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GC02 — Geomorphology

🌍 Physical Geography – GC02 CDS Level β˜… High Priority
πŸ“Œ CDS Focus: High-weightage topic. Questions focus on Earth's internal structure (layers, depths), plate tectonics (types of plate boundaries and resulting landforms), rock types (igneous, sedimentary, metamorphic with examples), volcanoes and earthquakes (types, seismic waves, scales), and landform identification (fold mountains, block mountains, plains, karst). Endogenic vs exogenic processes distinction is directly tested.

1. Earth's Interior

Fig. 1.1 β€” Earth's Internal Structure: Layers, Composition & Depth
INNER CORE CRUST Continental 30–70 km (SiAl) Β· Oceanic 5–10 km (SiMa) Β· Moho at base MANTLE 70–2,900 km Β· SiMa Β· Asthenosphere (semi-plastic) in upper mantle OUTER CORE 2,900–5,100 km Β· Liquid iron-nickel Β· Generates Earth's magnetic field INNER CORE 5,100–6,371 km Β· Solid iron-nickel Β· ~5,000–6,000 Β°C Β· Lehmann disc.
Topic AEarth's LayersDepths & Composition
Crust
Outermost solid layer. Continental crust (SiAl β€” Silicon + Aluminium): 30–70 km thick. Oceanic crust (SiMa β€” Silicon + Magnesium): 5–10 km thick, denser. Separated from mantle by the MohorovičiΔ‡ discontinuity (Moho).
Mantle
Largest layer (~84% of Earth's volume). Depth: 70–2,900 km. Composed of SiMa (silicates of magnesium). Upper mantle includes the asthenosphere (semi-plastic, on which tectonic plates "float"). Lower mantle is more rigid. Separated from outer core by the Gutenberg discontinuity.
Outer Core
Liquid iron-nickel. Depth: 2,900–5,100 km. Convection currents here generate Earth's magnetic field (magnetosphere). Generates Earth's protective shield against solar wind.
Inner Core
Solid iron-nickel despite very high temperatures (~5,000–6,000Β°C) because of immense pressure. Radius ~1,250 km. Separated from outer core by the Lehmann discontinuity.
⚠️ Discontinuities β€” CDS Direct Questions: Moho = crust-mantle boundary Β· Gutenberg = mantle-outer core boundary Β· Lehmann = outer-inner core boundary. Inner core is solid despite high temp (due to pressure). Outer core is liquid β€” generates Earth's magnetic field.
ROCKS & THE ROCK CYCLE

2. Rocks & Minerals

Fig. 2.1 β€” The Rock Cycle: How the three rock types transform into each other
IGNEOUS Cooled magma Β· Granite, Basalt SEDIMENTARY Deposits + fossils Β· Sandstone, Coal METAMORPHIC Heat + pressure Β· Marble, Slate Weathering & Erosion Heat + Pressure Melting β†’ Magma Magma molten rock
Rock TypeFormationExamplesCDS Fact
IgneousCooling of magma/lava (intrusive = slow/coarse; extrusive = fast/fine)Granite (intrusive), Basalt (extrusive), Obsidian, PumiceBasalt forms Deccan Traps; Granite β†’ forms peninsular India's base
SedimentaryDeposition, compaction & cementation of eroded material; contain fossilsSandstone, Limestone, Shale, Conglomerate, CoalFossil fuels found in sedimentary rocks; cover ~75% of Earth's surface
MetamorphicPre-existing rocks transformed by heat & pressure; no fossilsMarble (from limestone), Quartzite (from sandstone), Slate (from shale), Diamond (from carbon)Diamond is metamorphic; Marble used in Taj Mahal
GEOMORPHIC PROCESSES

3. Endogenic & Exogenic Processes

Fig. 3.1 β€” Endogenic vs Exogenic Processes: Origin & Key Examples
ENDOGENIC (Internal Forces) Source: Earth's interior heat & pressure β–Έ Diastrophism β€” folding & faulting of crust β–Έ Volcanism β€” lava flows, calderas β–Έ Earthquakes β€” at plate boundaries β–Έ Plate tectonics β€” convergent, divergent, transform Result: BUILD UP landforms EXOGENIC (External Forces) Source: Solar energy (water, wind, ice) β–Έ Weathering β€” physical, chemical, biological β–Έ Mass wasting β€” landslides, soil creep β–Έ Erosion β€” river, wind, glacier, wave β–Έ Deposition β€” deltas, fans, sand dunes Result: WEAR DOWN & reshape landforms
PLATE TECTONICS

