1.5 - Geography Notes 18

Elaborate Notes

Types of Mass Movements

Mass movement, also known as mass wasting, refers to the geomorphic process by which soil, regolith, and rock move downslope under the direct influence of gravity. The classification of these movements is based on the speed of movement and the type of material involved. Geomorphologists like Arthur Strahler and Carson and Kirkby (1972) have extensively studied these processes, emphasizing gravity as the primary driving force, often aided by triggers like water saturation or seismic activity.

Slow Movements

• Creep: This is the slowest, yet persistent, downslope movement of soil and weathered rock (regolith). Its motion is imperceptible on a day-to-day basis but becomes evident over years through its effects on surface objects.

  • Mechanism: Creep is primarily caused by cycles of expansion and contraction of the surface material. This can be due to freeze-thaw cycles (frost heave), where water freezing in pore spaces lifts particles perpendicular to the slope, and upon thawing, gravity pulls them vertically downward, resulting in a net downslope movement. Wetting and drying cycles have a similar effect.
  • Indicators: Evidence of soil creep includes tilted trees (with a characteristic “J” curve at the base), bent fences, leaning utility poles, and broken retaining walls.
  • Example: It is a common phenomenon on moderately steep, soil-covered slopes in temperate regions, such as the Appalachian Mountains in North America.

• Solifluction: This term, meaning “soil flow,” describes the slow downslope movement of water-saturated soil. It is a specific type of creep characteristic of periglacial (cold, non-glacial) environments.

  • Mechanism: In regions with permafrost (permanently frozen ground), the top layer of soil, known as the active layer, thaws during the summer. Since the underlying permafrost is impermeable, the meltwater cannot drain downwards, leading to the saturation of the active layer. This water-logged mass then flows slowly downslope as a viscous fluid.
  • Landforms: This process creates distinct landforms such as solifluction lobes and sheets.
  • Example: Solifluction is widespread in the tundra regions of Siberia, Alaska, and Northern Canada.

Rapid Movements

• Landslide: A general term for the rapid movement of a mass of rock, debris, or earth down a slope. In a landslide, the moving material remains in continuous or intermittent contact with the slope surface.

  • Mechanism: Landslides are often triggered by factors that reduce the shear strength of the slope material, such as intense rainfall saturating the ground, earthquakes providing a seismic shock, volcanic eruptions, or human activities like undercutting slopes for road construction or deforestation.
  • Historical Example: The Kedarnath landslide in Uttarakhand, India (2013), was triggered by a multi-day cloudburst, leading to catastrophic flash floods and landslides, highlighting the vulnerability of the Himalayas. The Huascarán debris avalanche in Peru (1970), triggered by the Ancash earthquake, buried the town of Yungay and resulted in tens of thousands of casualties.

• Avalanche: This is the rapid flow of snow down a sloping surface. It is analogous to a rock or debris landslide but involves snow and ice.

  • Types: Two main types are loose snow avalanches, which start at a point and gather more snow, and slab avalanches, where a cohesive layer of snow breaks away and slides as a single block. Slab avalanches are typically more dangerous.
  • Triggers: Triggers include heavy snowfall overloading the existing snowpack, warming temperatures weakening the snow layers, or external disturbances like skiers or loud noises.
  • Example: The Galtür avalanche disaster in Austria (1999) was a series of powder snow avalanches that descended into the village with immense force and speed, demonstrating their destructive power.

• Earthflow: This is the downslope viscous flow of fine-grained materials, such as clay or silt, that have been saturated with water.

  • Characteristics: They typically occur on hillsides in humid areas after heavy precipitation. Earthflows often leave a scarp at the head and form a tongue-shaped or lobate flow at the toe. Their movement can range from slow (meters per year) to moderately rapid (kilometers per hour).
  • Example: The Slumgullion Earthflow in Colorado, USA, is a famous natural laboratory for studying this phenomenon. It has been moving continuously for several hundred years.

• Mudflow: A rapid and fluid movement of a mixture of water and fine-grained earth materials (mud) and debris. When associated with volcanic eruptions, they are termed lahars.

