The ocean is never still. Its currents connect every shore. — Anonymous
13.1 Waves
Waves are oscillatory movements of water, usually wind-generated. Parts of a wave:
- Crest — highest point
- Trough — lowest point
- Wave height and wavelength depend on wind speed, duration, and fetch
Tsunamis are long-wavelength waves caused by underwater earthquakes or landslides. The 2004 Indian Ocean tsunami devastated coastal Tamil Nadu, Kerala, and Andaman & Nicobar Islands.
13.2 Tides
Tides are periodic rise and fall of sea level due to gravitational pull of the moon (primary) and sun (secondary).
| Type | Cause |
|---|---|
| Spring tide | Sun, moon, and earth aligned — highest range |
| Neap tide | Sun and moon at right angles — lowest range |
Tidal range varies: Gulf of Kachchh and Gulf of Khambhat have among the highest tides in India, useful for tidal energy projects.
13.3 Ocean Currents
Ocean currents are large-scale horizontal movements of water.
Surface currents — driven by wind and Coriolis force:
- Warm currents flow from equator to poles (e.g. Gulf Stream)
- Cold currents flow from poles to equator (e.g. Labrador Current)
Indian Ocean currents:
- Southwest monsoon current — flows east in summer
- Northeast monsoon current — reverses in winter
- Somali Current — strong flow off East Africa
Deep currents — driven by density differences (thermohaline circulation).
13.4 Effects of Ocean Movements
- Climate — warm currents warm coasts (e.g. western Europe); cold currents cool them
- Navigation — currents affect shipping routes
- Fishing — upwelling brings nutrients (Peru, Arabian Sea)
- Coastal erosion — waves reshape shorelines
Chapter Summary
- Waves (wind-driven) and tsunamis (seismic) move ocean water
- Tides caused by lunar and solar gravity; spring and neap cycles
- Surface currents driven by wind; Indian Ocean reverses with monsoon
- Ocean movements affect climate, fisheries, navigation, and coasts
- 2004 tsunami highlighted India's coastal vulnerability
Exercises (NCERT)
- Exercise 1 — formation of waves and tsunamis
- Exercise 2 — causes of tides; spring vs neap
- Exercise 3 — major ocean currents and their effects on climate