Major Geomorphic Processes

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Why This Matters

Earth's surface is constantly being sculpted by forces that break down, move, and rebuild materials. In Earth Surface Processes, you need to explain how landscapes form and why they look the way they do. That means understanding the mechanisms behind geomorphic processes: weathering versus erosion, gravity-driven versus fluid-driven transport, endogenic versus exogenic forces. These distinctions show up repeatedly in exam questions asking you to compare landforms or predict how landscapes will change over time.

The processes here connect directly to larger course themes like landscape evolution, hazard assessment, and human-environment interactions. When you see a U-shaped valley or a coastal cliff, you should immediately recognize the process responsible and the conditions that created it. Don't just memorize definitions. Know what agent of change drives each process, what landforms result, and how human activities modify natural systems.

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Breakdown and Preparation: Weathering

Weathering is the essential first step in landscape change. It prepares rock for transport by breaking it into smaller pieces or altering its chemistry. No weathering means no sediment supply for all the erosional processes that follow.

Weathering

• Breaks down rock in place. Unlike erosion, weathering doesn't transport material. It weakens and fragments rock at or near the surface.
• Three mechanisms typically operate simultaneously:
  • Physical (frost wedging, salt crystallization, thermal expansion/contraction)
  • Chemical (dissolution, oxidation, hydrolysis)
  • Biological (root wedging, burrowing organisms, organic acid production)
• Climate controls the dominant weathering type. Chemical weathering dominates in warm, wet environments because water and heat accelerate reactions. Physical weathering prevails in cold or arid regions where freeze-thaw cycles or salt growth are more active.

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Gravity-Driven Processes: Mass Wasting

When gravity alone moves material downslope without a primary transporting medium like water or wind, that's mass wasting. The steeper the slope and the more saturated the material, the greater the likelihood of failure.

Mass Wasting

• Gravity is the primary driving force. No wind, flowing water, or ice is required as a transport agent, though water often acts as a trigger by adding weight, increasing pore pressure, and reducing internal friction (shear strength).
• Speed and material vary widely. Soil creep operates at millimeters per year, while rockfalls and debris flows can travel at meters per second. Classification depends on material type (rock, debris, earth), movement style (fall, slide, flow), and velocity.
• Human triggers are common. Road cuts remove lateral support, deforestation eliminates root cohesion, and construction loads slopes with extra weight. All of these destabilize slopes and increase mass wasting frequency.

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Fluid Transport: Water, Wind, and Ice

These processes share a common mechanism: a moving fluid (liquid water, air, or glacial ice) picks up sediment, transports it, and deposits it elsewhere. The energy of the fluid determines how much material moves and how far it travels.

Fluvial Processes

Rivers are the dominant erosional agent on Earth's surface. Globally, flowing water moves more sediment than wind, ice, and gravity combined.

• Erosion occurs through three mechanisms: hydraulic action (force of flowing water), abrasion (sediment grinding against the bed and banks), and dissolution (chemical removal of soluble rock).
• Distinctive landforms result from erosion and deposition. V-shaped valleys, meanders, oxbow lakes, floodplains, alluvial fans, and deltas all reflect fluvial action at different points along a river's profile.
• Human modification is pervasive. Dams trap sediment (starving downstream reaches), channelization increases flow velocity, and land-use changes alter both runoff volume and sediment supply.

Glacial Processes

Glaciers erode, transport, and deposit vast quantities of material, but they operate on longer timescales than rivers and leave some of the most recognizable landforms on Earth.

• Ice erodes through plucking and abrasion. Plucking quarries blocks of rock from the bed by freezing onto them and pulling them away. Abrasion grinds surfaces smooth, leaving striations (scratches) that indicate ice flow direction.
• Distinctive landforms signal past glaciation. U-shaped valleys, cirques, arêtes, horns, fjords, moraines, drumlins, and erratics are all diagnostic features.
• Glaciers store freshwater and record climate. Ice cores provide paleoclimate data stretching back hundreds of thousands of years, and glacial melt contributes directly to global sea-level change.

Aeolian Processes

Wind is a selective transport agent. It can only move particles up to sand size under normal conditions, which means it dominates in environments where fine sediment is exposed and unprotected.

