Seasons on the Earth are caused mainly by:

Revolution around the Sun combined with the axial tilt

Varying distance from the Sun alone

Marble is formed from limestone by which process?

Metamorphism under heat and pressure

Geography — The Earth, Climate, Rocks, Earthquakes, India & Trade Routes

Geography in the GAT is the most map-dependent section, and it is also where a defence aspirant is expected to be strongest: knowing where the Himalayas run, which states touch which neighbour, and which strait carries the world's oil is professional knowledge, not just exam knowledge. The syllabus is precise — the earth and its grid, climate, rocks, earthquakes, regional India and trade routes — so this chapter follows it in that order and keeps the emphasis on facts you can locate on a map rather than definitions you can only recite.

The Earth, Latitudes & Longitudes

The grid — two lines that do different jobs
	Latitude	Longitude
Runs	East–west, parallel circles	North–south, meeting at the poles
Measures	Distance from the Equator (0°)	Distance from the Prime Meridian (0°, Greenwich)
Total	181 (90°N to 90°S)	360
Determines	Climate zones	Time — 1° = 4 minutes, 15° = 1 hour

🧠 Why Indian Standard Time is 82°30′E and 5½ hours ahead

IST is fixed on 82°30′E, which passes near Mirzapur in Uttar Pradesh. Since 15° of longitude equals one hour, 82.5 ÷ 15 = 5.5 hours ahead of GMT. One standard meridian is used because India spans about 30° of longitude — nearly two hours of real sun time between Gujarat and Arunachal — and running two zones would be impractical. The Tropic of Cancer (23°30′N) passes through eight Indian states, which is the other favourite grid question.

Earth's rotation on its axis takes 24 hours and causes day and night; its revolution around the Sun takes 365¼ days and causes seasons.
The axis is tilted 23½°, which is why the seasons exist at all — without the tilt there would be none.
The International Date Line runs roughly along 180° and bends to avoid land; cross it westward and you lose a day.
Earth's interior: crust, mantle, outer and inner core; the core is mostly iron and nickel — hence the magnetic field.

Rocks, Earthquakes & Volcanoes

Three rock families
Type	Formed from	Examples
Igneous ("primary")	Cooling of magma or lava	Granite, basalt (the Deccan Trap)
Sedimentary	Compression of sediment in layers; the only type with fossils	Sandstone, limestone, shale, coal
Metamorphic	Existing rock changed by heat and pressure	Limestone → marble; coal → diamond/graphite; granite → gneiss

🧠 Fossils only in sedimentary rock — and why

Igneous rock forms from molten material, which destroys any remains; metamorphic rock is crushed and heated, which erases them. Only sedimentary rock builds up gently in layers, so remains are buried intact. That is why coal and petroleum are found in sedimentary basins, and why a question that mentions fossils has already told you the rock type.

An earthquake starts at the focus underground; the point vertically above it on the surface is the epicentre, where damage is greatest.
Measured on the Richter scale (magnitude, logarithmic) and described on the Mercalli scale (intensity of damage); recorded by a seismograph.
Three wave types: P (primary, fastest, travel through solid and liquid), S (secondary, solids only), L (surface, most destructive).
Most earthquakes and volcanoes lie along plate boundaries — the Pacific Ring of Fire holds about two-thirds of the world's volcanoes.

Regional Geography of India

India by the numbers
Item	Fact
Area	32.87 lakh km², 7th largest country
Extent	8°4′N–37°6′N and 68°7′E–97°25′E
Land frontier	~15,200 km; coastline ~7,517 km
Longest border with	Bangladesh (~4,096 km); then China, Pakistan
Neighbours	Pakistan, Afghanistan, China, Nepal, Bhutan, Myanmar, Bangladesh; sea: Sri Lanka, Maldives
Highest peak in India	Kangchenjunga (8,586 m), Sikkim
Longest river	Ganga; the Brahmaputra carries more water
Southernmost point	Indira Point, Great Nicobar

India has six physical divisions: the Northern Mountains (Himalayas in three parallel ranges — Himadri, Himachal, Shiwalik), the Northern Plains built by the Indus, Ganga and Brahmaputra, the Peninsular Plateau (oldest landmass, Deccan basalt), the Thar Desert, the Coastal Plains, and the Islands — Andaman & Nicobar in the Bay of Bengal and Lakshadweep, which is coral, in the Arabian Sea.

🎯 The monsoon in one paragraph

In summer the land heats faster than the sea, creating low pressure over north-west India that pulls in moist air from the ocean — the south-west monsoon, arriving in Kerala around 1 June and covering the country by mid-July. In winter the flow reverses; the retreating north-east monsoon brings rain mainly to Tamil Nadu, which is why that state has a different rainy season from the rest of India. Mawsynram and Cherrapunji in Meghalaya are among the wettest places on Earth because the funnel-shaped hills force the air upward.