4. Plate Tectonics

Fig. 4.1 β€” Three Types of Plate Boundaries & Resulting Landforms
CONVERGENT Plates move TOWARD each other β–² Fold Mtn β†’ ← Landforms: β–Έ Fold mountains (Himalayas) β–Έ Ocean trenches (Mariana) β–Έ Volcanic arcs (Japan) β–Έ Earthquakes + volcanoes India + Eurasia β†’ Himalayas DIVERGENT Plates move APART Rift ← β†’ Landforms: β–Έ Rift valleys (E. African Rift) β–Έ Mid-ocean ridges β–Έ New oceanic crust forms β–Έ Submarine volcanoes TRANSFORM Plates slide PAST each other β†’ ← Fault plane Landforms: β–Έ Fault scarps β–Έ No new crust created β–Έ Shallow earthquakes β–Έ No volcanoes typically
Topic DContinental Drift & Plate Tectonics
Wegener
Alfred Wegener (1912) β€” Continental Drift Theory. All continents were once joined as a supercontinent called Pangaea (~200–300 million years ago), surrounded by a universal ocean called Panthalassa. Pangaea split into Laurasia (north) and Gondwanaland (south) first, then further fragmented. Evidence: matching coastlines, fossil correlation, coal in Antarctica, Glossopteris flora.
Seafloor
Seafloor Spreading (Hess, 1960s) β€” new oceanic crust forms at mid-ocean ridges as plates diverge; older crust is subducted at trenches. This explained the mechanism of continental drift that Wegener could not provide.
EARTHQUAKES & VOLCANOES

5. Earthquakes

🌊 Seismic Waves

  • P-waves (Primary/longitudinal) β€” fastest; travel through solids AND liquids; first to arrive
  • S-waves (Secondary/transverse) β€” slower; travel only through solids; do NOT pass through liquid outer core
  • L-waves (Surface/Love waves) β€” slowest; most destructive; travel on surface only
  • Shadow zone β€” area between 103°–143Β° from epicentre where no waves are received (S-waves absent beyond 103Β°)

πŸ“ Scales & Terms

  • Focus / Hypocentre β€” origin point inside Earth
  • Epicentre β€” point on surface directly above focus
  • Richter Scale β€” measures magnitude (logarithmic); each unit = 10Γ— more amplitude
  • Mercalli Scale β€” measures intensity (observed damage)
  • Isoseismal lines β€” connect points of equal intensity
  • Tsunami β€” seismic sea wave; caused by undersea earthquakes
⚠️ Seismic Wave Traps: (1) S-waves cannot pass through liquids β€” this is how we know the outer core is liquid. (2) P-waves travel through everything β€” solids AND liquids. (3) L-waves are the most destructive (surface waves). (4) Richter scale is logarithmic β€” a magnitude 7 is 10Γ— more than magnitude 6 in amplitude, but ~32Γ— more in energy.

6. Volcanoes

Topic FTypes of VolcanoesCDS Direct
Shield
Broad, gently sloping volcano. Low-viscosity lava flows freely. Non-explosive eruptions. Example: Mauna Loa, Hawaii (largest volcano by volume on Earth).
Composite
Stratovolcano β€” steep, conical shape. Alternating layers of lava and ash. Highly explosive. Examples: Mt. Fuji (Japan), Mt. Vesuvius (Italy), Mt. St. Helens (USA). Most dangerous type.
Cinder Cone
Smallest and simplest. Built from ejected lava fragments (cinders). Steep sides. Single vent. Example: ParicutΓ­n, Mexico.
Caldera
Collapsed volcano crater after a massive explosion empties the magma chamber. Can fill with water to form crater lakes. Example: Crater Lake, Oregon; Lonar Lake, Maharashtra.
Ring of Fire
Circum-Pacific Belt β€” zone around the Pacific Ocean with ~90% of world's earthquakes and ~75% of active volcanoes. Includes Japan, Philippines, Indonesia, west coast of Americas. Associated with subduction zones.
MAJOR LANDFORMS

7. Mountains, Plateaus, Plains & Special Landforms

LandformFormationTypeExample
Fold MountainsConvergent plate collision; rocks fold upwardYoung (steep) / Old (rounded)Himalayas, Andes, Alps, Rockies (young); Aravallis, Appalachians (old)
Block MountainsFaulting β€” central block (horst) rises; flanks (graben) dropHorst = uplifted block; Graben = down-droppedBlack Forest, Vosges (Europe); Vindhya range
Volcanic MountainsAccumulation of lava/ashShield / Composite / Cinder coneMt. Kilimanjaro, Mt. Fuji, Hawaiian islands
PlateausUplifted, flat-topped elevated areasDissected / Lava / IntermontaneDeccan Plateau (lava); Tibetan Plateau (highest, ~4,500m); Chota Nagpur
PlainsDeposition by rivers / glaciersAlluvial / Glacial / CoastalIndo-Gangetic Plain (alluvial); North European Plain

Karst Topography

Formed in areas of soluble rocks (limestone) by chemical weathering (carbonation). Underground water dissolves limestone creating characteristic features. Directly tested in CDS.

