Samacheer Class 9 Social Science - Lithosphere - II Exogenetic Processes

Greenland is at high latitude (near the Arctic) and much colder year-round, so the altitude at which snow persists year-round (snowline) is low (~600 m). The Alps are at lower latitudes and warmer, so permanent snow occurs only at higher altitudes (~2700 m). Local climate, temperature, and precipitation patterns control snowline.

Mixing baking soda (a fine powder) with shaving cream (foam) binds the particles into a light, moldable, white mixture that resembles artificial snow. This is a physical mixture (not a chemical reaction); the foam gives volume and cohesion.

Concise effects: (1) Sea level rise from melting ice and thermal expansion; (2) Increased frequency/intensity of extreme weather (storms, heatwaves, floods, droughts); (3) Melting glaciers and reduced snow cover; (4) Loss of biodiversity and habitat shifts; (5) Impacts on agriculture, water supply and human health; (6) Ocean acidification affecting marine life.

1. a. Rapids
2. b. Alluvial fan
3. c. Delta
4. d. Gorges

Delta is a depositional feature formed at the mouth (lower course) of a river where sediment is dropped as the river's velocity decreases on entering standing water. Rapids and gorges occur in upper course; alluvial fans form at mountain fronts.

1. a. Statement I is false and II is true
2. b. Statement I and II are false
3. c. Statement I is true and II is false
4. d. Statement I and II are true

Both statements are true: running water (rivers) is a major agent of gradation (erosion, transport, deposition); the erosive power and transport capacity of a river depend on the slope (gradient) of the land it flows over.

Upper course (youthful): steep gradient, dominant vertical erosion — landforms: V-shaped valleys, interlocking spurs, waterfalls, rapids.
Middle course (mature): moderate gradient, lateral erosion and transportation — landforms: meanders, river cliffs, point bars, river terraces, ox-bow lakes.
Lower course (old): gentle gradient, dominant deposition — landforms: wide flood plains, levees, deltas, estuaries, ox-bow lakes (old), alluvial plains.

Wind transports abrasive particles (sand, dust) in suspension and saltation which strike rock surfaces from many directions. Because wind direction can vary and particles are small enough to access sheltered surfaces, abrasion acts all around exposed rock. Deflation (removal of loose particles) also undermines and exposes all sides, making wind erosion effective from multiple directions.

Physical weathering: mechanical breakup (freeze–thaw, exfoliation, abrasion) with no chemical change; results in fragments and increased surface area. Chemical weathering: chemical reactions (hydrolysis, oxidation, carbonation) change mineral composition and produce new minerals or dissolved ions. Biological weathering can involve both mechanical and chemical effects.

Delta:
- Formed by deposition of river-borne sediments where a river enters a standing body of water (sea/lake).
- Generally has a triangular or fan-shaped wetland/landform built out into the sea.
- High silt/sediment accumulation; often has distributary channels.
- Water is fresh to brackish near river mouth; tidal influence may be small depending on river.
Estuary:
- A drowned river mouth where sea water floods the lower course of a river due to rising sea level or subsidence.
- Usually funnel-shaped and deeper; strong mixing of fresh and sea water (brackish).
- Low net sediment build-up compared to deltas; strong tidal currents may remove sediment.
- Important as tidal marshes, nurseries for fish and high biological productivity.

Stalactite:
- Hanging from cave ceilings.
- Formed by deposition of calcium carbonate from dripping water; ‘C’ in stalactite = ceiling (mnemonic).
- Grow downward.
Stalagmite:
- Built up from cave floor directly beneath drips.
- Formed from drops that fall and deposit calcite; grow upward.
- Often broader and more conical; stalactite + stalagmite may join to form a column.

Longitudinal dunes:
- Ridges parallel to the prevailing wind direction.
- Form where wind direction varies slightly or sand supply is moderate.
- Tend to be long straight or slightly sinuous ridges.
Transverse dunes:
- Ridges perpendicular (at right angles) to prevailing wind.
- Form where wind is fairly constant in direction and sand supply is abundant.
- Appear as a series of wave-like crests across the landscape.

Inselberg:
- Isolated residual hills or rock islands rising abruptly from a peneplain or plain (e.g., bornhardt).
- Formed by differential weathering and erosion; resistant rock left standing.
- Usually rounded or dome-shaped.
Yardang:
- Streamlined, elongated ridge sculpted by wind abrasion in arid regions.
- Aligned parallel to prevailing wind; sharp windward slope and tapered lee side.
- Formed by aeolian erosion rather than residual weathering.

Spit:
- A narrow ridge of sand or shingle projecting from the coast into the sea, formed by longshore drift; often with a curved hooked end.
- Connected to land at one end and open at the other.
Bar:
- A ridge of sand or shingle that extends across the mouth of a bay or river, connecting two headlands or across an inlet.
- Can enclose a lagoon (forming a bay-barrier) and may completely cut off the bay from the sea.

