Ocean relief (the shape of the sea floor)
- Continental shelf: the shallow, gently sloping edge of the land under the sea (up to about 200 m deep). Rich in fish and oil.
- Continental slope: the steep drop from the shelf into deep water.
- Deep sea (abyssal) plain: a wide, flat floor 3–6 km deep, covered with fine mud.
- Mid-ocean ridge: long underwater mountain chains where plates pull apart.
- Trench: long, very deep narrow valleys where one plate dives under another. The Mariana Trench is about 11 km deep, the deepest place on Earth.
Waves, tides and currents
Waves are made mostly by wind. The water mostly moves up and down in circles; the wave's energy moves forward. Near the shore the wave "breaks".
Tides are the regular rise (high tide) and fall (low tide) of the sea, usually twice a day. They are caused by the pull of the Moon (and the Sun). When Sun, Moon and Earth are in a line (full and new moon) tides are highest: spring tides. At half moon they are lowest: neap tides. Tides help ships enter ports and can make electricity.
Currents are steady flows of water in a fixed direction, like rivers in the sea. They are driven by winds, heat and saltiness. Warm currents flow from the equator towards the poles and warm the coasts; cold currents flow towards the equator. Where warm and cold currents meet, there is plenty of plankton and fish.
Marine resources and livelihoods
- Food: fish, prawns, crabs, seaweed.
- Minerals: salt from salt pans (Gujarat), oil and natural gas from the shelf (Mumbai High), sand, and metal nodules on the deep floor.
- Energy: tidal and wave power, offshore wind.
- Transport and trade: most goods between countries travel by ship; ports give jobs.
- Tourism: beaches, coral reefs.
Crores of people in coastal India are fishers, boat builders, salt workers and port workers. The blue economy means using the sea for jobs while keeping it healthy: no overfishing, no plastic, protecting mangroves and corals.
Cyclones, tsunamis and early warning
A tropical cyclone is a huge spinning storm that forms over warm sea water (above about 27 °C). Warm moist air rises, forms clouds, and the storm spins around a calm centre called the eye. It brings very strong winds, heavy rain and a storm surge (sea water pushed onto land). The Bay of Bengal gets many cyclones in May and October–November.
A tsunami is a series of giant waves caused by an undersea earthquake, landslide or volcano. In deep water it is low but very fast (up to 700–800 km/h); near the coast it slows down and rises very high.
Early warning: satellites and radars track cyclones (IMD); ocean buoys and seismographs detect tsunamis (INCOIS). Warnings on radio, TV and phones, cyclone shelters and mangrove belts save lives. If the sea suddenly pulls far back, run to high ground.
International maritime rules
The United Nations Convention on the Law of the Sea (UNCLOS, 1982) is the main rulebook for the seas. Distances are counted from the coast (baseline) in nautical miles (1 nm ≈ 1.852 km):
| Zone | Distance | Rights |
|---|---|---|
| Territorial sea | 0–12 nm | Full control, like land; other ships may pass peacefully. |
| Contiguous zone | 12–24 nm | Can check customs, pollution, immigration. |
| Exclusive Economic Zone (EEZ) | up to 200 nm | Only this country may fish and take oil and minerals. |
| High seas | beyond 200 nm | Open to all countries; shared by humankind. |
Key formulas and definitions
- Sea floor from coast: Shelf → Slope → Deep plain → Ridge / Trench
- Waves ← wind; Tides ← Moon and Sun pull; Currents ← wind, heat, saltiness
- Spring tide = Sun, Moon, Earth in line; Neap tide = at right angles
- Tsunami speed ≈ √(9.8 × depth) m/s
- UNCLOS: territorial sea 12 nm; contiguous zone 24 nm; EEZ 200 nm; 1 nm ≈ 1.852 km
Worked examples
1. Where is fishing best: on the continental shelf or the deep plain? Why?
On the shelf. The water is shallow, sunlight reaches the bottom, plankton grows and fish gather.
2. A tsunami crosses ocean 4,000 m deep. Find its speed.
√(9.8 × 4,000) = √39,200 ≈ 198 m/s ≈ 713 km/h (as fast as a jet plane).
3. At that speed, how long to cross 1,500 km of ocean?
1,500 ÷ 713 ≈ 2.1 hours. So a warning issued quickly gives about 2 hours to move people.
4. How many km is the EEZ limit of 200 nautical miles?
200 × 1.852 ≈ 370 km.
5. Why are tides highest on full-moon and new-moon days?
The Sun, Moon and Earth are in a line, so their pulls add up. These are spring tides.
6. Near the coast the depth is 10 m. Find the tsunami speed there.
√(9.8 × 10) = √98 ≈ 9.9 m/s ≈ 36 km/h. It slows down, so the water piles up into a tall wave.
Common mistakes
- Thinking waves carry the water itself across the ocean. Mostly the energy moves; water moves in small circles.
- Saying tides are made by wind. Tides come from the pull of the Moon and the Sun.
- Thinking a tsunami is always tall in the open ocean. It is low there and rises only near the coast.
- Thinking the EEZ is part of a country like land. It gives resource rights; other ships may still pass.