry dissolved material, and cloudy water need not have a large total transported mass if discharge is small.

Read the visual model

Compare markers near the bed with those within the water. The arrow indicates the selected flow direction, not a speed measurement.

Why should equal-size settling comparisons state particle density and shape?

Visual reasoning — after your attempt

Density contrast and resistance to motion depend on material and shape. Equal nominal size alone does not guarantee the same settling behavior or transport threshold.

Erosion involves removal and movement by water, wind, ice or gravity. Transport depends on fluid speed, particle size, density, shape and other conditions. A flowing stream can carry dissolved material, suspended particles and bed load simultaneously; transport evidence requires identifying the pathway.

When flow slows, its ability to transport some particles decreases. Larger or denser particles may settle sooner under the same conditions, but shape and turbulence can alter the simple size ordering.

Pause and explain

Why should a settling prediction state particle density and shape?

Check your reasoning after answering

They affect motion and settling along with size; size alone is not a complete model.

Part 3 · Link deposition to a changing environment

Deposition is accumulation of transported material, not necessarily a complete stop in fluid motion. Where a stream spreads into a basin, flow conditions change and some grains may settle while water continues through the system. Coarser or denser grains can behave differently from fine particles or dissolved material. Layering, sorting and structures can help test a depositional-environment interpretation, but sediment supply and later changes matter too. Explain what changed in transport capacity for the selected material rather than using deposition as a synonym for no motion.

Read the visual model

Trace water into the wider basin and identify the accumulating sediment wedge. Width and arrow lengths indicate a qualitative contrast only.

Why could some grains settle while fine material continues farther into the basin?

Visual reasoning — after your attempt

Different grain sizes, densities and shapes have different settling/transport behavior under the changed flow conditions. Depositing one fraction does not stop all water or all material transport.

Deposition occurs when transported material accumulates. Channel changes, entry into still water, reduced wind speed or melting ice can change transport conditions. Sorting and sediment structures can support an environmental inference but should be tested with multiple observations.

A river entering a lake can lose speed and deposit sediment. Coarser deposits near the entry and finer sediment farther out may support a gradient in transport, subject to flow and grain conditions.

Pause and explain

Does deposition mean that all water motion has stopped?

Check your reasoning after answering

No. Material can accumulate when transport conditions change even while water continues moving.

Part 4 · Use controls to test a landscape explanation

A runoff comparison should measure water volume as well as collected sediment mass. Here the bare tray yields 8 g in 80 mL, or 0.10 g/mL, while the covered tray yields 6 g in 40 mL, or 0.15 g/mL. The covered case exports less total mass but the collected water is more concentrated. These are distinct outcomes. Matched slope, rain input, material and collection time are needed to evaluate a cover effect. Real vegetation can alter infiltration, soil binding and storage, so a classroom tray models only selected processes.

Read the visual model

The upper bars measure volume and the lower bars mass. They are different units; do not compare their heights as one physical quantity.

Which tray exports less total sediment and which has the greater calculated concentration?

Visual reasoning — after your attempt

Covered exports less mass, 6 g rather than 8 g, but has greater concentration, 0.15 rather than 0.10 g/mL. State which outcome the design criterion concerns.

A classroom erosion comparison needs the same slope, applied water, material and collection method except for the variable being tested. Vegetation can affect runoff, infiltration and soil cohesion; a simplified tray represents only selected processes. Record both sediment and water so reduced runoff is not confused with reduced sediment concentration.

If one tray receives less water, a smaller sediment output cannot isolate the effect of plant cover. Compare outputs over equal rainfall input and explain remaining differences.

Pause and explain

Why measure runoff volume as well as sediment mass?

Check your reasoning after answering

Sediment mass and concentration answer different questions; volume helps distinguish transport amount from water amount.

Apply the whole unit · A controlled erosion comparison

Illustrative equal-rainfall trials: bare tray yields 2.0 L runoff and 12 g sediment; covered tray yields 1.0 L and 3 g.

Your task

Calculate sediment concentration for each tray, compare total loss and runoff, and explain what the result supports and what an actual experiment would need to control.

What your response should show

• Use the supplied evidence accurately and label units or locations.
• Explain a physical mechanism or evidence-to-claim relationship.
• Show calculations or a clearly labeled model when the task calls for one.
• Identify a meaningful assumption, uncertainty, or competing explanation.