Trade Routes & Strategic Waterways

The Silk Route was the ancient overland network linking China to the Mediterranean through Central Asia, carrying silk, spices, ideas and religion — Buddhism travelled to China along it. India's own maritime trade ran from Gujarat and the Coromandel coast to Arabia, East Africa and South-East Asia, using the monsoon winds in both directions.

Chokepoints a defence candidate must know
Waterway	Connects / significance
Suez Canal	Mediterranean ↔ Red Sea; saves the voyage round Africa. Opened 1869
Panama Canal	Atlantic ↔ Pacific; opened 1914
Strait of Hormuz	Persian Gulf ↔ Arabian Sea — the world's most important oil chokepoint
Strait of Malacca	Indian Ocean ↔ South China Sea; most of East Asia's trade and oil passes here
Palk Strait	India ↔ Sri Lanka
Bab-el-Mandeb	Red Sea ↔ Gulf of Aden; the southern gate to Suez

🎯 Why these straits are a naval question, not a trivia question

A chokepoint is any narrow passage that a large share of world trade must use, which makes it both economically vital and militarily vulnerable. India sits astride the sea lanes between Hormuz and Malacca, so the Indian Ocean Region is our primary area of maritime interest — this is exactly the reasoning an SSB interviewer is listening for if the conversation turns to naval strategy. Learn the map, then learn why the map matters.

Key takeaways

Latitude decides climate; longitude decides time — 15° of longitude = 1 hour.
IST is on 82°30′E near Mirzapur, giving GMT + 5½ hours.
Rotation gives day and night; revolution plus the 23½° tilt gives the seasons.
Fossils occur ONLY in sedimentary rock — hence coal and oil in sedimentary basins.
Focus is underground, epicentre on the surface; Richter measures magnitude.
India is 7th largest; longest border with Bangladesh; Kangchenjunga its highest peak.
South-west monsoon reaches Kerala about 1 June; Tamil Nadu gets the retreating one.
Hormuz is the oil chokepoint, Malacca the East Asia trade chokepoint.

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

The standard meridian of India passes through:
1. 82°30′E near Mirzapur
2. 80°E near Chennai
3. 75°E near Ahmedabad
4. 90°E near Kolkata
Show answer
Answer: A — 82°30′E near Mirzapur
82°30′E passes near Mirzapur in Uttar Pradesh. Since 15° equals an hour, 82.5 ÷ 15 gives IST as GMT + 5½ hours.
Seasons on the Earth are caused mainly by:
1. Rotation on its axis
2. Revolution around the Sun combined with the axial tilt
3. Varying distance from the Sun alone
4. The Moon's gravity
Show answer
Answer: B — Revolution around the Sun combined with the axial tilt
The 23½° tilt means each hemisphere leans towards the Sun for part of the orbit. Without the tilt, revolution alone would produce no seasons. Rotation causes day and night instead.
Fossils are found in which type of rock?
1. Igneous
2. Sedimentary
3. Metamorphic
4. All three equally
Show answer
Answer: B — Sedimentary
Only sedimentary rock builds up gently in layers, burying remains intact. Igneous rock forms from molten material and metamorphic rock is heated and crushed, both of which destroy fossils. This is also why coal and petroleum occur in sedimentary basins.
The point on the Earth's surface directly above the origin of an earthquake is called the:
1. Focus
2. Epicentre
3. Seismic zone
4. Fault line
Show answer
Answer: B — Epicentre
The focus (or hypocentre) is underground where the rupture begins; the epicentre is the surface point vertically above it, where shaking is strongest.
India shares its longest land border with:
1. China
2. Pakistan
3. Bangladesh
4. Nepal
Show answer
Answer: C — Bangladesh
Bangladesh, at roughly 4,096 km, followed by China and then Pakistan. Candidates often assume China or Pakistan because those borders are more often in the news.
Which strait is the world's most important chokepoint for oil shipments?
1. Strait of Malacca
2. Strait of Hormuz
3. Palk Strait
4. Bering Strait
Show answer
Answer: B — Strait of Hormuz
The Strait of Hormuz links the Persian Gulf to the Arabian Sea and carries a very large share of the world's seaborne crude. Malacca is the key chokepoint for East Asian trade rather than for oil at source.
Tamil Nadu receives most of its rainfall from:
1. The south-west monsoon
2. The retreating north-east monsoon
3. Western disturbances
4. Convectional rain in summer
Show answer
Answer: B — The retreating north-east monsoon
Tamil Nadu lies in the rain shadow of the Western Ghats during the south-west monsoon, and gets its main rain from the retreating north-east monsoon in October–December. Western disturbances bring winter rain to the north-west instead.
Marble is formed from limestone by which process?
1. Sedimentation
2. Cooling of magma
3. Metamorphism under heat and pressure
4. Weathering
Show answer
Answer: C — Metamorphism under heat and pressure
Heat and pressure convert the sedimentary rock limestone into the metamorphic rock marble. The parallel pairs are coal → graphite or diamond, and granite → gneiss.