Hydrosphere — Class 9 Social Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 9 Social Science, Geography — Chapter 4. Oceans, currents, tides, and the water world.
1. About this lesson
The hydrosphere covers about 71% of the Earth's surface. This chapter covers the five oceans, their characteristics, ocean currents (warm and cold), tides, salinity, ocean floor relief, and the importance of oceans.
2. The Five Oceans (by size)
| Rank | Ocean | Key Facts |
|---|---|---|
| 1 | Pacific Ocean | Largest and deepest; Mariana Trench (deepest point ~11 km); Ring of Fire |
| 2 | Atlantic Ocean | Second largest; busiest for trade; S-shaped; Mid-Atlantic Ridge |
| 3 | Indian Ocean | Warmest ocean; named after India; important for monsoon |
| 4 | Southern Ocean | Surrounds Antarctica; coldest |
| 5 | Arctic Ocean | Smallest and shallowest; around the North Pole |
3. Ocean Relief (Floor Features)
| Feature | Description |
|---|---|
| Continental Shelf | Gently sloping submerged land near continents; rich in fish and minerals; extends up to ~200 m depth |
| Continental Slope | Steep slope from shelf edge to deep ocean floor |
| Deep Sea Plain / Abyssal Plain | Vast, flat ocean floor (depth 3,000–6,000 m) |
| Oceanic Trench / Deep | Deepest parts; long, narrow depressions (e.g., Mariana Trench) |
| Mid-Oceanic Ridge | Underwater mountain chain (e.g., Mid-Atlantic Ridge) |
| Seamount / Guyot | Underwater volcanic mountain; flat-topped guyot |
4. Salinity
- Salinity: Amount of dissolved salts in seawater (measured in parts per thousand ‰).
- Average ocean salinity: 35‰.
- Factors affecting: evaporation (increases salinity), precipitation/river inflow (decreases), freezing (increases).
- Dead Sea: Highest salinity (~330‰) — no aquatic life.
5. Ocean Currents
Ocean currents are large-scale movements of ocean water in a definite direction.
Types
| Type | Direction | Temperature | Example |
|---|---|---|---|
| Warm Currents | Flow from equator → poles | Raise temperature of coastal areas | Gulf Stream, Kuroshio, Brazilian Current |
| Cold Currents | Flow from poles → equator | Lower temperature of coastal areas | Labrador, Canary, Benguela, Humboldt (Peru), California |
Important Currents
| Ocean | Warm Currents | Cold Currents |
|---|---|---|
| Atlantic | Gulf Stream, Brazilian | Labrador, Canary, Benguela |
| Pacific | Kuroshio, East Australian | California, Humboldt (Peru) |
| Indian | Agulhas, Mozambique | West Australian |
Effects of Ocean Currents
- Influence coastal climate (warm currents → warm, moist; cold currents → cool, dry).
- Cold currents create fishing grounds (mixing with warm → nutrients → plankton → fish). E.g., Grand Banks (Labrador + Gulf Stream), Peru coast (Humboldt).
- Aid navigation (ships use currents for faster travel).
6. Tides
Tides are the periodic rise and fall of sea level caused by the gravitational pull of the Moon and the Sun.
Types of Tides
| Type | When | Tidal Range |
|---|---|---|
| Spring Tide | Full Moon & New Moon (Sun + Moon aligned) | Highest high tide, lowest low tide |
| Neap Tide | First & Last Quarter Moon (Sun & Moon at right angles) | Lowest high tide, highest low tide |
Importance of Tides
- Navigation (ships enter/leave harbours at high tide).
- Fishing.
- Tidal energy generation.
7. Worked examples
Example 1. Which is the largest ocean? Pacific Ocean.
Example 2. What is the average ocean salinity? 35‰ (35 parts per thousand).
Example 3. Name a warm and a cold current in the Atlantic. Warm: Gulf Stream. Cold: Labrador Current.
Example 4. Where is the Great Barrier Reef? Australia (along the northeast coast).
8. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
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The deepest point in the Pacific Ocean is — (a) Tonga Trench / (b) Philippines Trench / (c) Mariana Trench / (d) Kuril Trench. Ans: (c) Mariana Trench.
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With increasing depth, the temperature of ocean water — (a) increases / (b) decreases / (c) remains constant / (d) none of the above. Ans: (b) decreases.
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Ocean currents are produced due to — (a) rotation of earth / (b) variation in temperature / (c) earth's movement / (d) all the above. Ans: (d) all the above.
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Consider the following statements:
- Most fishing grounds occur where the continental shelf is wide.
- Fishing is well developed in warm tropical waters.
- Mixing of warm and cold currents facilitates plant nutrients for fish.
- Inland fishing became significant in India. — (a) 1 and 2 correct / (b) 1 and 3 correct / (c) 2, 3 and 4 correct / (d) 1, 2 and 3 correct. Ans: (b). (2-wrong: fishing is better in cold/mixing waters; 4-wrong: marine fishing is more significant.)
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The oceanic ridge comes into existence due to — (a) convergence / (b) divergence of tectonic plates / (c) lateral movements / (d) shearing. Ans: (b) divergence of tectonic plates.
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Correct sequence of topography beneath the sea surface — (a) Continental shelf → Continental slope → Sea plain → Sea trench / (b) Continental slope → Continental shelf → Sea plain → Sea trench / (c) Sea plain → Continental slope → Continental shelf → Sea trench / (d) Continental slope → Sea plain → Continental shelf → Sea trench. Ans: (a).
