Geomorphology

William Morris Davis' Geomorphic Cycle of Erosion

William Morris Davis (1899), an American geographer, proposed the concept of the Geomorphic Cycle or Cycle of Erosion. According to Davis, landforms evolve through three sequential stages — Youth, Maturity, and Old Age — similar to the human life cycle.

Basic Assumptions of Davis’ Model:

• Rapid uplift of land.
• Long period of tectonic stability after uplift.
• Erosion mainly carried out by running water.
• Landscape development follows a time-dependent sequence.

Conceptually:

Stages of the Geomorphic Cycle

1. Youth Stage

• Dominant Process: Vertical erosion (down-cutting)
• Valley Shape: Deep, narrow V-shaped valleys
• River Gradient: Very steep
• Relief: Increasing relief
• Drainage: Poorly developed tributaries
• Interfluves: Broad and flat

Common Landforms:

• Gorges
• Canyons
• Waterfalls
• Rapids

2. Maturity Stage

• Dominant Process: Lateral erosion increases
• Valley Shape: Wider valleys with gentler slopes
• Relief: Maximum relief attained
• Drainage: Well-developed drainage system
• River Condition: Approaches base level
• Interfluves: Narrow and sharp

Common Landforms:

• Meanders
• Floodplains begin forming
• Valley widening

3. Old Age Stage

• Dominant Process: Deposition and lateral erosion
• Valley Shape: Very wide valleys
• Relief: Very low relief
• River: Sluggish flow with large meanders

Common Landforms:

• Peneplain (almost plain-like surface)
• Oxbow lakes
• Swamps
• Residual hills called Monadnocks

Final stage representation:

Important Concepts in Davis’ Model

• Uplift: The cycle begins with rapid tectonic uplift.
• Base Level: The lowest level to which a river can erode (usually sea level).
• Peneplain: Nearly level surface formed in old age stage.
• Monadnock: Resistant residual hill standing above peneplain.
• Rejuvenation: Renewed uplift restarts erosion cycle.

Mathematically:

Criticisms of Davis' Model

• Assumes rapid uplift followed by long stability (rare in reality).
• Ignores climatic variations.
• Too deterministic and time-dependent.
• Over-simplifies complex geomorphic processes.
• Later modified by Walther Penck and modern geomorphologists.

Conclusion

Davis’ Geomorphic Cycle was a pioneering model in geomorphology. Although later modified and criticized, it provided the first systematic explanation of landscape evolution based on time, erosion, and structural control.

Divergent Plate Boundaries

Divergent plate boundaries are zones where tectonic plates move away from each other. They are also called constructive plate margins because new crust is formed through volcanic activity. These boundaries occur in both oceanic and continental regions.

Major Landforms:

• Mid-Oceanic Ridges
  • Underwater mountain ranges formed by rising magma.
  • Example:
  • New oceanic crust is formed by seafloor spreading.
• Rift Valleys
  • Formed due to stretching and fracturing of continental crust.
  • Example:
  • Characterized by steep sides and flat floors.
• Horst and Graben Structures
  • Graben: Down-dropped block forming a valley.
  • Horst: Uplifted block forming a ridge.

Process of Formation:

• Upwelling of Magma: Hot magma rises due to mantle convection.
• Crustal Stretching: Plates move apart creating tension.
• Volcanic Activity: Magma erupts and forms new crust.
• Continuous Spreading: Ocean basins widen over time.

Landforms Produced by Wind Erosion

Wind erosion (aeolian erosion) is dominant in arid and semi-arid regions. The main processes include deflation, abrasion, and attrition.

Major Landforms:

• Mushroom Rocks (Pedestal Rocks)
  • Formed by abrasion of lower rock layers.
  • Lower part eroded more than upper part.
  • Resembles a mushroom shape.
• Yardangs
  • Streamlined ridges carved parallel to wind direction.
  • Steep windward side and tapering leeward side.
• Zeugen
  • Tabular rock masses with resistant cap rock.
  • Weaker rock beneath gets eroded.
  • Creates ridge-and-trough pattern.

Additional Features:

• Ventifacts: Polished stones shaped by wind.
• Desert Pavement: Pebble-covered surface after deflation.
• Blowouts: Depressions formed by removal of loose particles.