📑 Topics on this page (Jump to Topic):
📌 1. Oceans of the World
| Ocean | Feature |
|---|---|
| Pacific | Largest and deepest; ≈ 1/3 of Earth; most islands; Mariana Trench — Challenger Deep (≈ 10,994 m), Earth's deepest point |
| Atlantic | Second largest; "S"-shaped; busiest sea routes; deepest — Puerto Rico Trench |
| Indian | Third; only ocean named after a country; deepest — Java (Sunda) Trench |
| Southern (Antarctic) | Around Antarctica (south of 60° S) |
| Arctic | Smallest and shallowest; mostly frozen |
- Ocean-floor relief: continental shelf — shallow, rich in fish and oil (e.g. Mumbai High) → continental slope → abyssal plain → trenches.
- Depth unit fathom (1 fathom = 6 ft ≈ 1.83 m); measured by sonar / fathometer.
📌 2. Salinity
- Average ocean salinity ≈ 35‰ (35 g per 1000 g of water); main salt sodium chloride.
- Saltiest water bodies: Lake Van (Turkey) ≈ 330‰, Dead Sea ≈ 340‰ (Jordan-Israel; lowest land point); Great Salt Lake (USA). Among open seas the Red Sea is saltiest.
- Lowest salinity — Baltic Sea (much river water, low evaporation).
📌 3. Tides
- Caused mainly by the Moon's gravity (Sun's effect is under half); each place gets two high tides in ≈ 24 h 52 min — interval ≈ 12 h 26 min.
- Spring tide — on new/full moon, Sun-Moon-Earth in line. Neap tide — on quarter moons, at right angles.
- World's highest tides — Bay of Fundy (Canada); in India the Gulf of Khambhat has tidal energy potential.
📌 4. Ocean Currents
| Current | Type | Ocean |
|---|---|---|
| Gulf Stream | Warm — keeps W. Europe warm | North Atlantic |
| Kuroshio | Warm ("black current" of Japan) | North Pacific |
| Labrador | Cold | North Atlantic |
| Oyashio / Kurile | Cold | North Pacific |
| Humboldt (Peru) | Cold — among the richest fishing zones | South Pacific |
| Benguela | Cold — Namib Desert | South Atlantic |
| California, Canary, West Australian | Cold | — |
| Brazil, Agulhas, East Australian | Warm | — |
- Where warm and cold currents meet there is fog and plankton → major fishing grounds, e.g. Grand Banks (Newfoundland — Gulf Stream + Labrador), Dogger Bank (North Sea).
- Cold currents create deserts on the west coasts of continents (Atacama, Namib).
- Sargasso Sea — calm sea in the N. Atlantic bounded by currents, with no coastline; full of seaweed.
📌 5. Coral Reefs
- Coral polyps need warm (> 20–21 °C), clear, shallow water; types: fringing, barrier and atoll (ring-shaped).
- Largest — Great Barrier Reef (Australia, Queensland coast). In India: Lakshadweep (atolls), Gulf of Mannar, Gulf of Kutch, Andaman-Nicobar.
- Rising water temperature causes coral bleaching.
⚠️ Exam Traps:
- The deepest point is Challenger Deep (Pacific); the Atlantic's deepest is the Puerto Rico Trench.
- Spring tides occur on both new moon and full moon.
- Gulf Stream is warm, Labrador cold — they meet at the Grand Banks.
📌 6. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Which is the largest and deepest ocean? Practice
Correct answer: (B)
The Pacific covers about one-third of Earth.
The Pacific covers about one-third of Earth.
Q2. Earth's deepest point "Challenger Deep" is in the — Practice
Correct answer: (C)
The Mariana Trench (W. Pacific, near Guam) is ≈ 10.9 km deep.
The Mariana Trench (W. Pacific, near Guam) is ≈ 10.9 km deep.
Q3. The only ocean named after a country is the — Practice
Correct answer: (D)
The Indian Ocean is named after India.
The Indian Ocean is named after India.
Q4. The smallest ocean is the — Practice
Correct answer: (A)
The Arctic is the smallest and shallowest ocean.
The Arctic is the smallest and shallowest ocean.
Q5. The average salinity of oceans is about — Practice
Correct answer: (B)
Average salinity ≈ 35‰, i.e. 35 g per kg of water.
Average salinity ≈ 35‰, i.e. 35 g per kg of water.
Q6. Among open seas, the highest salinity is in the — Practice
Correct answer: (C)
High evaporation and little river inflow make the Red Sea very salty (≈ 40‰).
High evaporation and little river inflow make the Red Sea very salty (≈ 40‰).
Q7. Which sea has the lowest salinity? Practice
Correct answer: (D)
Due to heavy river inflow and low evaporation.
Due to heavy river inflow and low evaporation.
Q8. Tides are caused mainly by — Practice
Correct answer: (A)
Being closer, the Moon's tidal effect is more than twice the Sun's.
Being closer, the Moon's tidal effect is more than twice the Sun's.
Q9. When does a spring tide occur? Practice
Correct answer: (B)
Sun, Moon and Earth in line make tides highest.
Sun, Moon and Earth in line make tides highest.
Q10. The interval between two high tides at a place is about — Practice
Correct answer: (C)
Two high tides in a lunar day of ≈ 24 h 52 min → ≈ 12 h 26 min.
Two high tides in a lunar day of ≈ 24 h 52 min → ≈ 12 h 26 min.
Q11. The world's highest tides occur at — Practice
Correct answer: (D)
In the Bay of Fundy (Canada) the tidal range reaches ≈ 16 m.
In the Bay of Fundy (Canada) the tidal range reaches ≈ 16 m.
Q12. Which current keeps Western Europe warm? Practice
Correct answer: (A)
The Gulf Stream / North Atlantic Drift carries warm water to Europe.
The Gulf Stream / North Atlantic Drift carries warm water to Europe.
Q13. Which of these is a cold current? Practice
Correct answer: (B)
The Humboldt is a cold current along South America's west coast.
The Humboldt is a cold current along South America's west coast.
Q14. The Grand Banks fishing ground forms where which currents meet? Practice
Correct answer: (C)
Near Newfoundland the warm-cold mix boosts plankton.
Near Newfoundland the warm-cold mix boosts plankton.
Q15. The cold Benguela Current is linked to which desert? Practice
Correct answer: (D)
The cold current off SW Africa creates the Namib Desert.
The cold current off SW Africa creates the Namib Desert.
Q16. Which sea has no coastline and is bounded by currents? Practice
Correct answer: (A)
The Sargasso Sea lies inside the North Atlantic gyre.
The Sargasso Sea lies inside the North Atlantic gyre.
Q17. The world's largest coral reef is off — Practice
Correct answer: (B)
The Great Barrier Reef is ≈ 2,300 km long.
The Great Barrier Reef is ≈ 2,300 km long.
Q18. Lakshadweep islands are mainly — Practice
Correct answer: (C)
Lakshadweep consists of ring-shaped coral atolls.
Lakshadweep consists of ring-shaped coral atolls.
Q19. The fathom, a unit of sea depth, equals about — Practice
Correct answer: (D)
1 fathom = 6 ft ≈ 1.83 m.
1 fathom = 6 ft ≈ 1.83 m.
Q20. Which part of the ocean, being shallow, is richest in fish and petroleum? Practice
Correct answer: (A)
Sunlight reaches the shelf; Mumbai High lies on it.
Sunlight reaches the shelf; Mumbai High lies on it.