Response guidance — after your attempt

Bare concentration = 6 g/L; covered = 3 g/L. The covered tray has less runoff, total sediment and concentration. Equal rain supports a comparison, but slope, soil, cover type and repeated trials are needed to isolate causes and generalize.

Scientific references and adapted source material

• USGS — groundwater flow and the water cycle
• OpenSciEd 6.4 — public material examples
• OpenSciEd middle-school material — CC BY 4.0 license

Parent / teacher teaching notes

Use supplied data or a parent-approved contained model. Mark units, equal-input logic and limits; do not accept less water proves less weathering without identifying the measured process. Ask the learner to explain each visual label and causal step before calculating. Preserve a first model, attempt the changed independent case, then use feedback to revise. Diagrams state whether values are illustrative or observed. Selected OpenSciEd excerpts are middle-school source material used for supported foundations; the independent high-school explanations and supplements are not the complete OpenSciEd teacher editions or an endorsement.

Deposition (Accumulation) Rate

amount deposited = rate × elapsed time

rate is how fast sediment builds up (for example, meters per 100 years for a glacial moraine); multiply by elapsed time to find the total thickness or amount deposited.

Worked examples

Avoid these mistakes

• Weathering and erosion are different processes even when they occur together.

Words to know

weathering

The breakdown of rock into smaller pieces in place, without being moved, caused by physical forces or chemical reactions.

erosion

The transport of weathered sediment from one location to another by water, wind, ice, or gravity.

deposition

The settling and dropping of transported sediment when the agent carrying it — water, wind, or ice — loses energy and slows down.

sediment

Small, loose pieces of rock, mineral, or organic material produced by weathering.

Differential weathering carves dramatic landscapes

When a rock formation contains layers or sections of differing hardness, weathering doesn't wear them down evenly — softer rock erodes away faster, while harder, more resistant rock is left standing, a process called differential weathering. This is exactly how features like hoodoos (tall, thin rock spires), natural arches, and even waterfalls form: a resistant cap rock protects softer rock underneath until undercutting eventually causes collapse. Niagara Falls is a spectacular local example — a hard dolostone caprock protects softer shale beneath it, and as the softer rock erodes away, the waterfall's edge slowly retreats upstream.

Climate controls which kind of weathering wins

Physical and chemical weathering don't happen at equal rates everywhere — climate tips the balance. Chemical weathering reactions speed up dramatically with heat and moisture, which is why it dominates in hot, humid, tropical climates, sometimes breaking rock down to great depths. Physical weathering, especially frost wedging, dominates in cold climates with frequent freezing and thawing, like much of the Northeast United States in winter. This means a single rock type could weather completely differently depending on where on Earth it ends up — a direct link between this unit and the climate patterns covered elsewhere in this course.

The Hudson River: technically a fjord

By strict geologic definition, a fjord is a valley carved by a glacier that was later flooded by the sea — and the lower Hudson River, from roughly Troy down to New York Harbor, fits that definition. Ice age glaciers gouged the Hudson's valley far deeper than sea level at the time; when the glaciers melted and sea level rose, seawater flooded back into that deep, glacially-carved channel, making the lower Hudson a true, tidal, saltwater-influenced fjord — one of the few on the entire Eastern Seaboard, and part of why the river's flow near NYC is influenced by ocean tides all the way up to Troy.

Long Island: a giant pile of glacial leftovers

Long Island isn't bedrock at all — nearly the entire island is a depositional landform, built from debris that glaciers bulldozed forward and dumped as they advanced and then retreated at the end of the last ice age. Long Island's twin forks and its hilly spine are terminal and recessional moraines (ridges of unsorted rock and sediment dropped right at a glacier's edge), while the flatter land to the south is an outwash plain, built from sediment-choked meltwater spreading finer material out ahead of the ice. In other words, the entire island is a monument to deposition.

In the real world

The moraines, drumlins, and river valleys across Westchester County and the Hudson Valley are direct evidence of glacial erosion and deposition from the last Ice Age, shaping the hills and lakes we see in the region today.

Exit check

Separate weathering, erosion, and deposition in one river example.