Our Changing Earth

UPSC GS1 — Earthquakes:

Cause: Sudden release of energy when tectonic plates move, friction builds up, and rocks snap; vibrations travel through the Earth as seismic waves.

Key terms:

• Focus (Hypocenter): Point underground where earthquake originates
• Epicenter: Point on Earth's surface directly above the focus — feels maximum shaking
• Seismic waves: P-waves (travel through solids + liquids), S-waves (solids only), Surface waves (most destructive)
• Seismograph: Instrument that records seismic waves
• Richter scale: Logarithmic scale measuring earthquake magnitude; each step = 10× more amplitude (wave height) and ~31.6× more energy released (7.0 releases ~31.6× more energy than 6.0, not 10×)
• Mercalli scale: Measures intensity (felt effect on people and buildings) rather than energy

India's seismic zones:

• India divided into 4 seismic zones (II, III, IV, V) — Zone V is most dangerous
• Zone V (Very High): Entire Northeast India + Andaman & Nicobar, northern Bihar, eastern Uttarakhand, western HP (Himachal Pradesh), Kashmir valley (J&K), Rann of Kutch (Gujarat)
• Zone IV (High): Delhi, Jammu, parts of UP, Maharashtra, Bihar
• Significant Indian earthquakes: Bhuj (Gujarat) January 26, 2001 (magnitude 7.6, ~20,023 killed — India's deadliest earthquake since 1950); Uttarkashi 1991; Latur (Maharashtra) 1993; Nepal-Bihar 2015 (7.8)

Why Northeast India is most seismic:

• Convergence of Indian plate with Myanmar/Burmese plate in the east
• The curved fault systems of the Himalayas and Assam

[Additional] Coastal Landforms, India's Coasts, Chilika Lake, Coral Reefs, and Bleaching (GS1 — Physical Geography / GS3 — Environment):

Wave erosion landform sequence: Cliff → Wave-cut notch (hydraulic action + abrasion at cliff base) → Wave-cut platform (cliff retreats inland) → Sea cave (joint/fault exploited) → Sea arch (two caves meet through headland) → Sea stack (arch roof collapses) → Sea stump (stack eroded further)

Wave deposition landforms:

India's two contrasting coastlines:

Chilika Lake (Odisha):

• Asia's largest coastal lagoon (world's second-largest); area = ~1,100 sq km (monsoon) / ~900 sq km (dry)
• Ramsar designation: 1981 — India's first Ramsar site; first Ramsar site in the world designated under the Convention
• Placed on Montreux Record (threatened Ramsar sites) in 1993; removed 2002 — first site from Asia removed from Montreux Record
• Harbours Irrawaddy dolphins; over 1 million migratory waterbirds annually; critically important for the fishing community (~200,000 fisherfolk)

Vembanad Lake (Kerala):

• Ramsar designation: August 19, 2002 (designated as Vembanad-Kol Wetland); area = ~151,250 hectares
• Largest brackish wetland on India's southwest coast; fed by 10 rivers

India's coral reefs — four zones:

Coral and zooxanthellae relationship:

• Coral polyps have a mutualistic symbiotic relationship with photosynthetic algae called zooxanthellae (Symbiodinium) living inside coral tissue
• Zooxanthellae provide up to 90% of the coral's energy through photosynthesis; corals provide shelter + nutrients (CO₂, nitrogen, phosphorus)
• Corals get their colour from zooxanthellae; bleaching = expulsion of zooxanthellae when stressed

Coral bleaching:

• Trigger: Sea surface temperature exceeds the local maximum monthly mean (MMM) by ≥1°C for sustained periods → zooxanthellae experience photoinhibition → expelled → coral turns white (bleaches)
• NOAA metric: Degree Heating Week (DHW) — bleaching begins at ~4°C-weeks DHW; mortality at ~8°C-weeks DHW
• Global Coral Bleaching Event 4 (GCBE-4, 2023 onward): Worst ever — 70.7% of the world's coral reefs affected
• India (2023–24): Lakshadweep = worst affected in India; severe widespread bleaching since October 2023; more intense than 2015–16 event (CMFRI); Gulf of Mannar also affected; Andaman & Nicobar also significantly impacted