  • Mechanism: They are triggered by the sudden introduction of a large volume of water into unconsolidated soil or regolith, often from intense rainfall or snowmelt. They typically follow existing channels and can travel at high speeds over long distances.
  • Example: Following the 1991 eruption of Mount Pinatubo in the Philippines, seasonal monsoon rains repeatedly mobilized volcanic ash deposits, creating devastating lahars that buried numerous villages.

• Rock slide / Rockfall: These involve the movement of rock material.

  • Rockslide: This is the rapid transitional movement of a relatively intact mass of rock material along a planar surface of failure, such as a fault, joint, or bedding plane.
  • Rockfall: This involves the free-falling of individual rocks or fragments from a cliff or very steep slope. The accumulation of this fallen material at the base of the slope forms a talus slope or scree. This process is common in mountainous areas subject to mechanical weathering like frost wedging.
  • Example: The Frank Slide in Alberta, Canada (1903), was a massive rockslide where millions of tonnes of limestone slid from the face of Turtle Mountain, burying a portion of the town of Frank.

Landforms

A landform is a natural feature of the Earth’s solid surface. The study of their origin and evolution is called geomorphology. Landforms are created by various geomorphic agents, each producing a characteristic set of erosional and depositional topographies.

• Agent and its Topography:
  • River (Running Water) → Fluvial Topography: The work of rivers in eroding, transporting, and depositing material creates landscapes dominated by valleys, floodplains, and deltas.
  • Groundwater → Karst Topography: Formed from the dissolution of soluble rocks like limestone and dolomite. It is characterized by underground drainage systems with sinkholes, caves, and caverns. The term originates from the Karst Plateau region on the border of Slovenia and Italy.
  • Waves (Sea) → Marine/Coastal Topography: The action of sea waves, currents, and tides shapes coastal features like cliffs, beaches, spits, and bars.
  • Wind → Aeolian (Arid/Desert) Topography: Wind is a significant agent in arid regions where there is little vegetation cover. It creates landforms like sand dunes, loess plains, and mushroom rocks (yardangs/zeugens).
  • Glacier (Moving Ice) → Glacial Topography: The immense erosive power of glaciers carves out features like U-shaped valleys, cirques, and arêtes, and deposits material to form moraines and drumlins.

Riverine Topography

Fluvial topography is shaped by the geomorphic work of rivers. The concept of river evolution is classically described through three stages, a model famously proposed by William Morris Davis in his “Geographical Cycle” or “Cycle of Erosion” (1899). This model describes the sequential development of a river valley over geological time.

Stages in Riverine Topography

Index	Youth Stage	Mature Stage	Old Stage
Slope	Steep gradient, typically in mountainous or highland areas.	Moderate, gentle gradient as the river flows through wider valleys.	Very low or negligible gradient as the river traverses a broad, flat plain.
Energy & Velocity	High kinetic energy and velocity due to the steep slope.	Moderate energy and velocity.	Low energy and velocity.
Erosion	Dominated by vertical erosion (downcutting), leading to valley deepening.	Lateral erosion (bank cutting) becomes significant, leading to valley widening.	Minimal erosion; the river’s energy is almost entirely used for transportation.
Deposition	Very low, as high velocity allows the river to carry most of its sediment load.	Moderate deposition, especially on the inside of bends (point bars).	High deposition is the dominant process, building up floodplains, levees, and deltas.
Valley Shape	Narrow, deep, V-shaped valleys, gorges, and canyons.	Wider valleys with the beginnings of a floodplain.	Very wide, flat-bottomed valleys, often broader than the meander belt.
Meandering	River course is relatively straight, controlled by the original slope and geology.	Meanders begin to develop and become well-defined.	Pronounced and extensive meanders, ox-bow lakes are common.

Features of Riverine Topography

Fluvial Processes (Mechanisms of Erosion and Transport)

• Corrasion (Abrasion): The mechanical grinding of the river’s channel by the sediment load (sand, pebbles, boulders) it carries.
• Attrition: The process where transported rock particles collide with each other, breaking down and becoming smaller and more rounded.
• Hydraulic Action: The sheer force of the moving water dislodging loose material from the riverbed and banks.
• Corrosion (Solution): The chemical process of water dissolving minerals from the rocks, particularly in limestone areas.
• Cavitation: A powerful form of hydraulic action where air bubbles in turbulent water collapse, producing a shockwave that can erode solid rock.