πŸ•³οΈ Underground Karst Features

  • Caves β€” underground hollow spaces
  • Stalactites β€” hang from ceiling (calcium carbonate)
  • Stalagmites β€” rise from floor
  • Speleothems β€” general term for cave deposits

πŸŒ„ Surface Karst Features

  • Sinkholes (Dolines) β€” circular depressions
  • Uvala β€” coalesced sinkholes
  • Poljes β€” large flat-floored depression
  • Disappearing streams β€” rivers sink underground
πŸ’‘ Stalactite vs Stalagmite: Stalactite has a C β†’ hangs from Ceiling. Stalagmite has a G β†’ grows from Ground. When they meet, they form a column/pillar.

Glacial Landforms

πŸ”οΈ Erosional (Carved by ice)

  • Cirque β€” bowl-shaped depression at glacier head
  • ArΓͺte β€” sharp ridge between two cirques
  • Horn β€” pyramidal peak (3+ cirques meeting) β€” e.g., Matterhorn
  • U-shaped valley β€” carved by glacier (vs V-shape by river)
  • Fjord β€” U-valley flooded by sea

πŸͺ¨ Depositional (Left by ice)

  • Moraine β€” ridges of deposited debris (lateral, medial, terminal)
  • Drumlin β€” oval hill of glacial till; elongated in direction of ice movement
  • Esker β€” sinuous ridge deposited by subglacial streams
  • Outwash plain β€” sediment deposited beyond terminal moraine

πŸ“ Formula Sheet & Key Facts β€” GC02

Earth's Layers (Depth)
Crust: 0–70 km (continental)
Mantle: 70–2,900 km
Outer Core: 2,900–5,100 km (liquid)
Inner Core: 5,100–6,371 km (solid)
Discontinuities
Crust–Mantle: MohorovičiΔ‡ (Moho)
Mantle–Outer Core: Gutenberg
Outer–Inner Core: Lehmann
Seismic Waves Speed
Fastest: P-waves (through all media)
Medium: S-waves (solids only)
Slowest + most destructive: L-waves (surface)
Rock Examples
Igneous: Granite, Basalt, Obsidian
Sedimentary: Sandstone, Limestone, Coal
Metamorphic: Marble, Quartzite, Slate
Plate Boundaries
Convergent: fold mts + trenches
Divergent: rift valleys + mid-ocean ridges
Transform: fault scarps + shallow quakes
Pangaea & Continents
Supercontinent: Pangaea
Universal ocean: Panthalassa
Proposed by: Wegener (1912)
Split into: Laurasia + Gondwanaland

