15.2 Weathering, Erosion, and Deposition

Still in Domain IV.B.1, Praxis 5442 expects you to separate weathering from erosion, classify weathering processes, connect agents (water, ice, wind) to landforms, and reason about landslide hazards and mitigation. Teaching items often hinge on whether material was broken in place or moved.

Weathering vs. Erosion vs. Deposition

Exam trap: "A river carving a canyon is only weathering." Carving a canyon requires weathering plus erosion (and eventual deposition downstream). Weathering prepares loose material; erosion moves it; deposition builds deltas, beaches, and floodplains.

Types of Weathering

Physical (mechanical) weathering

Physical weathering breaks rock into smaller pieces without changing chemical composition. Surface area increases, which later speeds chemical attack.

Common mechanisms:

• Frost wedging: water freezes in cracks, expands (~9%), and pries rock apart—common in climates with freeze–thaw cycles.
• Abrasion: rock particles scrape and grind other rock (stream bedload, windblown sand, glacial ice).
• Exfoliation / unloading: overlying rock removed; pressure release causes sheets to peel (seen in some granitic domes).
• Thermal expansion (localized) and plant-root wedging (often classed with biological weathering) also crack rock.

Chemical weathering

Chemical weathering changes the minerals themselves through reactions with water, oxygen, acids, or other chemicals.

Warm, wet climates favor rapid chemical weathering; cold or arid climates often emphasize physical processes. Organic acids from soils amplify chemical breakdown.

Biological weathering

Biological weathering is breakdown aided by living organisms: root wedging, burrowing animals exposing fresh surfaces, lichens and microbes producing acids. It overlaps physical and chemical pathways—what matters on the exam is recognizing organisms as agents that accelerate rock breakdown.

Agents of Erosion: Water, Ice, and Wind

Waves along coastlines are a specialized water agent: they undercut cliffs, sort beach sand, and build or remove barrier features depending on storm energy and sediment supply.

Deposition — When Transport Energy Drops

Deposition begins when a transporting agent loses the energy needed to keep sediment moving. Coarser, denser particles settle first; finer silt and clay travel farther. That sorting explains many classroom landform examples:

• Deltas form where a river enters a quiet lake or ocean and drops its load.
• Alluvial fans spread at mountain fronts where steep streams suddenly flatten.
• Floodplains accumulate mud and sand during overbank floods.
• Moraines and unsorted till mark where glacial ice melts and dumps debris.
• Dunes and loess record wind deposition of sand versus fine dust.

Praxis teaching stems often show a labeled map (fan at a canyon mouth; spit along a coast) and ask which process dominated—erosion, transport, or deposition. The correct choice hinges on whether material was being picked up, moved, or dropped.

Valleys, Canyons, and Coastlines

Stream valleys typically begin as narrow, V-shaped cuts where downcutting dominates. Over time, lateral erosion widens floodplains. Canyons are deep, steep-walled valleys where a river cuts downward faster than walls can weather back—often in resistant rock and/or uplifted terrain (think arid plateau rivers).

Coastlines record a contest among wave energy, tides, sediment supply, and rock resistance:

• Headlands erode; bays may fill with sediment.
• Beaches and spits are depositional; sea cliffs and wave-cut platforms are erosional.
• Human structures (seawalls, groins) redistribute erosion rather than "stopping" coastal change permanently.

Glacial valleys contrast with stream valleys: ice scours a broader U-shape, often with truncated spurs and polished, striated bedrock.

Landslides — Prediction and Mitigation

A landslide (a form of mass wasting) occurs when gravity overcomes the strength of a slope. Praxis items may ask what increases risk or which mitigation strategy fits a scenario.

Factors that raise landslide likelihood:

• Steep slopes and undercutting (roads, rivers, waves)
• Saturation from heavy rain or rapid snowmelt (adds weight, reduces friction)
• Weak materials (clay layers, deeply weathered rock)
• Removal of vegetation that binds soil
• Earthquakes or human blasting that shake slopes

Retaining walls appear frequently as a concrete mitigation example: they do not stop weathering, but they reduce the chance that eroded or saturated material will fail catastrophically onto a roadway or building. Prediction combines geologic mapping, slope-angle analysis, rainfall thresholds, and monitoring of cracks or tilt—not a single "landslide forecast" like a daily weather map.

Classroom Scenario Pattern

Students photograph a crumbled hillside after a storm and label it "chemical weathering only." A strong instructional move asks: Which evidence shows breakdown in place (weathering)? Which evidence shows movement (erosion/mass wasting)? What role did water play (saturation, runoff)? Connecting observations to the weathering–erosion–deposition sequence builds SEP skills (analyzing data, constructing explanations) on a high-frequency Earth science distinction.

Quick Self-Check Before You Continue

• Weathering breaks down; erosion moves; deposition drops sediment.
• Physical = size change; chemical = composition change; biological = organism-aided.
• Water, ice, and wind leave different landform signatures (V vs. U valleys; dunes; wave-cut cliffs).
• Landslide risk rises with steepness, water, weak materials, and vegetation loss; retaining walls and drainage are key mitigations.