Notes Class 11 Chapter 7 Geography Maharashtra Board

Indian Ocean – Relief and Strategic Importance

Introduction

The Indian Ocean, the third largest ocean globally, spans about 20% of the world’s oceanic area. It is uniquely named after India and is bordered by Africa to the west, Asia to the north and east, Australia to the east, and the Southern Ocean to the south. Unlike the Pacific and Atlantic Oceans, it does not extend to the Arctic Ocean, creating a unique geographical setup that influences the monsoon climate of the Indian subcontinent.

Bottom Relief of the Indian Ocean

The Indian Ocean’s floor is complex, featuring continental shelves, mid-oceanic ridges, ocean basins, deeps, trenches, and islands. These features result from tectonic, volcanic, and denudation processes, similar to those on land. The average depth is approximately 4,000 meters, with several marginal seas.

1. Continental Shelf

Definition: The submerged portion of continents under oceanic water, hosting gulfs, seas, bays, and straits.

Characteristics:

Wide variation in width; extensive along Indian coasts, narrower along eastern Africa, Madagascar, and Indonesia (approx. 160 km).
Western India has wider shelves than the eastern coast.
Covered with sedimentary deposits from terrestrial erosion, forming sedimentary rocks over time, some of which are potential fossil fuel sources.

Fossil Fuels: Coal, oil, and natural gas.

2. Mid-Oceanic Ridges

Definition: Submerged mountain ranges separating deep ocean floor portions.

Key Features:

Mid-Indian Oceanic Ridge: Originates in the Gulf of Aden near Somalia, extends south, and splits near Madagascar into:
- Southwest Indian Ocean Ridge: Extends to Prince Edward Island.
- Southeast Branch: Reaches Amsterdam and St. Paul Islands.
Contains parallel ridges, discontinuous due to fracture zones (e.g., Owen, Amsterdam).
Ninety East Ridge: A north-south range in the Bay of Bengal, from west of Andaman and Nicobar Islands to near Amsterdam and St. Paul Islands. Named for its proximity to the 90°E longitude.

Other Plateaus:

Kerguelen Plateau (south Indian Ocean).
Madagascar Plateau (south of Madagascar).
Agulhas Plateau (south of Africa).
Chagos Plateau (west of India, extending to Mid-Indian Ridge).

3. Islands of the Indian Ocean

Types: Primarily deep-sea islands, excluding coastal ones. Grouped into:

Arabian Sea:
- Near African Coast: Madagascar (largest, 5.9 lakh sq. km, seismically active, separated from Africa and Indo-Australian plates), Comoros, Bassas de India, Europa Island, Reunion, Mauritius, Seychelles, Socotra (near African Horn).
- Lakshadweep-Chagos Ridge: Lakshadweep, Maldives, Chagos (coral atoll archipelagos).
- Near Pakistan/Iran: Bundal, Kish, Hendorabi, Lavan, Siri.
Bay of Bengal:
- Sri Lanka (largest), Andaman and Nicobar (volcanic, along converging plate boundaries), and islands west of Sumatra. Most are peaks of submerged mountains.
Australian Coast:
- Few islands, including Ashmore, Christmas, and Cocos (Keeling).
Near Antarctica: Smaller islands in the southern Indian Ocean.

4. Ocean Basins

Definition: Deep, flat areas on the ocean floor, terminal destinations for continental and oceanic sediments.

Key Basins in Indian Ocean (10 major):

Oman Basin, Arabian Basin, Somali Basin, Mauritius Basin, Mascarene Basin, Agulhas-Natal Basin, West Australian Basin, Mid-Indian Basin, Ganga Basin.

Characteristics: Vary in size, separated by ridges or plateaus, and receive sediments from rivers like the Ganga.

5. Deeps and Trenches

Definition: The deepest oceanic portions, mostly along eastern boundaries.

Key Trenches:

Java/Sunda Trench: 7,450 m deep, along the Indo-Australian and Pacific plate boundary.
Ob Trench: 6,875 m deep.

Characteristics: Seismically active due to plate convergence, fewer in number compared to other oceans.

Temperature and Salinity Distribution

Temperature

Importance: Influences marine life and oceanic water movement.

Seasonal Patterns:

Pre-Southwest Monsoon: High temperatures, especially near the equator, due to increased solar insolation.
Southwest Monsoon: Lower temperatures in the Arabian Sea due to monsoonal winds.
Northeast Monsoon: Reduced temperatures, with the Bay of Bengal recording around 24°C.

