Erosion Types

Why This Matters

Erosion is one of the most fundamental processes shaping Earth's surface, and you'll encounter it across nearly every geology topic—from landform development to sediment transport to environmental hazards. Understanding erosion means understanding the constant interplay between energy sources (gravity, flowing water, wind, ice) and material resistance (rock hardness, soil cohesion, vegetation cover). These concepts connect directly to plate tectonics, the rock cycle, and geomorphology.

You're being tested not just on naming erosion types, but on recognizing which agent is responsible for specific landforms, how erosion rates vary by climate and geology, and why human activities accelerate certain processes. Don't just memorize definitions—know what driving force powers each erosion type, what landforms it creates, and what conditions favor it over others.

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Fluid-Driven Erosion

These erosion types share a common mechanism: moving fluids (water or air) exert shear stress on surface materials, detaching and transporting particles. The key variables are fluid velocity, particle size, and surface resistance.

Water Erosion

• Most powerful erosive agent on Earth—responsible for more landscape modification than all other erosion types combined
• Two main categories: surface erosion (sheet wash and rills from overland flow) and channel erosion (rivers cutting into banks and beds)
• Creates diagnostic landforms including V-shaped valleys, canyons, gullies, and deltas where sediment deposits at base level

Coastal Erosion

• Wave energy concentrates at headlands—hydraulic action and abrasion attack coastlines through repeated wave impact
• Produces distinctive features: sea cliffs, wave-cut platforms, sea stacks, and arches through differential erosion of rock types
• Human acceleration from seawalls, jetties, and dredging disrupts natural sediment transport, often worsening erosion elsewhere

Wind Erosion

• Dominant in arid and semi-arid regions—requires loose, dry particles and minimal vegetation cover to operate effectively
• Two transport mechanisms: saltation (bouncing sand grains) and suspension (fine dust carried aloft)
• Creates deflation hollows and desert pavement through selective removal of fine particles, leaving coarse lag deposits behind

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Gravity-Driven Erosion

Gravity acts on all materials constantly, but these processes occur when gravitational stress exceeds the shear strength of slope materials. No transporting fluid required—just gravity and a slope.

Mass Wasting

• Driven purely by gravity—occurs when slope stability fails due to oversteepening, saturation, or loss of cohesion
• Range of speeds: from imperceptibly slow creep (mm/year) to catastrophic rockfalls and debris flows (m/second)
• Triggered by water saturation, earthquakes, undercutting, or human excavation—understanding triggers is essential for hazard assessment

Glacial Erosion

• Ice acts as a slow-moving conveyor belt—gravity pulls glaciers downslope while embedded rocks abrade the bedrock beneath
• Creates U-shaped valleys, cirques, arêtes, and fjords—these landforms are diagnostic evidence of past glaciation
• Plucking and abrasion work together: plucking quarries blocks from bedrock; abrasion polishes and scratches surfaces (striations)

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Chemical and Biological Erosion

These processes break down rock through chemical reactions or organic activity rather than mechanical force. They often work alongside physical erosion, weakening materials for later removal.

Chemical Erosion

• Dissolution dominates in soluble rocks—carbonic acid in rainwater reacts with limestone ($CaCO_3$), creating karst topography
• Produces caves, sinkholes, and disappearing streams—underground drainage networks form where chemical erosion exceeds surface erosion
• Critical for soil formation—chemical weathering releases nutrients and creates clay minerals essential for ecosystems

Biological Erosion

• Root wedging physically fractures rock—growing roots exploit cracks, expanding them through sustained pressure
• Organisms accelerate chemical weathering—lichens secrete acids; burrowing animals mix and expose soil to weathering agents
• Essential for soil development—biological activity transforms weathered rock into productive soil horizons

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Climate-Specific Erosion

These erosion types occur under specific climatic conditions and produce landforms unique to those environments.

Thermal Erosion

• Exclusive to permafrost regions—occurs when frozen ground thaws, losing the ice that cemented sediments together
• Creates thermokarst landscapes—subsidence produces irregular topography with lakes, sinkholes, and collapsed terrain
• Climate change accelerates this process—thawing permafrost releases stored carbon as $CO_2$ and $CH_4$, creating a positive feedback loop

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Quick Reference Table

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Self-Check Questions

1. Which two erosion types are both gravity-driven but operate at vastly different timescales? What landforms distinguish them?

2. A geologist finds polished bedrock with parallel scratches. Which erosion type created this, and what specific process within that type is responsible?

3. Compare and contrast water erosion and wind erosion: What do they share mechanically, and why does wind erosion dominate only in certain environments?

4. An FRQ asks you to explain how erosion contributes to soil formation. Which two erosion types would you emphasize, and why?

5. A coastal community installs a seawall to prevent erosion. Using your understanding of coastal erosion, predict what might happen to adjacent beaches and explain the mechanism.