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Which of the following is NOT correctly matched? (a) Gulf Stream – Pacific Ocean → Wrong (it's Atlantic) (b) Labrador current – North Atlantic Ocean → Correct (c) Canary current – Mediterranean Sea → Wrong (it's Atlantic) (d) Mozambique current – Indian Ocean → Correct Ans: (a) and (c).
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The amount of plankton in the ocean is determined by — 1. Depth of water. 2. Ocean currents. 3. Temperature and Salinity. 4. Length of day and night. — (a) 1 and 2 correct / (b) 1, 2 and 3 correct / (c) 1, 3 and 4 correct / (d) All correct. Ans: (b).
II. Assertion (A) and Reason (R) type
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Assertion (A): Oceans are always shown in blue in maps. Reason (R): It indicates the natural colour of the oceans. — (A) Both A and R are correct and R explains A.
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Assertion (A): Flat topped seamounts are known as Guyots. Reason (R): All guyot features are of volcanic origin. — (A) Both A and R are correct and R explains A.
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Assertion (A): Submarine canyons are deep gorges on the ocean floor. Reason (R): They are mainly restricted to continental shelf, slope and rise. — (B) Both A and R are correct but R does not explain A.
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Assertion (A): Atolls are more common in the Atlantic ocean. Reason (R): The marine population at depth is less. — A is false, R is correct. (Atolls are more common in the Pacific, not Atlantic.)
III. Match the following
| Column A | Column B |
|---|---|
| Mariana Trench | iii. Deepest point in the Pacific |
| Great Barrier Reef | iv. Australia |
| Sargasso Sea | vi. North Atlantic Ocean |
| Spring tides | vii. On full and new moon days |
| Heavy rains | i. Decreases salinity in the oceans |
| Kuroshio current | ii. Along the coast of Japan |
| Continental slope | v. Second order landform |
IV. Answer the following in brief
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What is Hydrosphere? — Hydrosphere consists of water in various forms on Earth. Over 97% of water is in oceans. Less than 3% is in glaciers, ice caps, groundwater, rivers, lakes, and water vapour.
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What is the hydrological cycle? — Water is always in motion. The continuous movement of water on, above, and below the Earth's surface is the Hydrological Cycle.
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Mention the relief features of the ocean floor. — Continental shelf, continental slope, continental rise, deep sea plain (abyssal plain), oceanic deep/trench, oceanic ridge.
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Factors generating ocean currents. — Earth's rotation, prevailing winds, differences in temperature and salinity.
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Brief note on sea waves. — Strongest ocean movements. Caused by winds. Height depends on wind speed, duration, and direction. Waves caused by ocean floor tremors are called Tsunami (very destructive).
V. Give reasons
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Northern hemisphere = land hemisphere; Southern = water hemisphere. — Northern Hemisphere: 61% land. Southern Hemisphere: 81% water. Hence the names.
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Continental shelf provides good fishing ground. — Shelf is shallower → sunlight penetrates → abundant growth of grass, seaweeds, and plankton → richest fishing grounds. Example: The Grand Bank of Newfoundland.
VI. Distinguish
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Spring tide vs Neap tide.
- Spring tide: Sun, Moon, Earth aligned → combined gravitational pull → highest tides. Occurs on Full Moon & New Moon.
- Neap tide: Sun & Moon at right angles → forces work against each other → lowest tides. Occurs at First & Last Quarter Moon.
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Abyssal plains vs Ocean deeps.
- Abyssal plains: Underwater plains on deep ocean floor; covered by thick sediments (clay, silt, sand); features: abyssal hills, seamounts, guyots, coral atolls.
- Ocean deeps (trenches): Deepest parts (~7% of ocean floor relief); V-shaped with steep sides; sediment-free; epicentres of great earthquakes found here.
VII. Answer in a paragraph
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Origin of oceans. — Oceans are a continuous body of salt water. Formed ~3 billion years ago. Initially no water on Earth. As Earth cooled, steam escaped from interior → formed clouds → incessant rains → water filled depressions for tens of thousands of years → super ocean formed.
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Continental shelf and continental slope.
- Continental Shelf: Shallow, gently sloping platform extending from land mass into sea. Almost uniform gentle gradient. Sunlight penetrates → rich fishing grounds.
- Continental Slope: Steep slope connecting the shelf to the deep ocean floor.
9. Common mistakes
- Mistake: Atlantic is the largest ocean. Fix: Pacific is the largest; Atlantic is second.
- Mistake: Spring tide occurs in the spring season. Fix: Spring tide occurs during Full/New Moon (when Sun and Moon are aligned).
- Mistake: Cold currents are useless. Fix: Cold currents create rich fishing grounds when mixing with warm currents.
9. Quick revision
- Geography Ch 4 · Hydrosphere — 71% of Earth.
- 5 oceans: Pacific (largest, deepest), Atlantic (busiest, S-shaped), Indian (warmest), Southern (coldest), Arctic (smallest).
- Ocean relief: Shelf → Slope → Abyssal Plain → Trench → Mid-Ocean Ridge.
- Salinity: avg 35‰. Dead Sea ~330‰ (highest).
- Currents: Warm (equator → poles: Gulf Stream, Kuroshio). Cold (poles → equator: Labrador, Humboldt).
- Tides: Spring (Full/New Moon, highest), Neap (Quarter Moon, lowest). Caused by Moon's gravity.