• Transport occurs by three modes: saltation bounces sand-sized grains along the surface, suspension carries silt and clay (which can form thick loess deposits) over hundreds of kilometers, and surface creep rolls larger grains pushed by saltating particles.
• Arid and coastal environments dominate. Dunes, deflation hollows, desert pavement, yardangs, and ventifacts form where vegetation is sparse and loose sediment is available.
• Human land use accelerates wind erosion. Overgrazing, deforestation, and poor agricultural practices strip protective ground cover and promote desertification. The 1930s Dust Bowl is a classic example.

Coastal Processes

Coastlines sit at the intersection of marine and terrestrial systems, shaped by waves, tides, currents, and the sediment they move.

• Waves, tides, and currents shape shorelines. Wave energy erodes resistant headlands, longshore drift transports sediment parallel to the coast, and tidal currents redistribute material in estuaries and inlets.
• Erosional and depositional landforms coexist. Sea cliffs, wave-cut platforms, sea stacks, and arches reflect high-energy erosion. Beaches, spits, barrier islands, and tombolos reflect deposition where energy decreases.
• Sea-level rise intensifies coastal hazards. Erosion rates increase as higher water levels reach previously stable landforms, and human infrastructure faces growing threats from storm surge and chronic flooding.

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Endogenic Forces: Tectonic and Volcanic Processes

These processes originate from Earth's internal heat and drive large-scale landscape construction. While exogenic processes tear landscapes down, endogenic processes build them up.

Tectonic Processes

Plate tectonics creates the first-order relief of Earth's surface. Without tectonic uplift, erosion would eventually reduce all land to near sea level.

• Three boundary types produce different features. Convergent boundaries build mountains and ocean trenches, divergent boundaries create mid-ocean ridges and continental rifts, and transform boundaries produce fault scarps and offset streams.
• Earthquakes and uplift reshape surfaces rapidly. A single earthquake can produce meters of vertical displacement. Over millions of years, tectonic activity creates the relief that exogenic processes then work to erode.
• Isostasy links erosion and uplift. As erosion removes mass from mountains, the crust rebounds upward. This feedback means that tectonic and exogenic processes are coupled, not independent.

Volcanic Processes

• Magma composition controls eruption style. Low-viscosity basaltic magma produces effusive eruptions and builds broad shield volcanoes (like Mauna Loa). High-viscosity silicic magma traps gas, creating explosive eruptions and steep stratovolcanoes (like Mount St. Helens).
• Volcanic landforms vary dramatically. Calderas, lava plateaus, cinder cones, and volcanic islands each reflect different eruptive histories and magma types.
• Eruptions impact climate and ecosystems. Large eruptions inject ash and $SO_2$ into the stratosphere, which can cool global temperatures for 1-2 years. Lava flows destroy existing landscapes while simultaneously creating new land.

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Chemical Dissolution: Karst Processes

Karst landscapes form when acidic water dissolves soluble bedrock over time. This is chemical weathering taken to an extreme, creating landforms entirely through rock removal rather than mechanical erosion.

Karst Processes

• Dissolution of soluble rock drives formation. Limestone, dolomite, and gypsum dissolve when exposed to carbonic acid ($H_2CO_3$), which forms naturally when $CO_2$ in the atmosphere and soil dissolves into rainwater and soil water.
• Surface and subsurface features are diagnostic. Sinkholes, disappearing streams, caves, speleothems (stalactites and stalagmites), and tower karst all indicate active or relict karst systems.
• Groundwater vulnerability is a key concern. Karst aquifers have large conduit networks instead of small pore spaces, so they lack natural filtration. This makes them highly susceptible to contamination from surface pollutants. About 25% of the world's population depends on karst groundwater.

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Quick Reference Table

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Self-Check Questions

1. Which two processes are both driven by fluid transport but differ in the size of particles they can move? Explain the difference.

2. How do endogenic and exogenic processes work together to create and then modify a mountain range over time?

3. Compare fluvial and glacial valleys: what diagnostic features would you use to distinguish between them in the field?

4. If a question asks you to explain why karst regions face unique groundwater management challenges, which processes and landforms would you discuss?

5. Identify three geomorphic processes that are significantly accelerated by human activities, and explain the mechanism of acceleration for each.