Weathering & Erosion: Glacial, Aeolian & Karst Landforms for UPSC

Weathering breaks rock. Erosion moves the broken material. The distinction sounds simple, but UPSC exploits the confusion between these two processes repeatedly. The 2017 Mains asked specifically about "the process of

formation" — students who conflated weathering with erosion lost marks on what should have been a straightforward answer.

Three erosion agents dominate UPSC Geography: glaciers, wind (aeolian), and groundwater (karst). Add fluvial processes and you have the complete geomorphology picture. Each agent produces a signature set of erosional and depositional landforms that you must know cold for both Prelims and Mains.

Types of Weathering

Weathering is the in-situ breakdown of rocks — no transportation involved. The moment material moves, it becomes erosion.

Physical (Mechanical) Weathering

Disintegrates rock without changing its chemical composition.

Frost Wedging (Freeze-Thaw): Water seeps into cracks, freezes, and expands by 9%. Repeated cycles shatter rock. Dominant in high-altitude and high-latitude regions. Produces angular rock fragments (scree/talus). Extremely active in the Himalayas — the reason mountain roads require constant maintenance.

Thermal Expansion and Contraction: Repeated heating and cooling causes differential expansion in minerals with different thermal coefficients. Most effective in deserts where diurnal temperature range exceeds 40 degrees C. Leads to granular disintegration and exfoliation (peeling of outer rock layers).

Salt Crystallization: Saline water enters pores. Water evaporates. Salt crystals grow and exert pressure up to 2,000 kg/sq cm, shattering rock from within. Active along coastlines and in arid regions. Responsible for tafoni (honeycomb weathering) seen on coastal rocks.

Pressure Release (Unloading/Exfoliation): When overlying rock is removed by erosion, the buried rock expands upward. This expansion produces curved fractures parallel to the surface, causing sheets of rock to peel off. Creates exfoliation domes like Half Dome in Yosemite.

Biological Weathering (Mechanical): Root wedging — tree roots penetrate cracks and widen them as they grow. Can split boulders over decades. Visible in ancient ruins where banyan and peepal trees have cracked stone walls (Angkor Wat, many Indian fort walls).

Chemical Weathering

Changes the chemical composition of rock minerals. Most effective in warm, humid tropical climates where water and heat accelerate chemical reactions.

Oxidation: Iron-bearing minerals react with oxygen and water. Produces iron oxides (rust). Turns rocks reddish-brown. The laterite soils of peninsular India result from intense oxidation and leaching.

Carbonation: Carbon dioxide dissolves in rainwater to form weak carbonic acid (H2O + CO2 = H2CO3). This acid dissolves limestone (CaCO3). The primary process behind karst topography. Responsible for caves, sinkholes, and stalactites worldwide.

Hydration: Minerals absorb water into their crystal structure, causing expansion and softening. Anhydrite becomes gypsum (volume increase of 30-60%). Feldspars in granite absorb water and weaken.

Hydrolysis: Water reacts chemically with minerals, particularly silicates. Feldspar + water = clay minerals (kaolinite). The most important chemical weathering process for silicate rocks, which make up 90% of the Earth's crust.

Solution (Dissolution): Rock minerals dissolve directly in water. Halite (rock salt) and gypsum are highly soluble. Limestone is soluble in acidic water. Creates underground channels and cavities.

Biological Weathering (Chemical)

Lichens secrete organic acids that dissolve rock surfaces. Burrowing animals (earthworms, rodents, ants) expose fresh rock to weathering agents. Decaying organic matter produces humic acid that accelerates chemical weathering. Bacteria can directly dissolve certain minerals.

Common student mistake: Treating biological weathering as purely mechanical. Organisms cause BOTH mechanical weathering (root wedging, burrowing) AND chemical weathering (lichen acids, humic acids). UPSC has tested this distinction.

Glacial Landforms

Glaciers cover about 10% of Earth's land surface today (mostly Antarctica and Greenland). During the Pleistocene Ice Ages, they covered 30%. Every glaciated landscape carries the signature of ice action — from the Himalayas to the Scottish Highlands.

Glacial Erosion Processes

• Plucking (Quarrying): Ice freezes onto bedrock and pulls out blocks as it moves
• Abrasion: Rock fragments embedded in the glacier base scrape the bedrock like sandpaper, producing striations (scratches) and glacial polish
• Freeze-thaw: Active above the glacier, shattering exposed rock faces

Erosional Landforms

Cirque (Corrie/Cwm): An armchair-shaped hollow at the head of a glacier. Formed by freeze-thaw weathering on the headwall and rotational sliding of ice. When filled with water after glacial retreat, it becomes a tarn (cirque lake). Example: numerous cirques in the Pir Panjal range.