Erosional and Depositional Features

Topography	Erosional Features	Depositional Features
Riverine (Fluvial Landforms)	V-Shaped Valleys, Gorges, Canyons: Formed by intense vertical erosion in the youth stage. Gorges are deep, narrow valleys (e.g., Indus Gorge), while canyons are grander versions (e.g., Grand Canyon, USA).	Alluvial Fans: Fan-shaped deposits of sediment formed where a fast-flowing stream emerges from a mountain onto a plain, causing a sudden decrease in velocity.
	Waterfalls, Cataracts, Rapids: Occur where a river flows over a resistant rock band or a sharp drop in the river bed. Niagara Falls is a prime example. Cataracts are large waterfalls; rapids are turbulent sections.	Natural Levees: Raised banks formed by the deposition of coarser sediment along the river channel during floods.
	Plunge Pools & Potholes: Plunge pools are deep depressions scoured at the base of waterfalls. Potholes are circular depressions drilled into the riverbed by the abrasive action of swirling pebbles.	Floodplains: The flat area of land adjacent to a river which is created by the deposition of fine sediment (alluvium) during floods. These are highly fertile (e.g., the Indo-Gangetic Plain).
	River Capture (Piracy) & Wind Gap: A powerful river erodes headward and captures the upper course of a weaker, adjacent river. The abandoned valley of the beheaded stream is called a wind gap.	Slip-off Slopes (Point Bars): Depositional features on the inside bend of a meander where water velocity is lower.
	Meanders & River Cliffs (Cut Banks): Sinuous bends in a river, prominent in mature and old stages. Erosion occurs on the outer bank (cut bank) due to higher velocity, creating a river cliff.	Ox-Bow Lakes: Crescent-shaped lakes formed when a river cuts through the narrow neck of a highly pronounced meander, abandoning the old loop.
		Delta: A landform created by deposition of sediment at the mouth of a river as it enters a body of standing water. Types include arcuate (Nile), bird’s-foot (Mississippi), and cuspate (Ebro).
		Estuary: A tidal mouth of a river where the tide meets the stream. Here, strong tidal currents prevent sediment from being deposited to form a delta (e.g., the mouth of the Narmada and Tapi rivers in India).

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Prelims Pointers

• Mass Movement: Downslope movement of rock and soil due to gravity.

• Creep: Slowest form of mass movement; indicators include tilted trees and fences.

• Solifluction: Slow soil flow over permafrost in periglacial regions.

• Landslide: Rapid movement of earth/rock mass down a slope.

• Avalanche: Rapid movement of snow and ice down a slope.

• Lahar: A type of mudflow composed of pyroclastic material and water, originating from a volcano.

• Talus Slope (or Scree): The accumulation of rock debris at the base of a cliff from rockfalls.

• Geomorphic Agents & Topographies:

  • River → Fluvial
  • Groundwater → Karst
  • Waves → Marine/Coastal
  • Wind → Aeolian/Arid
  • Glacier → Glacial

• River Stages (W.M. Davis):

  1. Youth Stage: Dominant process is vertical erosion (downcutting).
  2. Mature Stage: Dominant process is lateral erosion (valley widening).
  3. Old Stage: Dominant process is deposition.

• Fluvial Erosional Landforms:

  • V-Shaped Valley, Gorge, Canyon
  • Waterfall, Rapids, Cataract
  • Pothole, Plunge Pool
  • River Capture (Piracy), Wind Gap
  • Meanders, River Cliff (Cut Bank)

• Fluvial Depositional Landforms:

  • Alluvial Fan, Delta
  • Floodplain, Natural Levee
  • Ox-Bow Lake
  • Point Bar (Slip-off Slope)
  • Estuary (A feature of the river mouth where deposition is inhibited by tides).

• Examples:

  • Canyon: Grand Canyon (Colorado River, USA)
  • Bird’s-foot Delta: Mississippi River Delta
  • Arcuate Delta: Nile River Delta, Ganga-Brahmaputra Delta
  • Estuary-forming rivers in India: Narmada, Tapi.