πŸ“ Topic-Wise PYQs & Tricky Questions β€” GC02

Q1. The liquid outer core of the Earth is primarily composed of: CDS PYQ
(a) Silicon and Aluminium(b) Iron and Nickel(c) Silicon and Magnesium(d) Carbon and Sulphur
βœ” Answer: (b) Iron and Nickel
Both the outer core (liquid) and inner core (solid) are composed primarily of iron and nickel. The outer core's liquid iron-nickel generates Earth's magnetic field through convection. The crust is composed of SiAl (continental) or SiMa (oceanic). The mantle is SiMa (silicates of magnesium).
Q2. Which type of seismic waves cannot pass through liquid? CDS PYQ
(a) P-waves(b) S-waves(c) L-waves(d) All of the above
βœ” Answer: (b) S-waves
S-waves (Secondary/transverse waves) can only travel through solids. They cannot pass through the liquid outer core β€” this is the key evidence that helped scientists determine the outer core is liquid. P-waves (Primary) travel through solids AND liquids. L-waves travel only on the surface. This is a frequently tested direct question.
Q3. The Himalayas are an example of which type of mountain? CDS PYQ
(a) Block mountains(b) Volcanic mountains(c) Fold mountains(d) Residual mountains
βœ” Answer: (c) Fold mountains
The Himalayas formed ~50 million years ago when the Indian Plate collided with the Eurasian Plate (convergent boundary), folding the sedimentary rocks upward. They are young fold mountains and are still rising. Block mountains (e.g., Black Forest, Vindhyas) are formed by faulting. The Aravallis are old fold mountains.
Q4. Which of the following rocks is correctly matched with its type? ⚑ Tricky
(a) Marble β€” Igneous(b) Limestone β€” Metamorphic(c) Granite β€” Igneous(d) Coal β€” Metamorphic
βœ” Answer: (c) Granite β€” Igneous
Granite is intrusive igneous rock (cooled slowly underground β†’ coarse-grained). Marble is metamorphic (formed from limestone under heat/pressure). Limestone is sedimentary (from marine organism shells). Coal is sedimentary (compressed plant material). Diamond is metamorphic carbon β€” a common CDS trap.
Q5. The term "horst" in geology refers to: ⚑ Tricky
(a) A fold mountain(b) An uplifted block between two faults(c) A down-dropped block(d) A volcanic caldera
βœ” Answer: (b) An uplifted block between two faults
In block mountain formation, tensional forces along faults cause the central block to be uplifted β€” called a horst. The down-dropped blocks on either side are called graben. Graben form rift valleys (e.g., East African Rift Valley, Rhine Valley). Black Forest and Vosges Mountains in Europe are examples of horsts flanking the Rhine Graben.
Q6. Lonar Lake in Maharashtra was formed by: ⚑ Tricky
(a) Tectonic activity(b) Meteorite impact(c) Volcanic caldera(d) Glacial erosion
βœ” Answer: (b) Meteorite impact
Lonar Lake (Maharashtra) was formed by a meteorite impact ~52,000 years ago β€” it is a saline impact crater lake, not a volcanic caldera. It is situated in the Deccan Trap basalt. This is a frequently asked CDS trap. Crater Lake, Oregon (USA) is a volcanic caldera example. Lonar is one of only four known hyper-velocity impact craters in basaltic rock worldwide.
Q7. Stalactites in a cave are formed by: CDS PYQ
(a) Glacial deposition(b) Wind erosion(c) Chemical deposition of calcium carbonate from dripping water(d) Volcanic activity
βœ” Answer: (c) Chemical deposition of calcium carbonate
Stalactites hang from cave ceilings; stalagmites grow from the cave floor. Both are speleothems formed by the deposition of calcium carbonate (CaCO₃) from dripping groundwater. This occurs in limestone (karst) regions where water dissolves and re-deposits the calcium carbonate. Memory aid: stalacTite β†’ Top; stalagMite β†’ Mound on floor.

🧠 Quick Memory Chart β€” GC02

🌍 Earth's Layers
  • Crust–Mantle: Moho
  • Mantle–Outer Core: Gutenberg
  • Outer–Inner Core: Lehmann
  • Outer core: liquid Fe-Ni β†’ magnetic field
  • Inner core: solid (high pressure)
  • Largest layer: Mantle
πŸͺ¨ Rocks Quick Match
  • Granite β†’ Igneous (intrusive)
  • Basalt β†’ Igneous (extrusive)
  • Limestone β†’ Sedimentary
  • Coal β†’ Sedimentary
  • Marble β†’ Metamorphic (←limestone)
  • Diamond β†’ Metamorphic (←carbon)
πŸ”οΈ Mountain Types
  • Himalayas β†’ Fold mountains
  • Black Forest β†’ Block mountain
  • Aravallis β†’ Old fold mountains
  • Vindhyas β†’ Block mountains
  • Mt. Fuji β†’ Composite volcano
  • Mauna Loa β†’ Shield volcano
⚑ Seismic Waves
  • P-waves: fastest; all media
  • S-waves: solids only
  • L-waves: surface; most destructive
  • S-wave absence β†’ outer core = liquid
  • Richter: logarithmic magnitude
  • Mercalli: intensity (damage)
πŸŒ‹ Plate Tectonics
  • Convergent β†’ Fold mts + Trenches
  • Divergent β†’ Rift valleys + Ridges
  • Transform β†’ Faults + Quakes
  • Pangaea (Wegener, 1912)
  • Ring of Fire β†’ 90% quakes
  • India-Eurasia β†’ Himalayas
πŸ•³οΈ Karst & Glacial
  • Stalactite β†’ from ceiling
  • Stalagmite β†’ from ground
  • Sinkhole = doline (karst)
  • Horst = uplifted block
  • Graben = down-dropped block
  • U-valley = glacial; V-valley = river
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