GS Notes

1.5 - Geography Notes 19

17 min read

Elaborate Notes

Karst Topography / Limestone Topography

Karst topography is a distinctive landscape shaped by the dissolution of soluble bedrock, primarily limestone, dolomite, and gypsum. The term originates from the ‘Karst’ plateau region of Slovenia (formerly part of Yugoslavia), which was one of the first areas where this topography was systematically studied. The primary geomorphic agent is groundwater, which becomes weakly acidic through chemical processes.

  • Essential Conditions for Karst Development:

    1. Presence of Soluble Rock: A significant thickness of soluble rocks like limestone (calcium carbonate), dolomite, or gypsum near the surface is essential.
    2. Permeability and Porosity: The rock should be dense, highly jointed, and thinly bedded. This allows water to penetrate along the joints and bedding planes, facilitating dissolution from within, rather than just on the surface.
    3. Moderate to Heavy Rainfall: Ample water is required to drive the dissolution process. The water, upon absorbing atmospheric carbon dioxide, forms a weak carbonic acid (H2O + CO2 → H2CO3).
    4. Entrenched Valleys: The presence of deep valleys below the upland limestone surface allows for the lowering of the water table and the circulation of groundwater, which is crucial for the formation of extensive cave systems.

  • Geomorphic Processes:

    • Corrosion/Solution: This is the dominant process. Carbonic acid in rainwater reacts with calcium carbonate in limestone, converting it into soluble calcium bicarbonate (H2CO3 + CaCO3 → Ca(HCO3)2), which is then carried away in the solution.
    • Carbonation: A specific type of chemical weathering involving the reaction of carbonate and bicarbonate with minerals.
    • Corrasion (Abrasion): Mechanical erosion by rock fragments carried by underground streams, though this is a secondary process compared to corrosion.
    • Attrition: The wearing down of rock particles as they collide with each other in underground streams.
    • Hydraulic Action: The force of moving water dislodging rock particles, particularly in well-developed underground river systems.

  • Erosional Features (Surface and Sub-surface):

    • Sinkholes (or Cenotes): These are circular depressions formed either by the dissolution of the surface rock (solution sinks) or by the collapse of the roof of an underground cave (collapse sinks). In the Yucatan Peninsula of Mexico, these features, known as ‘cenotes’, were vital water sources for the Mayan civilization, which flourished in an area largely devoid of surface rivers. The Chicxulub crater, from the asteroid impact that led to dinosaur extinction, is located here, and the crater’s rim influenced the formation of a ring of cenotes.
    • Swallow Hole: A hole through which a surface stream ‘disappears’ or flows underground.
    • Doline: A larger depression formed by the coalescence of several sinkholes.
    • Uvala: A very large, complex depression formed by the merger of multiple dolines.
    • Polje: A very large, flat-floored, steep-sided elongated depression that may have a river flowing through it, which disappears into a swallow hole. It is the largest of these solutional depressions.
    • Blind Valley/Karst Valley: A valley whose river disappears into a swallow hole at its lower end. The valley floor is dry beyond this point.
    • Caves and Caverns: Large, hollow spaces formed beneath the ground by the dissolution of limestone along joints and bedding planes. The Borra Caves in Andhra Pradesh, India, and the Mammoth Cave in Kentucky, USA (the world’s longest known cave system) are prime examples.

  • Depositional Features (Speleothems): These features are formed within caves by the precipitation of calcium carbonate from dripping water. As water seeps through the cave roof, it loses carbon dioxide to the cave atmosphere, causing the calcium bicarbonate to revert to insoluble calcium carbonate.

    • Stalactites: Icicle-like formations that hang from the ceiling of a cave. They are formed by the deposition of calcite from dripping water.
    • Stalagmites: Formations that grow upwards from the cave floor, directly below a stalactite. They are typically thicker and more rounded than stalactites.
    • Cave Pillars (or Columns): Formed when a stalactite and a stalagmite grow to meet each other, forming a continuous column from floor to ceiling.

  • Global Distribution:

    • Europe: Karst Plateau (Slovenia, Croatia), Pennines (England), Causses region (France).
    • Asia: Meghalaya Plateau (India, famous for Mawsynram and Cherrapunji, featuring some of the world’s deepest caves like Krem Liat Prah), parts of Thailand and Malaysia.
    • North America: Yucatan Peninsula (Mexico), Kentucky and Florida (USA).
    • Australia: Nullarbor Plain in Western Australia.

