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Class 9 Oceans and Life: cheat sheet
CBSE Social Science. The ocean floor, waves, tides and currents, tsunamis and cyclones, maritime zones and the Blue Economy.
Key ideas
- The ocean floor has the continental shelf, the steep continental slope, the flat abyssal plain, deep trenches and underwater mountains such as seamounts and guyots.
- The Mariana Trench is the deepest place on Earth, and the Mid-Ocean Ridge is the longest mountain range.
- A wave’s high point is the crest and its low point the trough; waves carve caves, arches and stacks and build beaches.
- Spring tides, when the Sun, Earth and Moon are in a line, have the biggest range; neap tides, at quarter moons, the smallest.
- Currents are driven by winds, the Coriolis force, differences in temperature and saltiness, and the shape of the basins; where warm and cold currents meet, fishing is rich.
- Tsunamis are giant waves caused mainly by undersea earthquakes; India’s warnings use seismic stations, ocean-bottom recorders and tide gauges.
- The Blue Economy means using the ocean for growth and jobs while keeping it healthy.
Remember
| Nautical mile | 1,852 m = 1.852 km |
| Contiguous zone | up to 24 nautical miles from the baseline |
| Exclusive Economic Zone | up to 200 nautical miles |
| High seas | beyond every EEZ, about 64% of the oceans |
| Warm and cold currents | Gulf Stream (warm); Labrador Current (cold) |
| Coriolis force | turns currents to the right in the Northern Hemisphere |
Watch out
- Mixing up spring tides (biggest range) and neap tides (smallest): spring tides have nothing to do with the season.
- Using nautical miles as kilometres: multiply by 1.852 to convert.
- Thinking tsunamis are caused by wind: most come from undersea earthquakes.
Worked example
Two fishing boats are 163 and 372 nautical miles from India’s baseline. (a) Convert both to km (1 nautical mile = 1.852 km), to one decimal place. (b) Which boat is inside India’s Exclusive Economic Zone? (c) Why are such zones agreed under UNCLOS?
Working: 163 × 1.852; 372 × 1.852
Answer: (a) 301.9 km and 688.9 km (b) the boat at 163 nautical miles (the EEZ reaches 200) (c) So that countries share the seas fairly and avoid disputes.
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