2.3 Weathering, Erosion, Deposition & Soil

2.3 Weathering, Erosion, Deposition & Soil

Earth's surface is constantly reshaped by external geological processes driven by solar energy and gravity. These surface processes—collectively known as exogenic processes—include weathering, erosion, deposition, and soil formation. For the Praxis 5005 exam, candidate teachers must master the precise scientific distinctions between these processes and understand how soil horizons develop.

Weathering vs. Erosion vs. Deposition

Elementary students frequently confuse weathering and erosion. Geologists distinguish these three sequential stages as follows:

• Weathering: The physical breakdown or chemical alteration of rocks in situ (at or near Earth's surface, without movement).
• Erosion: The physical removal and transport of weathered rock fragments and soil particles from one location to another by mobile agents (water, wind, ice, or gravity).
• Deposition: The process by which eroded sediments settle out and drop to the surface when the transporting agent loses kinetic energy.

Physical (Mechanical) Weathering

Physical weathering (also called mechanical weathering) breaks solid rock into smaller fragments without altering the rock's chemical or mineralogical composition. Physical weathering increases the overall surface area of the rock, accelerating subsequent chemical weathering.

Key Physical Weathering Mechanisms

1. Frost Wedging (Freeze-Thaw): Water seeps into fractures and pores in rock. As water freezes into ice, it expands by approximately 9% in volume, exerting immense pressure (up to 2,000 kg/cm²) against fracture walls. Repeated freeze-thaw cycles wedge rock fractures open, eventually shattering the rock into angular scree/talus slopes at mountain bases.
2. Exfoliation (Unloading / Pressure Release): Deeply buried plutonic igneous rocks (like granite batholiths) form under intense confining pressure. As overlying rock erodes away, the reduced pressure causes the exposed granite to expand outward, fracturing into thin concentric curved sheets that peel away like onion layers (e.g., Enchanted Rock in Texas, Half Dome in Yosemite).
3. Root Wedging (Biological Activity): Plant roots grow into existing rock cracks in search of moisture. As roots grow thicker, they exert outward mechanical force, widening cracks and splitting rock.
4. Thermal Expansion: Daily heating by sunlight causes rock surfaces to expand, while nightly cooling causes contraction. In arid deserts with extreme diurnal temperature ranges, differential expansion between different mineral grains eventually causes surface spalling.

Chemical Weathering

Chemical weathering transforms original rock minerals into new chemical compounds through chemical reactions with atmospheric gases, rain, and groundwater. Warm, humid tropical climates accelerate chemical weathering, whereas cold or arid climates retard it.

$\text{Carbonic Acid Formation: } \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{H}_2\text{CO}_3$ $\text{Limestone Dissolution: } \text{CaCO}_3 + \text{H}_2\text{CO}_3 \rightarrow \text{Ca}^{2+} + 2\text{HCO}_3^{-}$

Agents of Erosion & Depositional Landforms

Erosion transports weathered particles across Earth's surface. As transporting agents slow down or lose energy, sediment is deposited, creating characteristic landforms.

1. Water (Fluvial & Coastal Erosion)

Running water is the single most dominant agent of erosion on Earth.

• Erosional Features: High-gradient mountain streams cut deep V-shaped valleys and river canyons through hydraulic action and abrasion (e.g., Grand Canyon). Meandering rivers cut outer banks (cut banks) while depositing sediment on inner banks (point bars).
• Depositional Features:
  • Deltas: When a sediment-laden river enters a quiet body of water (ocean or lake), its velocity abruptly drops to zero. Suspended sediments settle, forming fan-shaped depositional deltas (e.g., Mississippi River Delta).
  • Alluvial Fans: Fan-shaped sediment deposits formed where steep, narrow mountain streams emerge abruptly onto flat valley floors in arid regions.

2. Wind (Aeolian Erosion)

Wind erosion is most active in dry deserts and coastal areas lacking vegetation cover.

• Deflation: Wind lifts and removes fine silt and clay particles, leaving behind a coarse lag deposit of pebbles called desert pavement.
• Depositional Features: Sand dunes (barchan, transverse dunes) formed when wind-driven sand encounters obstacles, and loess—thick blanket deposits of fine, wind-blown silt that create extremely fertile agricultural soils (e.g., Midwest U.S., Loess Plateau in China).

3. Ice (Glacial Erosion)

Glaciers are massive rivers of ice moving slowly downhill under the influence of gravity.

