Exogenetic Forces & Geomorphic Cycles
1. Exogenetic Forces (Denudation)
Exogenetic forces originate externally, deriving their energy from the Sun and the tectonic gradients. Their primary role is destructionalтАФwearing down elevated landforms through a general process called Denudation. Denudation comprises weathering, mass wasting, erosion, and transportation.
Weathering
Weathering is the in-situ (on-site) disintegration and decomposition of rocks. No significant movement is involved.
- Physical (Mechanical) Weathering: Mechanical breakdown. Includes frost wedging (water freezing and expanding in cracks), exfoliation (peeling of rock layers due to thermal expansion/contraction or unloading), and salt crystal growth.
- Chemical Weathering: Alteration of rock chemistry. Includes hydration (addition of water), oxidation (reaction with oxygen, e.g., rusting of iron-rich rocks), carbonation (reaction with carbonic acid forming caves in limestone), and solution. Heavily dependent on water and temperature.
- Biological Weathering: Action of plants (roots breaking rocks), animals (burrowing), and human activity.
Mass Wasting (Mass Movement)
The downslope movement of rock debris, regolith, and soil under the direct, predominant influence of gravity. Geomorphic agents like flowing water do not carry the load, though water may act as a lubricant.
- Slow movements: Soil creep (imperceptibly slow down-slope movement of soil).
- Rapid movements: Earthflow, mudflow, landslide, rockfall, and debris avalanche.
2. Concepts of Geomorphic Cycles
A geomorphic cycle describes the sequential stages of landscape evolution from an uplifted landmass to a featureless plain.
W.M. Davis: Normal Cycle of Erosion (1899)
The most famous concept. Davis stated that "landscape is a function of structure, process, and time (stage)."
- Trio of Davis: Structure (lithology, folding, faulting), Process (agents like rivers, wind, glaciers), and Stage (Youth, Maturity, Old Age).
- Assumption: Rapid short-period upliftment, followed by a long period of crustal stability where erosion acts.
- Youth: Deep V-shaped valleys, rapid down-cutting, waterfalls, maximum relief.
- Maturity: Lateral erosion begins, valleys widen, floodplains form, meandering starts. Maximum distinct landforms.
- Old Age: Slopes become gentle, almost featureless plain called a peneplain, dotted with isolated residual resistant hills called monadnocks.
Walther Penck's Morphological System (1924)
Penck challenged Davis's assumption of rapid uplift. He argued that upliftment and erosion are continuous and overlapping processes. Landscape evolution is a result of the ratio between the rate of uplift and the rate of erosion.
- Aufsteigende Entwicklung (Waxing development): Uplift exceeds erosion. Deepening of valleys.
- Gleichformige Entwicklung (Uniform development): Uplift matches erosion. Steady state.
- Absteigende Entwicklung (Waning development): Erosion exceeds uplift. Valleys flatten.
- Instead of a peneplain, Penck proposed an Endrumpf (an end surface) and Inselbergs (residual hills).
L.C. King's Arid Cycle of Erosion (Pediplanation)
Developed for arid/savanna regions (derived from his study of African landscapes). He emphasized parallel retreat of slopes.
- Relies on scarp retreat (backward wearing of steep slopes while maintaining a constant angle).
- Leaves behind a gently sloping erosional rock surface at the base called a pediment. The coalescence of multiple pediments forms an extensive flat plain known as a Pediplain.