Class 9 Social Science · Chapter 10 NotesOceans and Life

Just like land, the ocean floor has mountains, valleys, plains, and trenches. These features are called submarine relief or ocean relief. The continental shelf is the shallow, submerged edge of a continent and is rich in marine life and resources such as oil and gas. Beyond it lies the steep continental slope, marking the true edge of the continent. The abyssal plain is a vast, flat, and deep ocean floor. The mid-ocean ridge is the world’s longest mountain range, formed by volcanic activity. Seamounts are underwater volcanic mountains; when their tops are flattened by wave action, they are called guyots. Ocean trenches are the deepest parts of the ocean, such as the Mariana Trench in the Pacific. Some underwater mountains, like Mauna Kea in Hawai‘i, are taller than Mount Everest.

• Continental shelfshallow, resource-rich edge of a continent.
• Continental slopesteep drop marking the continent’s edge.
• Abyssal plainvast, flat, deep ocean floor.
• Mid-ocean ridgelongest underwater mountain range.
• Seamountunderwater volcanic mountain; guyot: flat-topped seamount.
• Trenchdeepest part of the ocean, e.g., Mariana Trench.

When the ocean surface rises and falls alternately, it forms a wave. Waves are mainly caused by winds blowing across the water and carry energy across long distances. The high point of a wave is the crest, and the low point is the trough. Wavelength is the horizontal distance between two consecutive crests or troughs. Wave frequency is the number of crests passing a point each second, and wave period is the time for one crest to reach the next point. Waves shape the coastline through erosion and sediment transport. They create erosional features such as sea cracks, caves, arches, stacks, and stumps, and depositional features such as beaches. During storms, strong winds generate huge waves. Underwater earthquakes can cause giant waves called tsunamis, which may reach a run-up height of up to 30 metres or more.

• Cresthighest point of a wave.
• Troughlowest point of a wave.
• Wavelengthdistance between two consecutive crests or troughs.
• Wave frequencynumber of crests passing a point per second.
• Wave periodtime for a crest to travel from one point to the next.
• Tsunamigiant wave caused by underwater earthquakes.

Tides are the regular rise and fall of ocean water, occurring twice a day in most places. High tide (flood tide) is when water rises to its highest level, and low tide (ebb tide) is when it falls to its lowest. Tides are caused by the gravitational pull of the Sun and the Moon. When the Sun, Earth, and Moon are in a straight line—on full moon and new moon days—their combined pull causes spring tides, with unusually high high tides and low low tides. When the Moon is in its first and third quarters, the pulls act in different directions, producing neap tides with a smaller difference between high and low tide. High tides help ships enter harbours and support fishing. Tidal energy can generate electricity; in India, the Gulf of Kutch, Gulf of Khambhat, and Sundarbans are identified as suitable sites.

• Spring tideduring full moon and new moon, when Sun, Earth, and Moon align.
• Neap tideduring first and third quarter moon, when tidal range is smaller.
• High tides aid navigation and fishing.
• Tidal energy sites in IndiaGulf of Kutch, Gulf of Khambhat, Sundarbans.

Ocean currents are continuous streams of water flowing across the ocean surface in definite directions. They are caused by winds, the Earth’s rotation (Coriolis force), differences in temperature and salinity, and the shape of continents and ocean basins. Currents flowing from warmer regions are warm currents, while those from colder regions are cold currents. On maps, warm currents are usually shown in red and cold currents in blue. The Gulf Stream, a warm current from the Gulf of Mexico, keeps the waters near Norway relatively warm, so sea ice does not form easily there. The Labrador Current, a cold current along Canada’s east coast, carries freezing water and supports sea ice and icebergs. Regions where warm and cold currents meet are rich in nutrients and form important fishing grounds.

• Warm currentflows from warmer regions.
• Cold currentflows from colder regions.
• Coriolis forcedeflection of moving objects due to Earth’s rotation.
• Fishing groundsareas where warm and cold currents meet.

Marine resources are materials and features from the ocean that have intrinsic or economic value. They are broadly classified into living resources (fish, aquatic vegetation, bioactive products) and non-living resources (minerals, oil, natural gas, sand and gravel, energy). Living resources include coastal or neritic fish (within the continental shelf, 0–200 m deep) and oceanic fish (beyond the shelf, deeper than 200 m). Non-living resources include submerged minerals and energy sources such as tidal, wave, and ocean thermal energy. Marine resources are vital for food security, livelihoods, trade, and tourism. For countries like India, fishing, trade, and tourism support millions of people. The Blue Economy concept promotes sustainable use of ocean resources for economic growth while preserving marine ecosystem health.

• Living resourcesfish, aquatic vegetation, bioactive products.
• Non-living resourcesminerals, oil, gas, sand, gravel, energy.
• Neritic fishlive within the continental shelf (0–200 m).
• Oceanic fishlive beyond the continental shelf (deeper than 200 m).
• Blue Economysustainable use of ocean resources for growth and jobs while preserving ecosystem health.

Maritime zones are areas of the ocean defined by international law, determining a country’s rights and responsibilities. The territorial sea extends up to 12 nautical miles from the baseline, where the country has near-complete control, subject to innocent passage for foreign ships. The contiguous zone extends up to 24 nautical miles, where the country has limited control over customs, fiscal matters, and illegal immigration. The Exclusive Economic Zone (EEZ) extends up to 200 nautical miles, where the country has special rights to use ocean resources such as fish, oil, and gas, though other ships may pass peacefully. Beyond the EEZs lie the high seas, covering about 64 per cent of the world’s oceans, which belong to no single country and are part of the global commons. On the high seas, the laws of the flag state apply.

• Territorial seaup to 12 nautical miles; near-complete control.
• Contiguous zoneup to 24 nautical miles; limited control.
• Exclusive Economic Zone (EEZ)up to 200 nautical miles; special resource rights.
• High seasbeyond EEZs; global commons; flag state laws apply.
• Innocent passageright of foreign ships to pass through territorial waters peacefully.

Cyclones and tsunamis are sudden ocean hazards. Early warning systems detect them, predict their path, and issue timely alerts so people can evacuate. India uses advanced technologies, including the National Cyclone Risk Mitigation Project (NCRMP) and the Storm Surge Early Warning System (SSEWS) by INCOIS and IMD. The Indian Tsunami Early Warning Centre (ITEWC) operates a real-time network of tsunami detection buoys. The GEMINI device, a handheld tool for fishermen, receives satellite data and provides alerts in local languages. International maritime rules, such as COLREGs for preventing collisions and MARPOL for preventing pollution from ships, keep the oceans safe and fair. These rules help resolve disputes, protect marine life, and ensure sustainable use of ocean resources.

• NCRMPbuilds cyclone shelters and strengthens warning systems.
• SSEWSforecasts storm surges and coastal inundation.
• ITEWCoperates tsunami detection buoys.
• GEMINIhandheld device for fishermen to receive alerts.
• COLREGsrules to prevent collisions at sea.
• MARPOLconvention to prevent pollution from ships.