• Erosional Features: Alpine glaciers scour V-shaped river valleys into steep, flat-bottomed U-shaped valleys, leaving behind sharp mountain peaks (horns), bowl-shaped basins (cirques), and steep ridges (arêtes).
• Depositional Features: Glaciers deposit unsorted mixtures of rock debris called till. Unsorted till builds ridge landforms called moraines (terminal, lateral, and medial moraines) and tear-drop shaped hills called drumlins.

4. Gravity (Mass Wasting)

Gravity acts continuously on all slopes, driving mass wasting—the downslope movement of rock and soil under direct gravitational influence.

• Fast mass movement: Landslides, rockfalls, debris flows, and mudslides (often triggered by heavy rainfall or earthquakes).
• Slow mass movement: Soil creep, the extremely slow (millimeters per year) downhill movement of soil evidenced by curved tree trunks, tilted fence posts, and retaining wall displacement.

Soil Horizons & Composition

Soil is a dynamic natural body composed of mineral particles, organic matter, water, and air, capable of supporting plant life. Soil forms over hundreds to thousands of years through the interaction of five factors: parent material, climate, organisms, topography, and time.

The Four Essential Soil Components

1. Mineral Matter (45%): Weathered rock particles classified by size into sand (0.05–2.0 mm), silt (0.002–0.05 mm), and clay (<0.002 mm). The relative proportion of these three sizes defines soil texture (e.g., loam is an ideal balanced agricultural mixture of sand, silt, and clay).
2. Water (25%): Fills pore spaces, dissolving mineral nutrients so plant roots can absorb them.
3. Air (25%): Fills un-saturated pore spaces, providing essential oxygen for plant root respiration and soil microbes.
4. Organic Matter / Humus (5%): Dark, decayed organic debris provided by plants and animals. Humus enhances nutrient storage, improves water retention, and binds soil minerals into stable aggregates.

Soil Horizons (The Soil Profile)

As soil matures, vertical weathering and chemical leaching create distinct horizontal layers called soil horizons. A vertical cross-section displaying these horizons is called a soil profile.

Soil Profile Layering (Top to Bottom):
[ O Horizon ] - Dark Organic Litter (Leaves, twigs, decaying organisms)
[ A Horizon ] - Topsoil (Humus + Minerals; high biological activity)
[ E Horizon ] - Zone of Eluviation / Leaching (Light-colored; acidic soils)
[ B Horizon ] - Subsoil (Illuviation Zone; clay & iron oxide accumulation)
[ C Horizon ] - Weathered Parent Material (Regolith; partially broken bedrock)
[ R Horizon ] - Unweathered Solid Bedrock (Parent rock)

Horizon Descriptions

• O Horizon (Organic Layer): The top layer composed of fresh and partially decomposed organic matter (leaf litter, humus).
• A Horizon (Topsoil): A dark-colored layer rich in humus and mineral matter. It contains the highest concentration of soil organisms (earthworms, bacteria, fungi) and plant roots.
• E Horizon (Zone of Eluviation/Leaching): Found in mature acidic forest soils. Water percolating downward dissolves and leaches out fine clays, iron, and organic acids, leaving a light-colored horizon rich in resistant quartz.
• B Horizon (Subsoil / Zone of Illuviation): Accumulation zone where leached materials (clays, iron oxides, calcium carbonate) transported downward from A/E horizons precipitate and collect.
• C Horizon (Parent Material / Regolith): Partially weathered bedrock or sediment. Contains large rock fragments; lacks significant organic material.
• R Horizon (Bedrock): Unweathered solid bedrock beneath the soil profile.

Classroom Pedagogical Strategies

When teaching surface processes to elementary students (Praxis 5005), teachers should emphasize active physical simulation and environmental stewardship:

Interactive Classroom Labs

• Stream Table Labs: Students use sand and water flow tables to observe how water velocity affects erosion rates, meander formation, and delta deposition.
• Soil Texture / Jar Test: Mixing soil samples with water in a mason jar and allowing it to settle reveals distinct layers of sand (bottom), silt (middle), and clay (top), letting students calculate soil texture percentages.

Addressing Common Misconceptions

• Misconception: Weathering and erosion are the same thing. Correction: Weathering is the breaking of rock in place; erosion is the moving of rock pieces to a new location.
• Misconception: Soil is just "dirt" (dead dirt without structure). Correction: Soil is a living ecosystem containing complex organic humus, soil micro-organisms, air, water, and structured horizons essential for planetary life.