Weathering is the in-situ breakdown and disintegration of rocks at or near the Earth's surface by physical, chemical and biological processes without movement of the material. Main types:
1. Physical (mechanical) weathering:
- Breakdown of rock into smaller pieces by physical forces.
- Processes: freeze–thaw (ice wedging), thermal expansion and contraction, pressure release (exfoliation), salt crystallization.
- Produces angular fragments and increases surface area for further weathering.
2. Chemical weathering:
- Decomposition or alteration of minerals by chemical reactions with water and gases.
- Processes: solution (especially of carbonates), hydrolysis, oxidation, carbonation.
- Produces clays, soluble ions and changes rock chemistry; common in warm, wet climates.
3. Biological weathering:
- Caused by plants, animals and microbes.
- Roots widen cracks, burrowing animals expose rock to weathering, and organisms produce organic acids that chemically alter minerals.
Weathering is a key preparatory step for erosion and soil formation; climate, rock type and vegetation control its rate and nature.

Underground water, especially in soluble rocks like limestone, erodes by chemical solution and mechanical removal to form characteristic landforms:
- Caves and caverns: enlarged joints and bedding planes where groundwater dissolves rock; may form extensive underground chambers.
- Sinkholes (dolines): circular or funnel-shaped depressions formed when roof of an underground cave collapses or by solution at the surface.
- Swallow holes/ponors: openings where surface streams sink into subterranean channels.
- Underground drainage channels and conduits: networks that carry water below ground instead of on the surface, often leading to spring outlets.
- Karst valleys and dry valleys: surface valleys formed by collapse or the capture of surface streams into underground systems.
Note: Stalactites and stalagmites are secondary depositional features formed from dripping cave water.

A glacier is a persistent body of dense ice that forms on land from the accumulation and compaction of snow and moves under its own weight.
Types:
- Valley (alpine) glaciers: flow down valleys in mountainous areas, confined by valley walls.
- Cirque glaciers: small glaciers in bowl-shaped hollows (cirques) on mountainsides; source areas for valley glaciers.
- Piedmont glaciers: form when valley glaciers spill out onto lowlands and spread into broad lobes.
- Ice caps: dome-shaped ice masses covering less than 50,000 km2, burying underlying topography.
- Ice sheets: very large continental ice masses over 50,000 km2 (e.g., Antarctica, Greenland).
- Tidewater glaciers: valley glaciers that terminate in the sea and calve to form icebergs.
Each type differs by size, setting and pattern of flow.

Depositional work of wind (aeolian deposition):
- Processes: wind transports sand-sized particles by saltation (bouncing), fine dust in suspension, and larger grains by surface creep. When wind velocity falls below carrying capacity, particles settle and accumulate.
- Main depositional landforms:
- Dunes: ridges or mounds of sand formed downwind of obstacles. Types include barchan (crescentic), transverse, longitudinal (seif), parabolic and star dunes, each reflecting wind regime and sand supply.
- Sand sheets: flat expanses of sand with little dune development, formed where wind moves sand more uniformly.
- Loess deposits: thick blankets of fine silt and dust deposited downwind of deserts or glacial outwash plains; form fertile but easily eroded soils.
- Factors controlling deposition: wind velocity, availability and size of sediment, presence of obstacles, and vegetation. Aeolian deposits are important for soil formation and landscape development in arid and semiarid regions.

1. Two deltas (mark on world map):
- Nile Delta (northern Egypt, Mediterranean coast)
- Ganges–Brahmaputra Delta (northeastern India / Bangladesh, Bay of Bengal)
2. A karst region (mark):
- Dinaric Karst (Balkans) or Guilin–Yangshuo area (southern China) or Yucatan Peninsula (Mexico).
3. Two hot deserts (mark):
- Sahara Desert (North Africa)
- Arabian Desert (Arabian Peninsula)
Two cold deserts (mark):
- Gobi Desert (Mongolia / northern China) — cold semi-arid
- Antarctica (Antarctic polar desert) or Arctic (Greenland/Arctic region) as polar deserts.
Instruction: locate and label approximate positions on the outline map: deltas at river mouths; karst region in chosen region; deserts in their continental locations.

Limestone dissolves easily in slightly acidic water (carbonation), enlarging joints and bedding planes to create conduits and voids. This increases permeability so precipitation infiltrates and is carried underground rather than flowing on the surface. The result is extensive subterranean drainage (karst systems), sinkholes and springs; thus surface runoff is reduced compared to non-karst terrains.

a. Chemical alteration/solution of carbonate rocks in limestone regions is called karstification (or solution weathering), producing karst landforms.
b. Flat surfaces near cliffs, especially coastal cliffs, are often called wave-cut platforms (or benches).
c. The result of erosion and transportation is deposition, also called sedimentation.
d. The lower limit of a permanent snow field is commonly termed the snowline (sometimes the firn line where compacted snow begins to recrystallize into firn).