15.1 Weathering, Erosion, Deposition & Soil Horizons

Weathering, Erosion, Deposition & Soil Horizons

Quick Answer: The sediment cycle sculpts Earth's crust in three continuous phases: weathering breaks rock down in place (in situ), erosion transports sediment via water, ice, wind, or gravity, and deposition drops sediment when fluid velocity declines. Mechanical weathering fractures rock physically without changing mineral composition, multiplying surface area; chemical weathering decomposes minerals through oxidation, hydrolysis, and carbonation (favored in warm, humid climates). Over time, weathered rock mixes with organic humus to form stratified soil horizons ($O, A, E, B, C, R$).

The HiSET Science subtest tests your ability to distinguish rock breakdown from sediment transport, evaluate how fragmentation accelerates chemical weathering, identify depositional landforms by sediment sorting, and analyze vertical soil profiles.

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The Sediment Cycle: Weathering vs. Erosion vs. Deposition

Earth's crust is continuously transformed by three sequential exogenic processes:

1. Weathering: The physical disintegration or chemical decomposition of rocks and minerals at or near Earth's surface in situ (no transport involved).
2. Erosion: The removal and kinetic transport of weathered rock clasts, soil, and ions by mobile agents (liquid water, glacial ice, wind, or gravitational mass wasting).
3. Deposition: The dropping or precipitation of transported sediments when the transporting medium loses velocity and carrying capacity.

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Mechanical vs. Chemical Weathering

Mechanical (Physical) Weathering

Mechanical weathering shatters rock into smaller fragments without altering mineral chemistry:

• Frost Wedging (Freeze-Thaw): Water enters rock fissures. Upon freezing at $0^\circ\text{C}$, water expands by $9%$ in volume, exerting outward pressure exceeding $200\text{ MPa}$. Repeated freeze-thaw cycles pry rock apart, forming piles of angular debris called talus slopes.
• Abrasion: Physical grinding of rock surfaces. Windblown sand polishes stones into ventifacts, river currents tumble gravel into rounded pebbles, and glaciers gouge linear striations in bedrock.
• Exfoliation (Pressure Release): Deep intrusive granite batholiths expand upward as overlying strata erode away, fracturing into concentric sheets that peel off like onion layers.
• Biological Physical Action: Expanding plant roots (root wedging) pry open rock joints.

The Surface Area Principle

Mechanical fracturing dramatically accelerates chemical weathering rates. Consider a solid rock cube measuring $10\text{ cm}$ on each edge:

• Initial surface area: $6 \times (10\text{ cm})^2 = 600\text{ cm}^2$
• Fracturing into 1,000 smaller cubes ($1\text{ cm}$ each) yields: $1{,}000 \times [6 \times (1\text{ cm})^2] = 6{,}000\text{ cm}^2$

While volume remains unchanged ($1{,}000\text{ cm}^3$), exposed surface area increases tenfold, providing ten times more reactive contact area for chemical attack.

Chemical Weathering Mechanisms

Chemical weathering transforms unstable minerals into stable surface compounds:

• Oxidation: Reaction of dissolved oxygen ($O_2$) with ferrous iron ($Fe^{2+}$) in minerals, forming ferric iron oxides like hematite ($Fe_2O_3$). This "rusting" weakens the rock matrix.
• Hydrolysis & Carbonation: Rainwater absorbs atmospheric $CO_2$ to form dilute carbonic acid ($H_2CO_3$):

$H_2O + CO_2 \rightleftharpoons H_2CO_3$

Carbonic acid dissolves calcite ($CaCO_3$) in limestone, carving caverns and sinkholes (karst topography):

$CaCO_3 + H_2CO_3 \longrightarrow Ca^{2+} + 2HCO_3^-$

Hydrolysis also alters feldspar in granite into insoluble kaolinite clay.

• Climate Controls: Chemical weathering thrives in warm, humid climates (heat and water accelerate kinetics). Mechanical weathering dominates in cold or arid climates with frequent freeze-thaw cycling.

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Agents of Erosion & Depositional Sorting

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Soil Horizons: Soil Profile Anatomy

Soil forms through the interaction of climate, organisms, relief/topography, parent material, and time (CLORPT). Percolating water and biota differentiate mature soil into distinct layers:

• O Horizon (Organic Layer): Surface layer of fresh and decomposing plant litter, leaves, and dark humus.
• A Horizon (Topsoil): Dark, fertile blend of mineral particles and decayed humus; zone of intense biological activity.
• E Horizon (Zone of Eluviation): Pale, bleached horizon where acidic water leaches out organic matter, clays, and iron oxides (eluviation), leaving quartz sand.
• B Horizon (Subsoil / Zone of Illuviation): Dense accumulation zone where clays, aluminum oxides, and iron compounds washed down from upper layers collect (illuviation).
• C Horizon (Substratum / Regolith): Partially weathered parent rock with minimal organic matter.
• R Horizon (Bedrock): Solid, unweathered parent bedrock.

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Common HiSET Pitfalls & Exam Traps

[!CAUTION] Trap 1: Weathering vs. Erosion. Weathering breaks rock in place. Erosion transports sediment. Ice cracking a boulder is weathering; river water washing the sand downstream is erosion.

[!WARNING] Trap 2: Glacial vs. Fluvial Sorting. Fluvial deposits (deltas) are strictly sorted by grain size. Glacial till (moraines) is unsorted and unstratified because melting ice drops all grain sizes simultaneously.

[!NOTE] Trap 3: Eluviation vs. Illuviation. Remember: Eluviation = Exiting (leached out of the E horizon); Illuviation = Into (accumulating in the B horizon).