Marine Topography

Marine topography is shaped by the action of sea waves, currents, and tides along coastlines. The energy for these processes is primarily derived from wind, which is a product of solar energy causing differential heating of the Earth’s surface.

  • Geomorphic Agent: Sea Waves

  • Processes:

    • Hydraulic Action: The sheer force of waves crashing against the coast, compressing air in cracks and crevices, which expands explosively, shattering the rock.
    • Corrasion (Abrasion): The grinding and wearing action of rock fragments (sand, pebbles, boulders) hurled by waves against the coastline.
    • Attrition: The wearing down of the rock fragments themselves as they are dashed against each other and the coast.
    • Corrosion (Solution): The chemical action of sea water dissolving certain types of rock, particularly limestone and chalk.
    • Cavitation: The rapid formation and collapse of vapor bubbles in water, which can exert high pressures and cause rock to fracture.

  • Erosional Features (Common on high-energy, emergent coastlines like India’s West Coast):

    • Sea Cliffs: Steep rock faces along the coast, formed by the undercutting action of waves at their base.
    • Wave-Cut Platform (or Shore Platform): A flat, gently sloping platform at the base of a cliff, representing the area eroded by waves. It is often visible only at low tide. This feature is a key element in W.M. Davis’s (1899) cycle of erosion for coastlines.
    • Sea Caves: Caves formed at the base of cliffs by wave action targeting zones of weakness in the rock.
    • Sea Arch: A natural archway formed when two sea caves on opposite sides of a headland erode and meet, or when waves cut through a narrow headland.
    • Sea Stack: An isolated pillar of rock that is left standing after a sea arch collapses. The ‘Twelve Apostles’ in Australia are famous examples, though erosion has reduced their number.
    • Sea Stump: A shorter, worn-down sea stack that may only be visible at low tide.
    • Bays and Headlands: Formed by differential erosion. Softer, less resistant rocks are eroded more quickly to form bays (inlets), while harder, more resistant rocks remain projecting into the sea as headlands (promontories).
    • Blowholes (or Gloups): A vertical shaft that connects a sea cave to the surface inland. During high seas, powerful waves can force water and spray to shoot upwards through the blowhole.
    • Hanging Valley: While predominantly a glacial feature, a coastal hanging valley can occur where a river valley’s mouth is truncated by rapid cliff erosion, leaving it hanging high above the current sea level. The coastlines of Norway, shaped by both glacial and marine forces, exhibit such features.

  • Depositional Features (Common on low-energy, submergent coastlines like India’s East Coast):

    • Beaches: Accumulations of sand, shingle, or pebbles deposited by waves in sheltered areas like bays. The swash (forward movement of water) is stronger than the backwash, leading to net deposition. Marina Beach in Chennai is the longest urban beach in India.
    • Sand Bars: Elongated ridges of sand built by waves and currents, parallel to the coast. If not connected to land, they are called offshore bars.
    • Barrier Islands: Larger, more permanent offshore bars. Sriharikota, the location of India’s satellite launch center, is a classic example of a barrier island separating Pulicat Lake from the Bay of Bengal.
    • Spit: A sand bar that is attached to the land at one end and extends out into the sea, often across a bay or estuary.
    • Tombolo: A sand bar that connects an island to the mainland or two islands to each other.
    • Lagoons: A shallow body of water separated from a larger body of water (usually the sea) by barrier islands or reefs. Chilika Lake in Odisha and Pulicat Lake on the Andhra-Tamil Nadu border are major lagoons in India. The backwaters of Kerala (e.g., Vembanad Lake) are a network of lagoons and lakes parallel to the Arabian Sea coast.

Arid / Desert Topography

Arid topography is primarily shaped by the action of wind (aeolian processes) and occasional, but intense, rainfall. Physical weathering, driven by large diurnal temperature ranges, is dominant over chemical weathering.

  • Geomorphic Agent: Wind

  • Processes:

    • Deflation: The process of lifting and removing loose particles (sand, silt) from the surface. This can lower the land surface over large areas, creating deflation hollows.
    • Abrasion (or Sandblasting): The erosive action of wind-blown sand particles against rock surfaces. This action is most effective within a meter of the ground, as larger sand grains cannot be lifted higher.
    • Attrition: The wearing down of sand particles as they collide with each other during transport. This process is responsible for the rounded nature of desert sand grains.