Arete: A knife-edged ridge formed when two cirques erode back-to-back on opposite sides of a mountain. Extremely sharp, jagged crest. Example: the ridges between glacial valleys in the Alps.

Horn (Pyramidal Peak): Formed when three or more cirques erode around a single mountain peak. The classic pointed peak. The Matterhorn (Switzerland-Italy border) is the textbook example. Shivling peak in Uttarakhand has horn-like characteristics.

U-Shaped Valley (Glacial Trough): Glaciers transform V-shaped river valleys into U-shaped profiles — wide flat floors with steep, straight sides. The valley floor is overdeepened by ice erosion. Kashmir Valley shows glacial trough characteristics.

Hanging Valley: A tributary glacier valley that enters the main valley at a higher elevation. When ice retreats, the tributary stream forms a waterfall plunging to the main valley floor. Yosemite Falls drops from a hanging valley.

Fjord: A deep, narrow coastal inlet created when a glacial trough is partially submerged by the sea. Depths can exceed 1,000 meters. Norway's Sognefjord is 204 km long and 1,308 meters deep. No true fjords in India, but the concept is UPSC-relevant.

Roche Moutonnee: An asymmetric bedrock hill. The upstream (stoss) side is smoothed by abrasion. The downstream (lee) side is rough and steep from plucking. Indicates ice flow direction.

Depositional Landforms

Moraines: Accumulated glacial debris (till). Four types you must know:

• Lateral moraine: Along the sides of a glacier
• Medial moraine: Where two glaciers merge, their lateral moraines combine into a central ridge
• Terminal (end) moraine: Deposited at the glacier's farthest advance. Marks the maximum extent of glaciation
• Ground moraine: Sheet of till deposited beneath the glacier as it retreats

Drumlins: Elongated, egg-shaped hills of glacial till. Steep end faces the direction the ice came from, tapered end points in the direction of ice movement. Often found in groups ("swarms"). The drumlin fields of Ireland and New York state are classic examples.

Eskers: Long, winding ridges of sand and gravel deposited by meltwater streams flowing within or beneath the glacier. Can extend for hundreds of kilometers. Serpentine shape distinguishes them from moraines.

Outwash Plain (Sandur): Flat area of stratified sand and gravel deposited by meltwater streams beyond the terminal moraine. Well-sorted material (unlike till, which is unsorted).

Kettle Lakes: Depressions formed when blocks of ice buried in outwash deposits melt, causing the surface to collapse. Fill with water. The Minnesota "Land of 10,000 Lakes" has thousands of kettle lakes.

Erratics: Large boulders transported by glaciers and deposited far from their source rock. Their foreign rock type proves glacial transport. Some erratics weigh thousands of tonnes.

Aeolian (Wind) Landforms

Wind is the dominant geomorphic agent in arid and semi-arid regions — deserts covering about 33% of Earth's land surface. Wind erodes by deflation (lifting loose particles), abrasion (sandblasting with carried particles), and attrition (particles wearing each other down).

Erosional Landforms

Mushroom/Pedestal Rocks: Rocks undercut at the base by wind abrasion (strongest near ground level where sand concentration is highest). The top remains wide while the base narrows, creating a mushroom shape. Found throughout the Thar Desert and Sahara.

Yardangs: Elongated ridges of rock aligned parallel to the prevailing wind direction. Wind-carved channels separate them. Can be meters to kilometers long. Found in the Sahara, Central Asian deserts.

Ventifacts: Individual rocks shaped and faceted by wind-blown sand. Display flat, polished faces (facets) oriented toward the prevailing wind. Small-scale but diagnostic of wind erosion.

Deflation Hollows (Blowouts): Depressions formed when wind removes loose, fine-grained material from the surface. Can range from small hollows to the massive Qattara Depression in Egypt (19,605 sq km, 133 meters below sea level).

Desert Pavement (Reg): A surface layer of closely packed pebbles and gravel left behind after wind removes all fine sand and dust. Acts as a protective armor against further deflation.

Depositional Landforms

Sand Dunes: The signature aeolian landform. Classified by shape:

• Barchan: Crescent-shaped dune with horns pointing downwind. Forms with moderate sand supply and unidirectional wind. The most common dune type. Found extensively in the Thar Desert (Jaisalmer district)
• Seif / Longitudinal Dunes: Long, narrow ridges parallel to the wind direction. Can extend for hundreds of kilometers. Formed by bidirectional winds. Dominant in the Sahara and Australian deserts
• Transverse Dunes: Long ridges perpendicular to the wind direction. Form where sand supply is abundant and wind is unidirectional. Found along coastlines and in sand seas
• Star Dunes: Central peak with arms radiating outward. Formed by multidirectional winds. Tallest dune type — some exceed 300 meters. Found in the Grand Erg Oriental (Algeria)

Loess Deposits: Fine wind-blown silt deposited far from its source. The Chinese Loess Plateau is the world's largest loess deposit — up to 335 meters thick, covering 640,000 sq km. Loess is extremely fertile but highly erodible. Also found in Central Europe (from Pleistocene glacial outwash) and the Mississippi Valley.