Observations:

Isotherms are not drawn on continents as they depict sea surface temperatures.
Arabian Sea is cooler than the Bay of Bengal due to monsoonal winds and upwelling.
Southern Indian Ocean is warmer pre-monsoon due to equatorial proximity.

Salinity

Definition: Measure of salts in seawater, expressed as parts per thousand (‰), with an average of 35‰.

Patterns:

Arabian Sea: Higher salinity (high temperature, low rainfall, fewer river inflows near Somali Peninsula and Saudi Arabia).
Bay of Bengal: Lower salinity due to freshwater from Ganga and peninsular rivers, especially during the southwest monsoon.
Minimum Salinity: Lowest in Bay of Bengal during the southwest monsoon.

Reasons for High Arabian Sea Salinity: Limited river inflow, high evaporation, and arid climate.

Ocean Currents

Definition: Bodies of water maintaining distinct flow under wind influence, classified as warm or cold based on temperature.

Patterns:

Northern Indian Ocean: Seasonally reversing due to monsoon winds:
- Southwest Monsoon: Clockwise flow.
- Northeast Monsoon: Anticlockwise flow.
Southern Indian Ocean: Forms a large gyre with:
- South Equatorial Current: East to west (easterlies).
- West Wind Drift: West to east (westerlies).
- Mozambique-Agulhas Currents: Western arm.
- West Australian Current: Eastern arm.

Characteristics:

North Equatorial Current is weaker than South Equatorial Current.
Few cold currents due to the ocean’s enclosed northern boundary and warm equatorial climate.
Gyre circulation is a major global feature.

Strategic and Economic Importance

Global Significance

Trade and Transport:

Connects Asia, Africa, and Australia with Europe and the Americas.
Carries heavy petroleum traffic from the Persian Gulf and Indonesia via straits like Hormuz, Malacca, and Bab-el-Mandeb.
Supports economies of India, ASEAN, and Gulf countries, with India’s trade with ASEAN doubling from $1,484 million (1993) to $10,942 million (2004).

Chokepoints:

Strait of Hormuz: Links Persian Gulf to Arabian Sea, 33-95 km wide, handles 30% of global oil exports.
Strait of Malacca: World’s busiest chokepoint, critical for East-West trade.

Island Economies: Maldives, Seychelles rely on marine ecosystems and tourism.

Mineral and Energy Resources

Poly-metallic Nodules: Found on abyssal plains, contain nickel, copper, manganese, cobalt. India has 2 million sq. km for research (International Seabed Authority).
Hydrocarbons: 40% of global offshore oil from Saudi Arabia, Iran, India, Western Australia.
Heavy Minerals: Beach sands of India, South Africa, Indonesia, Sri Lanka.

India’s Strategic Role

Geographical Advantage:

Central location in the Indian Ocean, linking key maritime routes.
Proximity to straits enhances control over trade and energy routes.

Economic Dependence:

70% of India’s oil imports from the Gulf region.
Growing trade with ASEAN (Thailand, Singapore FTAs).

Security Concerns:

Rising piracy, maritime terrorism, and naval tensions (e.g., Diego Garcia’s military base).
India’s navy protects sea lanes and counters regional conflicts.

Foreign Policy:

Promotes peace via Indian Ocean Rim Association, BIMSTEC, Mekong-Ganga Cooperation.
Aims to counter superpower rivalry and secure littoral cooperation.

Historical Context:

Western coast forts (Janjira, Alibaug) linked India to Arabian Sea trade.
Ratnagiri (“Gem Mountain”) reflects historical gem trade significance.

Key Features on Map

Sunda Trench: Eastern Indian Ocean, near Java/Sumatra.
Diego Garcia: Central Indian Ocean, Chagos Archipelago.
Southwest Monsoon: Northern Indian Ocean, affecting South Asia.
Agulhas Current: Western Indian Ocean, South Africa’s east coast.
West Australian Current: Eastern Indian Ocean, Australia’s west coast.
Ninety East Ridge: Eastern Indian Ocean, north-south ridge.
Strait of Hormuz: Northern Indian Ocean, Persian Gulf-Arabian Sea link.
Chabahar Port: Southeastern Iran, Gulf of Oman.