  • Erosional Features:

    • Mushroom Rocks (or Gours): Isolated rock formations with a narrow base and a broad top, resembling a mushroom. They are formed by intense abrasion near the ground level on a resistant rock mass.
    • Zeugen: Tabular rock masses with a layer of resistant rock overlying a layer of softer rock. Wind erodes the softer rock more rapidly, especially along joints, creating ridge and furrow landscapes. The layers are arranged horizontally.
    • Yardangs: Elongated ridges of resistant rock separated by troughs, aligned parallel to the prevailing wind direction. They are formed in regions with alternating vertical bands of hard and soft rock.
    • Ventifacts & Dreikanter: Pebbles and stones that have been faceted, polished, and shaped by wind abrasion. A Dreikanter is a classic ventifact with three polished faces.
    • Inselbergs (or Monadnocks): Isolated, steep-sided hills of resistant rock rising abruptly from a level plain. They are remnants of ancient mountains worn down by erosion. Uluru (Ayers Rock) in Australia is a famous inselberg.
    • Pediplains: Extensive, gently sloping plains formed by the coalescence of pediments (gently sloping erosional surfaces at the base of mountains in arid regions). This concept was developed by geomorphologist L.C. King in his theory of Pediplanation (1953).

  • Depositional Features:

    • Sand Dunes: Hills or ridges of sand deposited and shaped by the wind. Their shape depends on wind direction, wind speed, and sand supply.
      • Barchans: Crescent-shaped dunes with ‘horns’ or ‘limbs’ pointing downwind. They form in areas with a moderate sand supply and a unidirectional wind.
      • Parabolic Dunes: U-shaped dunes with arms pointing upwind. They are often found in coastal areas and are anchored by vegetation.
      • Seif Dunes: Long, narrow, knife-edged ridges of sand, often a variation of a barchan with one elongated limb.
      • Transverse Dunes: Long, wavy ridges of sand that form perpendicular to the prevailing wind direction, where sand supply is abundant.
      • Longitudinal Dunes (or Ergs): Long, parallel ridges of sand aligned with the prevailing wind direction. They form where sand supply is high and winds are strong and consistent. The Rub’ al Khali in the Arabian Peninsula is a vast erg.
    • Loess: Extensive blankets of fine-grained, wind-deposited silt. Loess deposits are typically unstratified and highly porous. The Loess Plateau in China is the most significant example, with deposits originating from the Gobi Desert.
    • Playas (or Bolson): Temporary lakes that form in enclosed depressions (bolsons) in desert basins after rainfall. When the water evaporates, it leaves behind a flat bed of salt and fine silt, known as an alkali flat or playa.
    • Oasis: A fertile area in a desert where the water table is close to the surface, supporting vegetation. Oases are often found in association with playas or deflation hollows.

Glacial Topography

Glacial topography is created by the action of moving ice masses called glaciers. Glaciers are powerful agents of erosion, capable of quarrying and abrading vast amounts of rock and transporting them over long distances.

  • Geomorphic Agent: Glaciers (moving ice)

  • Processes:

    • Plucking (or Quarrying): The process where a glacier freezes onto loosened blocks of bedrock and pulls them away as it moves. This is most effective at the base of the glacier where meltwater seeps into cracks and refreezes.
    • Abrasion: The grinding and scouring action of rock debris embedded in the base and sides of the glacier against the valley floor and sides. This process creates polished surfaces and fine grooves called striations.

  • Erosional Features (Alpine/Mountain Glaciation):

    • Cirque (or Corrie/Cwm): An armchair-shaped, steep-sided hollow at the head of a glacial valley, formed by nivation (erosion under a snow patch) and glacial plucking. After the glacier melts, a small lake called a tarn may occupy the cirque basin.
    • Arête: A sharp, knife-edged ridge formed between two adjacent cirques that have eroded back to back.
    • Horn: A sharp, pyramidal peak formed where three or more cirques erode back to back, leaving a pointed summit. The Matterhorn in the Swiss Alps is the classic example.
    • Col: A saddle-like pass or gap in a mountain ridge, formed when two cirques on opposite sides of a divide erode and meet, lowering the arête between them. Indira Col, the northernmost point of India, is a col.
    • U-Shaped Valley (or Glacial Trough): A valley with a characteristic U-shaped cross-section, formed when a glacier widens, deepens, and straightens a pre-existing V-shaped river valley. Yosemite Valley in California is a famous example.
    • Hanging Valley: A smaller, tributary glacial valley that hangs above the main U-shaped valley. It often has a waterfall or cascade where it joins the main valley.
    • Fjords (or Fiords): Deep, narrow, U-shaped coastal inlets that are submerged glacial troughs. They are common in Norway, New Zealand, and Chile.