UPSC synthesis: Coastal Landforms = GS1 Physical Geography. Key exam facts: wave erosion sequence = cliff → notch → cave → arch → stack → stump; longshore drift = zigzag sediment movement = responsible for spit formation; west coast rivers = estuaries (Narmada, Tapi); east coast rivers = deltas (Godavari, Krishna, Cauvery); Chilika Lake = Asia's largest coastal lagoon = Ramsar 1981 = India's first Ramsar site; Vembanad = Ramsar 2002; Gulf of Mannar = India's first Marine Biosphere Reserve = 1989 (NOT 1986); Lakshadweep = coral atolls (NOT fringing reefs — atolls are the ring-shaped ones formed as islands subsided); zooxanthellae = provide up to 90% of coral's energy; bleaching = expulsion of zooxanthellae when temperature ≥1°C above MMM for sustained period; GCBE-4 (2023) = 70.7% of world's reefs affected; India's worst = Lakshadweep. Prelims trap: Gulf of Mannar MBR = 1989 (NOT 1986 — many coaching notes incorrectly say 1986; the notified year is 1989); Lakshadweep = atolls (NOT fringing reefs — Gulf of Kutch and Gulf of Mannar have fringing reefs; Lakshadweep has atolls); Chilika = coastal lagoon (NOT freshwater lake — it is a brackish water coastal lagoon); Ramsar 1981 = Chilika = India's first (NOT Keoladeo Ghana/Bharatpur — that was also 1981 but Chilika is listed as first); estuary = west coast (Narmada, Tapi) = NOT delta.

[Additional] Aeolian Landforms, Thar Desert, and Desertification (GS1 — Physical Geography / GS3 — Environment):

Aeolian erosion processes:

• Deflation: Fine particles (clay, silt, fine sand) blown away → land surface lowered → deflation hollow/blowout; leaves behind coarser material = desert pavement (reg); Qattara Depression (Egypt) = world's largest deflation hollow
• Abrasion: Sand-laden wind sandblasts rock; most intense within ~1–2 m of ground (sand rarely rises higher); undercuts rock bases → mushroom rock

Aeolian erosion landforms:

Aeolian deposition landforms:

India's Thar Desert:

Desertification in India — ISRO SAC 2021 data (Desertification and Land Degradation Atlas):

• Trend: Increasing — land degradation is growing despite policy measures
• Rajasthan has highest absolute area under desertification (62.90% of its geographical area in 2011–13)
• Dominant cause: Wind erosion (44.2%) > water erosion (11.2%) > vegetation degradation (6.25%) > salinisation (1.07%)
• India's NAPCD: Prepared in 2001 (after ratifying UNCCD)

UNCCD:

UPSC synthesis: Aeolian landforms + desertification = GS1 Physical Geography + GS3 Environment. Key exam facts: Deflation = fine particles removed by wind = deflation hollow; abrasion = sandblasting = mushroom rock (base undercut); Yardang = parallel to prevailing wind = elongated ridge; Barchan = crescent dune = horns point downwind = moves ~10–30 m/year; Seif = parallel to wind (longitudinal); Transverse = perpendicular to wind; Loess = fine wind-deposited silt = China Loess Plateau = 50–300 m thick; Thar Desert Indian portion = ~2 lakh sq km = mainly Rajasthan + parts Gujarat, Haryana, Punjab; IGNP = ~1.86 million hectares under cultivation but caused waterlogging + salinisation; ISRO 2021 = India land degradation = 29.7% / 97.85 mha (2018–19) + 29.32% / 96.40 mha (2011–13) = increasing trend = Rajasthan most affected; UNCCD = adopted June 17, 1994 + in force December 26, 1996 + 197 parties + definition = arid + semi-arid + dry sub-humid (NOT hyperarid); India's NAPCD = 2001. Prelims trap: Barchan horns = point downwind (NOT upwind — parabolic dune has horns trailing upwind; barchan and parabolic are often confused in MCQs); Seif dune = parallel to wind (NOT perpendicular — transverse is perpendicular); Yardang = parallel to wind (same direction as seif dunes, but yardangs are erosional, seif dunes are depositional); UNCCD adopted = 1994 (entered into force = 1996 — two different years, both tested); desertification definition excludes hyperarid areas (true deserts are already desert — desertification = dry but NOT yet desert land being degraded).