Exogenic Processes & Agents
Kinetic Energy ($E_k = \frac{1}{2}mv^2$) · 5 Mobile Geomorphic Agents · Graded Sorting · Climatic Zonation (28 Micro-Realms)
The Five Mobile Sculptors of the Crust (NCERT Chapter 6)
While weathering and mass movements prepare and shift rock debris locally, dynamic Erosion and Transportation require a mobile Geomorphic Agent powered by Kinetic Energy ($\frac{1}{2}mv^2$). Operating across three physical states—gaseous (Wind), liquid (Running Water, Groundwater, Waves/Currents), and solid (Glaciers)—these five agents strip highlands and deposit graded sediments (coarsest first, finest last) whenever slope gradients flatten and velocity drops.
I. Exogenic Mechanics & The Kinetic Energy Law ($E_k = \frac{1}{2}mv^2$)
In-Situ vs. Mobile Exogenic
- Weathering is static (in-situ)
- Erosion & transport are dynamic
- Requires a moving fluid/solid agent
Process vs. Agent Coupling
- Process = Force applied on rock
- Agent = Mobile medium carrying it
- Inseparable in active erosion (NCERT)
Kinetic Energy ($E_k = \frac{1}{2}mv^2$)
- Sole driving force of erosion (NCERT)
- $m$ = Mass of moving medium
- $v$ = Velocity of flow along slope
Velocity-Squared ($v^2$) Power
- Energy scales with square of velocity
- Doubling river/wind speed quadruples $E_k$
- Explains catastrophic flood erosion
Debris Acquisition
- Erosion = Acquisition + Transport
- Hydraulic lift, plucking, deflation
- Scoops loose regolith & bedrock
4 Modes of Transport
- Traction: Rolling boulders on bed
- Saltation: Bouncing sand grains
- Suspension (silt) & Solution (ions)
Abrasion vs. Attrition
- Abrasion (Corrasion): Clasts scrape bed
- Attrition: Clasts collide & round out
- Cavitation: Bubble implosion shock
II. The Five Mobile Geomorphic Agents & Their Three Physical States (NCERT)
3 States of Matter (NCERT)
- 1. Gaseous state: Wind
- 2. Liquid state: Running water, Waves, GW
- 3. Solid state: Glaciers (moving ice)
1. Running Water (Fluvial)
- Most important single agent globally
- Overland sheet flow + linear streams
- Sculpts valleys, gorges, plains, deltas
2. Groundwater (Karst)
- Percolating subsurface water
- Acts via chemical solution/precipitation
- Requires soluble Limestone/Dolomite
3. Glaciers (Moving Ice)
- Masses of ice moving as sheets/streams
- Very low velocity ($v$), colossal mass ($m$)
- Tremendous plucking & abrasion power
4. Wind (Aeolian)
- Sole gaseous geomorphic agent
- Low mass density ($m$), high speed ($v$)
- Deflation, sandblasting & dune building
5. Waves & Currents
- Breaking surf, tides & longshore drift
- Confined to narrow coastal interface
- Carves sea cliffs, stacks, bars & spits
Climatic vs. Non-Climatic
LITHOLOGY
COASTLINE
- Water, Ice, Wind = Controlled by climate
- Groundwater = Controlled by rock type
- Waves = Controlled by land-sea margin
III. Slope, Competence, Hjulström Curve & Graded Deposition
Slope-Velocity Control
- Steep slope = High $v$ = Net Erosion
- Gentle slope = Low $v$ = Net Deposition
- Erosion degrades relief, lowering slope
Glacier ($m$) vs. Wind ($v$)
HIGH m · LOW v
LOW m · HIGH v
- Glaciers move at cm/day but carry boulders
- Wind moves at km/h but lifts only sand/dust
- Both satisfy $E_k = \frac{1}{2}mv^2$
Competence vs. Capacity
- Competence: Largest grain size ($D \propto v^6$)
- Capacity: Total sediment mass load
- Glaciers have infinite competence!
Hjulström Curve Paradox
- Sand (0.1–0.5 mm) erodes easiest
- Boulders need high $v$ due to weight
- Clay needs high $v$ due to cohesion!
Deposition Logic (NCERT)
- Not the work of any separate agent
- Triggered when velocity & energy drop
- Agent can no longer carry its load
Coarse-to-Fine Sorting
- Coarser materials settle out first
- Finer silt & clay deposit last (NCERT)
- Creates graded alluvial fan beds
Sorted vs. Unsorted Till
WELL SORTED
UNSORTED
- Water & wind stratify by grain size
- Melting glacier drops everything at once
- Till = Unsorted boulders + clay mix
IV. Morphoclimatic Zonation & Indian Geomorphic Agent Mosaic
Climatic Agent Shifts
- Dominant agent shifts with latitude
- And with altitude (mountain belts)
- Peltier morphoclimatic model
Humid Fluvial Regimes
- Equatorial, Monsoon & Temperate
- Rainfall exceeds evaporation
- Running water dominates landscape
Arid & Semi-Arid Regimes
- Sparse vegetation exposes loose sand
- Free wind deflation & dune migration
- Sudden ephemeral cloudburst wadis
Cryospheric Ice Regimes
- Polar caps & high mountain peaks
- Precipitation falls as solid snow
- Glaciers & frost-wedging dominate
Karst Lithological Window
- Groundwater needs two conditions:
- 1) Thick jointed carbonate rocks
- 2) Abundant rainfall for $\text{H}_2\text{CO}_3$
Azonal Coastal Margins
- Operates across all latitudes
- From tropical reefs to polar fjords
- Driven by wind-waves, swell & tides
India's 5-Agent Mosaic
ICEGANGA
RIVERTHAR
WINDBORRA
KARSTKONKAN
WAVESCHILIKA
LAGOON
- India hosts all 5 geomorphic agents!
- Himalayan glaciers to Thar dunes
- Gangetic rivers to 7,516-km coast
Core Distinctions
• Gaseous State: Wind.
• Liquid State: Running water, Groundwater, Waves and Currents.
• Solid State: Glaciers (moving ice).
All five are powered by Kinetic Energy ($E_k = \frac{1}{2}mv^2$) controlled by mass ($m$) and slope-driven velocity ($v$).
Action of running water, glaciers, and wind is controlled primarily by climatic regimes (arid, humid, frigid). By contrast, the work of groundwater is determined by lithological character (soluble limestone/dolomite), and the work of waves is determined by location along the coastal land-sea interface.
3. Sorted Fluid Deposits vs. Unsorted Glacial TillFluid agents (running water and wind) deposit load progressively as velocity drops—coarser materials first, finer silt/clay last (graded sorting). Solid glaciers melt in place and dump boulders, pebbles, sand, and clay together as unsorted, unstratified Till.
The 5 Geomorphic Agents Matrix
State, Control & Indian Exemplar Key