  • Depositional Features (Alpine and Continental Glaciation):

    • Moraines: Ridges or blankets of unsorted glacial debris (till) deposited by a glacier.
      • Lateral Moraines: Ridges of till deposited along the sides of a valley glacier.
      • Medial Moraines: A single ridge of till formed when two valley glaciers merge and their lateral moraines combine.
      • Terminal (End) Moraines: A ridge of till deposited at the snout (end) of a glacier, marking its furthest advance.
      • Ground Moraines: Till deposited as a thin layer under the melting ice sheet.
    • Drumlins: Elongated, oval-shaped hills of glacial till, streamlined in the direction of ice flow. They often occur in large groups called ‘drumlin fields’ or ‘basket of eggs topography’.
    • Eskers: Long, sinuous ridges of sand and gravel, deposited by meltwater streams flowing in tunnels beneath, within, or on top of a glacier.
    • Tillites: Lithified (hardened into rock) glacial till from past ice ages. The presence of tillites in various continents provides key evidence for the theory of continental drift and ancient glaciations (e.g., the Karoo Ice Age during the Permo-Carboniferous period).
    • Moraine-dammed Lakes: Lakes formed when a terminal or lateral moraine acts as a natural dam, blocking meltwater. The 2013 Kedarnath disaster was exacerbated by the breach of a moraine-dammed lake, Chorabari Tal.

Prelims Pointers

  • Karst Topography:

    • Origin of term: Karst plateau in Slovenia (erstwhile Yugoslavia).
    • Primary Rock Type: Limestone (Calcium Carbonate), Dolomite.
    • Dominant Process: Corrosion/Solution by carbonic acid.
    • Erosional Features: Sinkholes, Swallow Holes, Dolines, Uvalas, Poljes, Caves.
    • Depositional Features (Speleothems): Stalactites (from ceiling), Stalagmites (from floor), Cave Pillars.
    • Indian Example: Borra Caves (Andhra Pradesh), Mawsmai Cave (Meghalaya).
    • Global Example: Yucatan Peninsula (Mexico) with its ‘cenotes’.

  • Marine Topography:

    • Agent: Sea Waves.
    • Energy Source: Wind.
    • Erosional Features: Sea Cliffs, Wave-Cut Platforms, Sea Caves, Sea Arches, Sea Stacks, Sea Stumps, Bays.
    • Depositional Features: Beaches, Sand Bars, Spits, Tombolos, Lagoons, Barrier Islands.
    • India’s East Coast is primarily depositional (e.g., Marina Beach, Chilika Lagoon).
    • India’s West Coast is primarily erosional (more cliffs and headlands).
    • Sriharikota, India’s satellite launch center, is located on a barrier island.
    • Chilika and Pulicat are major lagoons in India. Vembanad Lake (Kerala) is part of a lagoon system known as backwaters.

  • Arid/Desert Topography:

    • Agent: Wind (Aeolian processes).
    • Erosional Processes: Deflation, Abrasion, Attrition.
    • Erosional Features: Mushroom Rocks (Gours), Zeugen (horizontal hard/soft layers), Yardangs (vertical hard/soft bands), Inselbergs, Ventifacts.
    • Depositional Features: Sand Dunes (Barchan, Parabolic, Seif, Transverse, Longitudinal), Loess, Playas.
    • Barchans: Crescent-shaped dunes with horns pointing downwind.
    • Hundar Sand Dunes (cold desert) are in Nubra Valley, Ladakh.
    • Rub’ al Khali (Arabian Peninsula) is the world’s largest sand sea (erg).

  • Glacial Topography:

    • Agent: Moving Ice (Glaciers).
    • Erosional Processes: Plucking and Abrasion.
    • Alpine Erosional Features: Cirque (with Tarn lake), Arête, Horn, Col, U-Shaped Valley, Hanging Valley, Fjords.
    • Famous Horn: Matterhorn in the Alps.
    • Northernmost point of India: Indira Col.
    • Depositional Features: Moraines (Lateral, Medial, Terminal), Drumlins (‘basket of eggs topography’), Eskers.
    • GLOF: Glacial Lake Outburst Flood. Example: Chorabari Tal breach (2013 Kedarnath tragedy).
    • Tillite: Hardened glacial till, evidence of past ice ages.

