13.3 Uplift, subsidence, and their geomorphic effects

Uplift and subsidence shape Earth's surface, creating diverse landscapes. These processes, driven by plate tectonics, alter elevation and relief, influencing erosion rates, river systems, and coastal features. Understanding their effects is crucial for grasping landscape evolution.

Tectonic forces cause vertical movements that interact with erosion, sedimentation, and sea-level changes. This dynamic interplay creates landforms like mountains, valleys, and coastal plains. Isostatic adjustments further complicate the picture, adding another layer to Earth's ever-changing surface.

Tectonic Uplift vs Subsidence

Vertical Displacement and Tectonic Settings

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• Tectonic uplift vertically displaces Earth's surface upward due to plate tectonic forces, increasing elevation
• Subsidence moves Earth's surface downward, decreasing elevation
• Uplift typically occurs in compressional tectonic settings (convergent plate boundaries)
• Subsidence often happens in extensional settings or sedimentary basins
• Rate and duration of uplift or subsidence determine resulting landforms and geomorphic processes

Geomorphic Consequences

• Uplift increases relief, steepens slopes, and enhances erosion rates
  • Forms deeply incised valleys, gorges, and rejuvenated landscapes
• Subsidence creates sedimentary basins, coastal plains, and submerges land areas
  • Develops accommodation space for sediment accumulation
• Both processes change base level, affecting river systems
  • Alters erosional and depositional patterns
• Examples of uplift-induced landforms (Rocky Mountains, Andes)
• Examples of subsidence-induced features (North Sea Basin, Mississippi Delta)

Uplift and Subsidence Impacts on Erosion

Erosion Rates and Sediment Transport

• Uplift increases erosion rates by steepening slopes and increasing stream gradients
  • Enhances stream power and sediment transport efficiency
• Subsidence decreases erosion rates in subsiding areas
  • Increases erosion in adjacent uplifted regions
  • Creates accommodation space for sediment deposition
• Interaction between uplift/subsidence rates and erosion rates determines landscape equilibrium state
  • Dynamic equilibrium vs. disequilibrium
• Alters sediment transport patterns from source to sink
  • Affects sediment distribution across landscapes

River Systems and Drainage Patterns

• Uplift causes river incision and terrace formation
• Subsidence leads to river aggradation and alluvial plain development
• Base level changes shift erosional and depositional zones within drainage basins
• Influences drainage pattern development
  • Antecedent streams (Colorado River through Grand Canyon)
  • Drainage reversals (Amazon River)
• Examples of uplift-induced river changes (Yangtze River gorges)
• Examples of subsidence-induced river changes (Mississippi River delta)

Coastal Landscapes: Uplift vs Subsidence

Coastal Landform Development

• Coastal uplift forms marine terraces
  • Flat, elevated surfaces representing former sea levels
  • Record past sea-level changes and tectonic activity
• Uplift causes emergence of wave-cut platforms and sea cliff formation
• Subsidence develops extensive coastal plains
  • Submerges former land surfaces
  • Creates estuaries and drowned river valleys
• Subsidence leads to drowning of coastal features and barrier island development
• Interplay between uplift/subsidence rates and sea-level changes determines coastline transgression or regression
• Examples of uplifted coasts (California coast)
• Examples of subsiding coasts (Louisiana Gulf Coast)

Preservation and Abrupt Changes

• Uplift/subsidence rate relative to sea-level change influences coastal landform preservation
• Tectonic activity causes abrupt coastal landscape changes
  • Coseismic uplift events create new land
  • Sudden subsidence submerges coastal areas
• Impact varies based on rock type, sediment supply, and wave energy
• Examples of preserved uplifted coastlines (Napier, New Zealand)
• Examples of rapidly subsiding coastlines (Venice, Italy)

Isostatic Adjustments in Response to Tectonics

Isostatic Principles and Tectonic Influences

• Isostasy maintains gravitational equilibrium between Earth's crust and mantle
  • Crust "floats" on denser mantle
• Isostatic adjustments occur to maintain equilibrium when crustal loading or unloading changes
• Tectonic processes cause isostatic adjustments as crust thickens or thins
  • Mountain building or rifting leads to vertical surface movements
• Adjustment rate depends on mantle viscosity and load change magnitude
  • Results in delayed landscape responses to tectonic or erosional events

Erosional and Glacial Isostatic Adjustments

• Erosional unloading of mountain ranges triggers isostatic uplift (isostatic rebound)
  • Prolongs mountain range lifespan
  • Influences geomorphic evolution
• Glacial isostatic adjustment responds to ice sheet loading and unloading
  • Causes vertical surface movements continuing long after ice melts
• Isostatic adjustments influence regional drainage patterns and sediment transport pathways
  • Redistributes erosional and depositional zones
• Examples of erosional isostatic rebound (Appalachian Mountains)
• Examples of glacial isostatic adjustment (Scandinavia)