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Mains Insights

Mass Movements: A Human-Induced Hazard (GS-I Geography, GS-III Disaster Management)

• Cause-Effect Relationship: While mass movements are natural processes, their frequency and intensity are increasingly influenced by anthropogenic activities. Deforestation removes the binding action of roots, making slopes unstable. Unscientific construction of roads and buildings in hilly terrains, especially by undercutting slopes, destabilizes them. Quarrying and mining activities create steep, unsupported slopes prone to failure. These factors, combined with climate change-induced extreme rainfall events, create a recipe for disaster, as seen frequently in the Himalayas and Western Ghats.
• Development vs. Environment Debate: In fragile ecosystems like the Indian Himalayan Region, there is a constant tension between the need for infrastructure development (for connectivity, tourism, and strategic purposes) and environmental conservation. The Char Dham Pariyojana has faced scrutiny for its potential to destabilize slopes through extensive hill-cutting. A sustainable approach requires a balance, incorporating advanced geological surveys, bio-engineering techniques (e.g., using vetiver grass), and strict adherence to environmental impact assessment (EIA) norms.
• Disaster Management Framework: Effective management requires a multi-pronged approach beyond post-disaster relief. This includes:
  1. Hazard Zonation Mapping: Identifying and mapping vulnerable areas to regulate and restrict construction.
  2. Early Warning Systems (EWS): Using rainfall thresholds, satellite imagery, and ground sensors to predict and warn about potential landslides.
  3. Community Preparedness: Training local communities in mitigation and response, as they are often the first responders.
  4. Policy and Governance: Strict enforcement of building codes and land-use policies based on scientific assessments, as recommended by the National Disaster Management Authority (NDMA) guidelines.

Riverine Topography: Interface of Geomorphology and Human Civilization (GS-I Geography)

• Civilizational Cradles and Geomorphic Settings: Major ancient civilizations (Mesopotamian, Egyptian, Indus Valley) flourished on the fertile floodplains created by rivers in their old stage. These depositional plains offered fertile alluvium for agriculture and a reliable water source. However, this also exposed them to the recurrent hazard of floods, leading to the development of early flood management techniques and complex social structures.
• Human Intervention and Alteration of River Systems: Modern engineering has profoundly altered natural fluvial processes.
  • Dams and Reservoirs: While providing hydroelectricity, irrigation, and flood control, dams trap sediment. This leads to sediment starvation downstream, causing deltas to shrink (e.g., the Nile delta after the Aswan High Dam) and increasing coastal erosion. The reservoir itself can sometimes induce seismic activity.
  • Embankments and Levees: These are built to confine rivers and protect floodplains. However, they prevent the natural deposition of fertile silt on the plains and can lead to a gradual raising of the riverbed (aggradation). This increases the flood risk catastrophically if the embankment breaches, as seen in the Kosi river floods in Bihar.
  • River Interlinking: Projects like the proposed Indian Rivers Inter-linking Project are debated from a geomorphological perspective. Critics argue that altering the natural flow, sediment load, and basin dynamics of rivers can have unpredictable and potentially irreversible ecological and geomorphic consequences, including changes in erosion and deposition patterns.

Historiographical Perspective: Evolution of Geomorphological Thought (GS-I Geography Optional)

• Davisian Cycle of Erosion: W. M. Davis’s model (1899) was a revolutionary concept that provided a simple, elegant framework for understanding landform evolution as a function of “Structure, Process, and Stage.” It dominated geomorphological thought for decades. Its strength lies in its explanatory power and simplicity.
• Critique and Alternative Models: Davis’s model was criticized for its oversimplification, particularly its assumption of a rapid initial uplift followed by a long period of tectonic quiescence, which is geologically rare.
  • Walther Penck’s Model (1924): Penck proposed a model where landform development is a continuous interaction between the rates of uplift and denudation. He argued that the shape of a slope (convex, straight, or concave) is a reflection of this dynamic relationship, rather than a function of a specific “stage.”
  • Modern Geomorphology: Contemporary approaches move beyond these cyclical models to embrace concepts like Dynamic Equilibrium (J.T. Hack), where landscapes are seen as being in a state of balance, constantly adjusting to changing conditions. The focus is now more on quantifiable processes, field measurements, and understanding the complex, non-linear responses of geomorphic systems to various forces, including climate change and human impact. This shift represents a move from historical/explanatory models to more process-based/functional studies.