Mains Insights

Human-Environment Interaction and Economic Significance (GS-I, GS-III)

  1. Karst Regions and Resource Management:

    • Water Scarcity and Unique Solutions: Karst landscapes often lack surface water, posing challenges for settlement. The Mayan civilization adapted by using cenotes (sinkholes) as their primary water source in the Yucatan Peninsula. This highlights human ingenuity in adapting to unique geographical constraints.
    • Resource Extraction and Tourism: Limestone is a crucial raw material for the cement industry. However, quarrying can destroy unique karst landscapes. Cave systems like Borra Caves are major tourist attractions, creating an economy but also posing risks of degradation if not managed sustainably.
    • Groundwater Pollution: The high permeability of karst regions makes aquifers extremely vulnerable to pollution from agriculture or industry, as contaminants can travel quickly through underground channels without natural filtration.

  2. Coastal Landforms and Strategic Importance:

    • Ports and Harbours: Natural bays and estuaries provide ideal locations for ports and harbours, crucial for trade (e.g., Mumbai, Kochi). In contrast, depositional coasts with sand bars can hinder port development.
    • Strategic Locations: Barrier islands like Sriharikota provide secure and isolated locations for strategic installations like satellite launch centers, away from dense populations and with a safe launch trajectory over the ocean.
    • Coastal Livelihoods: Depositional features like beaches and lagoons support massive economies through tourism (Goa, Kerala) and fishing (Chilika Lake). However, they are highly vulnerable to sea-level rise and coastal erosion.

  3. Desert Landforms and Human Adaptation:

    • Resource Potential: Arid regions, while seemingly barren, are often rich in mineral resources (e.g., gypsum, salt from playas). They are also ideal for solar and wind energy generation due to high insolation and consistent winds.
    • Challenges to Infrastructure: The shifting nature of sand dunes poses a significant threat to roads, railways, and settlements. Engineering solutions are required to stabilize dunes along transport corridors.
    • Cultural Significance: Deserts like the Thar have unique cultures and traditional knowledge systems for water conservation (e.g., ‘tankas’) and pastoralism, representing centuries of human adaptation to extreme environments.

Disaster Management and Climate Change (GS-III)

  1. Glacial Lake Outburst Floods (GLOFs):

    • Cause and Effect: Climate change is causing rapid glacial melting, increasing the volume of water in moraine-dammed lakes. The instability of moraine dams makes them prone to breaching, leading to catastrophic downstream floods like the 2013 Kedarnath and 2021 Chamoli events.
    • Management: This requires a multi-pronged strategy: systematic monitoring of glacial lakes using remote sensing, developing early warning systems for vulnerable downstream communities, and exploring engineering solutions to lower lake levels safely. This is a critical aspect of Himalayan ecology management.

  2. Coastal Vulnerability:

    • Erosion vs. Deposition Debate: The dynamic equilibrium of a coast is being disrupted by climate change (sea-level rise, increased storm intensity) and human activities (dam construction reducing sediment supply to deltas, coastal construction). This turns a natural process into a hazard, threatening coastal communities and infrastructure.
    • Policy Implications: There is a need for robust Coastal Regulation Zone (CRZ) norms, promotion of nature-based solutions like mangrove restoration, and planned relocation of vulnerable populations (managed retreat).

Historiographical and Geomorphological Perspectives (GS-I, Optional)

  • Cyclic vs. Acyclic Development: The study of landforms has been shaped by debates. W.M. Davis’s concept of the ‘Geographical Cycle’ (1899) proposed a sequential evolution of landforms through stages of youth, maturity, and old age. For example, he described a cycle for arid regions ending in a ‘peneplain’. In contrast, Walther Penck’s model (1924) emphasized the continuous interaction between tectonic uplift and erosion, suggesting landforms are a product of this dynamic relationship rather than a fixed cycle. These contrasting views show the evolution of geomorphological thought from a historical-descriptive approach to a more dynamic, process-oriented one.
  • Climate Geomorphology: The landforms discussed (Karst, Arid, Glacial) are prime examples of ‘climatic geomorphology,’ which argues that specific climatic regimes produce characteristic suites of landforms and processes. The global distribution of these topographies serves as evidence for this school of thought.

Graph View