Indian Context: The Thar Desert in Rajasthan and Gujarat displays the full range of aeolian landforms — barchans near Jaisalmer, longitudinal dunes in the eastern Thar, deflation hollows, and pedestal rocks in the Kutch region.

Karst Landforms

Karst topography develops where soluble rock (primarily limestone, also dolomite, gypsum, and rock salt) is dissolved by slightly acidic groundwater. The process is carbonation — carbonic acid in rainwater reacts with calcium carbonate.

Named after the Kras/Karst region in Slovenia-Italy, where this landscape type was first scientifically studied.

Surface Karst Features

Sinkholes (Dolines): Circular depressions formed by dissolution of limestone at the surface or collapse of underground cavities. Range from meters to hundreds of meters wide. Two types: solution sinkholes (gradual dissolution) and collapse sinkholes (sudden cave roof failure — can swallow buildings).

Uvala: Formed when multiple sinkholes merge. Larger, irregular depression with an uneven floor.

Polje: The largest karst depression — a flat-floored valley several kilometers long. May have a temporary lake during wet season. Classic examples in the Dinaric Alps (Croatia, Bosnia).

Karren/Lapies: Grooves and channels dissolved into exposed limestone surfaces. Creates a furrowed, fluted rock surface.

Disappearing Streams: Rivers that flow into sinkholes or cave entrances and continue underground through dissolved passages. May re-emerge kilometers away as springs. Common in Meghalaya's limestone areas.

Underground Karst Features

Caves/Caverns: Underground chambers dissolved along joints and bedding planes. Can extend for hundreds of kilometers. Mammoth Cave (Kentucky) is the world's longest at 676 km mapped.

Stalactites: Calcium carbonate formations hanging from cave ceilings. Formed when dissolved CaCO3 in dripping water precipitates as carbon dioxide escapes. Grow downward. Growth rate: roughly 1 cm per century.

Stalagmites: Calcium carbonate formations growing upward from the cave floor. Formed from the same dripping water. Generally thicker and more rounded than stalactites.

Pillars/Columns: When a stalactite and stalagmite meet and fuse. Indicates long, stable geological conditions.

Underground Rivers: Water flowing through dissolved cave passages. Can be substantial — the Puerto Princesa Underground River (Philippines) is 8.2 km long and navigable by boat.

Karst in India

Meghalaya has India's most developed karst landscape. The Mawsmai Caves, Krem Liat Prah (longest cave in India at over 34 km), and numerous other cave systems are formed in Cretaceous and Tertiary limestones. The extreme rainfall (Mawsynram and Cherrapunji) accelerates carbonation.

Chhattisgarh's Kutumsar Cave and Kailash Cave in the Bastar district are significant karst features.

Panchmarhi, Madhya Pradesh has caves formed in sandstone with some karst characteristics.

Comparison Table: Erosion Agents and Their Landforms

For detailed coverage of river-formed landforms, see the fluvial landforms guide.

Connections Between Weathering, Erosion, and Climate

Humid Tropical: Chemical weathering dominates. Intense hydrolysis produces deep laterite soils. Biological weathering also strong. Thick regolith. Connected to biosphere and ecosystem dynamics.

Arid/Semi-Arid: Physical weathering dominates (thermal expansion, salt crystallization). Wind erosion active. Thin, poorly developed soils.

Periglacial (Cold): Frost weathering dominates. Permafrost-related processes. Solifluction, frost heave, patterned ground.

Temperate: Balanced physical and chemical weathering. Moderate soil development. Frost action significant in winters.

Common student mistake: Writing that "chemical weathering is most active in deserts because of high temperatures." Wrong. Chemical weathering requires WATER. High temperatures accelerate reactions, but without moisture, chemical weathering is minimal. The humid tropics (Amazon, Congo, Southeast Asia) have the highest chemical weathering rates, not deserts.

UPSC Previous Year Connections

2017 Mains: “What are the geomorphic processes? Discuss briefly the process of soil formation.” — Required discussing weathering as the foundation of soil formation.

2019 Prelims: Questions on specific landform identification — cirques, moraines, and karst features have appeared.

2020 Mains: "Discuss the geomorphological features of the Deccan Plateau." — Required connecting basaltic weathering with laterite formation and landform evolution.

The topic connects to environmental conservation when discussing soil erosion, desertification, and land degradation under UNCCD.

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