Geophysical Phenomena — Quick Revision

A last-week revision sheet for geophysical phenomena. Prelims tests definitions and matching; Mains tests distribution, vulnerability and preparedness.

Earthquakes

Term	One-liner
Focus (hypocentre)	Point of origin inside the Earth
Epicentre	Point on the surface directly above the focus
Magnitude	Energy released — Richter (logarithmic; +1 = 10× amplitude, ~32× energy), moment magnitude (Mw)
Intensity	Effects at a place — Modified Mercalli, I–XII
Deep-focus quakes	300–700 km; only in subduction zones
Liquefaction	Saturated sand loses strength and behaves like a liquid

Belts: Circum-Pacific (Ring of Fire) → Alpine–Himalayan → mid-ocean ridges.

Indian earthquake	Year
Great Assam (Shillong)	1897
Bihar–Nepal	1934
Assam–Tibet	1950
Koyna (reservoir-induced)	1967
Latur (Killari)	1993
Bhuj	2001
Kashmir	2005
Nepal (Gorkha) — Operation Maitri	2015

India’s seismic hazard map is part of the BIS code IS 1893 (long-used map: zones II–V); check the current edition, as BIS revises it.

Tsunamis

High-yield facts

Need vertical displacement of water — subduction earthquakes (usually large and shallow), submarine landslides, volcanic eruptions.
Deep ocean: fast, long, low. Near shore: slows and grows (shoaling); the sea often retreats first.
26 December 2004: Mw 9.1 off northern Sumatra; worst-hit Indian areas: Tamil Nadu (Nagapattinam) and the Andaman and Nicobar Islands.
ITEWC, INCOIS Hyderabad (Ministry of Earth Sciences) — operational since 2007.
11 March 2011: Tōhoku earthquake and tsunami, Japan — led to the Fukushima nuclear accident.

Volcanoes

Type	Lava	Example
Shield	Fluid basalt; gentle	Mauna Loa (Hawaii)
Composite (strato)	Viscous andesite; explosive	Fuji, Vesuvius, Mount St Helens
Caldera	Collapse after explosive eruption	Krakatoa
Flood basalt	Fissure eruptions	Deccan Traps, Columbia Plateau
Mid-ocean ridge	Fissure, underwater	Mid-Atlantic Ridge, Iceland

Intrusive form	Shape
Batholith	Huge deep granite mass
Laccolith	Dome with flat base
Lopolith	Saucer
Phacolith	Wavy, along folds
Sill	Horizontal sheet
Dyke	Vertical sheet

India: Barren Island (active), Narcondam (dormant) — Andaman Sea; Baratang (Andamans) has mud volcanoes.
Stromboli — “Lighthouse of the Mediterranean”.
Big eruptions can cool climate (Pinatubo, 1991); ash disrupts aviation (Eyjafjallajökull, 2010).
Hunga Tonga–Hunga Ha’apai (Tonga), January 2022 — global pressure wave, Pacific-wide tsunamis.

Tropical Cyclones

Item	Fact
Conditions	SST > 27 °C; Coriolis; low wind shear; pre-existing low; upper divergence
Not at	The Equator (no Coriolis); rare in the South Atlantic
Eye	Calm, descending air
Eye wall	Strongest winds
Names	Cyclone (Indian Ocean), hurricane (Atlantic/E. Pacific), typhoon (W. Pacific), willy-willy (NW Australia)
IMD: cyclonic storm	62–88 km/h — named from here
IMD: super cyclonic storm	≥ 222 km/h
Naming	RSMC New Delhi; 13 countries; 169 names (2020); not reused
Deadliest hazard	Storm surge
Indian seasons	April–May and October–December
Bay vs Arabian Sea	Bay far more cyclone-prone

Changing Critical Geographical Features

Change	Example	Effect
Glacier retreat	Himalayan glaciers	Glacial lakes grow; risk of glacial lake outburst floods (GLOFs); long-term water security
GLOFs	Chorabari lake breach above Kedarnath (June 2013); South Lhonak Lake, Sikkim (October 2023), which flooded the Teesta valley	Sudden floods, dam and road damage
Shrinking lakes	Aral Sea — diversion of the Amu Darya and Syr Darya for Soviet-era irrigation; Lake Chad	Loss of fisheries, dust storms, salinisation, health effects
Sea-ice and ice-sheet loss	Arctic sea ice; Greenland and Antarctic ice sheets	Sea-level rise; albedo feedback; new Arctic shipping routes
Coastal change	Erosion, sinking deltas, sea-level rise	Threat to low islands (Maldives, Pacific islands) and the Sundarbans
Ecological shifts	Coral bleaching from marine heatwaves; species moving upslope and poleward	Biodiversity loss

Common Traps

Traps to avoid

Magnitude ≠ intensity.
Tsunamis are low in the deep ocean.
Not every undersea earthquake causes a tsunami.
The Deccan Traps are an ancient flood basalt, not an active volcanic area.
The cyclone eye is calm; the eye wall is the most violent part.
Cyclones weaken after landfall, though rain may continue inland.