Quick Summary
The hydrosphere covers 71% of Earth's surface. It includes oceans, seas, lakes, rivers, groundwater, glaciers, and water vapour. This chapter focuses on oceans, their movements (waves, tides, currents), and freshwater bodies.
Key Terms
| Term | Meaning |
|---|---|
| Salinity | Amount of salt dissolved in water |
| Tide | Rise and fall of sea level due to Moon/Sun gravity |
| Ocean current | Large mass of ocean water moving in fixed direction |
| Spring tide | Very high/low tides at full and new moon |
| Neap tide | Less extreme tides at quarter moon |
The Oceans
| Ocean | Area | Notable Feature |
|---|---|---|
| Pacific | Largest (165 million km²) | Deepest (Mariana Trench, 11,034 m) |
| Atlantic | Second largest | S-shaped; most shipping |
| Indian | Third largest | Surrounded by Asia, Africa, Australia |
| Southern | Around Antarctica | Strong westerly winds |
| Arctic | Smallest | Mostly covered by ice |
Ocean Salinity
- Average salinity: 35 parts per thousand (ppt)
- Causes: evaporation, rivers bringing salts, volcanic activity
- High salinity: Dead Sea (~340 ppt), Red Sea
- Low salinity: Baltic Sea, near river mouths
Ocean Movements
Waves
- Caused by wind blowing over the ocean surface
- Energy moves through water; water itself moves in circles
- Tsunami: giant wave caused by underwater earthquake or volcano
Tides
- Spring tides: Sun, Earth, Moon in line (full/new moon) → highest tides
- Neap tides: Sun, Moon at right angles to Earth (quarter moon) → lowest range
- Tides important for fishing, navigation, tidal power generation
- Mumbai, Chennai, and other coastal cities affected by tidal patterns
Ocean Currents
- Warm currents: Gulf Stream (Atlantic), Kuroshio (Pacific), Agulhas (Indian Ocean)
- Cold currents: Labrador Current, Canary Current, Benguela Current
- Warm currents bring rain to adjacent coasts
- Cold currents bring fog and dry conditions
Freshwater Bodies
Rivers
- Source: mountains/glaciers; mouth: sea/ocean/lake
- Important Indian rivers: Ganga, Yamuna, Brahmaputra, Godavari, Krishna, Cauvery
- Cauvery is the most important river for Tamil Nadu
Lakes
- Natural or man-made water bodies surrounded by land
- Natural lakes: Dal Lake (Kashmir), Chilika Lake (Odisha), Wular Lake
- Man-made: Mettur Dam reservoir (Tamil Nadu)
Groundwater
- Water stored in underground aquifers
- Important for drinking water and irrigation in Tamil Nadu
- Over-extraction causing depletion in many areas
Important Questions
One Mark
1. Which is the largest ocean? 2. What is spring tide? 3. Name one warm ocean current. 4. What is the average salinity of ocean water? 5. Name the deepest point in the ocean.
Short Answer (3 marks)
1. How are tides formed? Explain spring and neap tides. 2. What are ocean currents? Explain their effect on climate. 3. Name three important rivers of Tamil Nadu and their significance.
Long Answer (5 marks)
1. Describe the movements of ocean water — waves, tides, and currents. 2. Explain the importance of freshwater bodies for human life.
Map Work
- Mark and label the five oceans on a world map.
- Locate the Mariana Trench on a map.
- Mark the major rivers of Tamil Nadu: Cauvery, Palar, Vaigai.
Common Mistakes
- Pacific is largest AND deepest — students sometimes think Atlantic is largest.
- Spring tides are the STRONGEST (biggest range) — not the weakest.
- Dead Sea has very HIGH salinity, not low.
Revision Checklist
- I can list the five oceans in order of size.
- I know the difference between spring and neap tides.
- I can name one warm and one cold current with their locations.
- I can name 3 rivers of Tamil Nadu.
Frequently Asked Questions
Ocean currents are large masses of ocean water moving continuously in a definite direction. Warm currents flow from equatorial to polar regions; cold currents flow from polar to equatorial regions. They influence climate of coastal areas.