Oceans: Floor, Temperature & Salinity – NDA Geography Notes

The oceans cover about 71% of the Earth’s surface. Yet until very recently, we knew less about the ocean floor than we knew about the surface of the Moon. The deep ocean is dark, cold, and under enormous pressure. Over the past century, advances in technology have mapped the ocean floor and discovered a landscape as varied and dramatic as anything on land, with towering underwater mountain ranges, vast flat plains, deep trenches, and active volcanoes.

NDA tests this chapter through trench-ocean matching questions, ocean zone definitions, salinity patterns, the halocline, and coral reef types. These are direct identification questions with very little ambiguity. A student who knows the key facts scores full marks.

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The World’s Oceans

The Earth has five oceans: all connected as one global body of water.

Pacific Ocean: largest and deepest; covers about 46% of Earth’s water surface; contains the deepest point on Earth.

Atlantic Ocean: second largest; most important for global trade; bounded by Americas to the west and Europe and Africa to the east.

Indian Ocean: third largest; bounded by Africa, Asia, and Australia; important for Indian and Arabian trade.

Southern Ocean: surrounds Antarctica; defined by the Antarctic Circumpolar Current.

Arctic Ocean: smallest and shallowest; largely ice-covered.

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Structure of the Ocean Floor

Moving from the coastline outward toward the deep ocean, you cross several distinct zones.

Continental Shelf

The continental shelf is the submerged extension of the continent, a shallow, gently sloping platform of continental crust extending from the shoreline to the edge of the continent. Depths are generally less than 200 metres.

The continental shelf is important because sunlight penetrates to the bottom, supporting rich marine life. The major fishing grounds of the world are on continental shelves because plankton and fish thrive in shallow, sunlit water. Most offshore oil and gas reserves are found here.

Grand Banks fishing grounds: off Newfoundland, Canada: one of the world’s most productive fishing grounds because of a vast continental shelf where sunlight reaches the bottom, allowing enormous plankton growth. Both statements are correct and the second explains the first: (1) Grand Banks is a major fishing ground; (2) Planktons grow in shallow, sunlit waters.

Continental Slope

Beyond the edge of the continental shelf, the sea floor drops steeply. This is the continental slope. It is the real boundary between the continent and the deep ocean basin. Depth increases rapidly from 200 m to several thousand metres.

Continental Rise

At the base of the continental slope, the gradient lessens and sediment accumulates. This is the continental rise, sloping gently toward the abyssal plain.

Abyssal Plain

The vast, flat, deep ocean floor: typically 3,000–6,000 metres depth. This is the largest habitat on Earth by area, covered by a thick layer of fine sediment (ooze) that has settled slowly over millions of years.

Mid-Ocean Ridges

Long, continuous underwater mountain ranges running through the middle of the ocean basins. They form at divergent plate boundaries where new oceanic crust is being created.

The Mid-Atlantic Ridge runs down the middle of the Atlantic Ocean from Iceland to the South Atlantic, with a central rift valley and active volcanic activity. Iceland is the exposed surface portion of the Mid-Atlantic Ridge, built above sea level by exceptional volcanic activity.

Hawaiian Islands are NOT associated with mid-oceanic ridges. They are formed by a hot spot. The Azores, Ascension Islands, and Tristan da Cunha ARE associated with mid-oceanic ridges. [NDA 2025-II]

Oceanic Trenches

The deepest parts of the ocean floor. Trenches form at convergent plate boundaries where an oceanic plate is subducted beneath another plate, creating a long, narrow, very deep depression. Oceanic trenches are associated with active volcanoes and strong earthquakes. They are significant in the study of plate movements. [NDA 2025-II]

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Major Ocean Trenches

Trench	Ocean	Depth	Key Fact	NDA Tag
Mariana Trench (Challenger Deep)	Western Pacific Ocean	~11,034 m	Deepest point on Earth	[NDA 2017-II]
Milwaukee Deep / Puerto Rico Trench	Atlantic Ocean	~8,376 m	Deepest point in Atlantic	—
Sunda Trench (Java Trench)	Indian Ocean	~7,455 m	Deepest point in Indian Ocean	[NDA 2008-II]
South Sandwich Trench	Southern Ocean	~7,235 m	—	—

Mariana Trench is in the western Pacific Ocean: not the Atlantic. [NDA 2017-II]

Sunda (Java) Trench is in the Indian Ocean: south of Java, Indonesia. Despite Indonesia being geographically in the Pacific region, this trench is in the Indian Ocean. [NDA 2008-II]

Milwaukee Deep = deepest in Atlantic. Sunda/Java Trench = deepest in Indian Ocean. Challenger Deep (Mariana) = deepest anywhere on Earth.

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Special Ocean Areas

The Sargasso Sea

The Sargasso Sea is unique: it is the only sea with no coastline. It is bounded not by land but by four ocean currents: Gulf Stream (west) | North Atlantic Current (north) | Canary Current (east) | North Equatorial Current (south).

Key characteristics: Warm, calm, clear blue water | thick mats of floating Sargassum seaweed (which give the sea its name) | anticyclonic (clockwise) circulation | relatively high salinity but NOT the highest salinity in the Atlantic Ocean.

The Sea of Azov

The Sea of Azov is a very shallow inland sea in Eastern Europe, connected to the Black Sea through the Strait of Kerch.

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Ocean Temperature

Ocean temperature varies with latitude, depth, and season.

Variation with latitude: The ocean surface is warmest near the equator (~27–28°C) and coolest near the poles (~0°C). Solar radiation is most intense at the equator.

Variation with depth:

Surface layer (0–200 m): Warm, well-mixed by waves and wind.

Thermocline (200–1,000 m): Temperature decreases rapidly with depth. The thermocline is the zone of sharp temperature change with depth. [NDA 2018-II: distinguish from halocline]

Deep ocean (below 1,000 m): Very cold, uniform: near 2–4°C everywhere in the world regardless of latitude.

Annual range: The ocean has a much smaller annual temperature range than land, because water absorbs and releases heat slowly (high specific heat capacity). This is why coastal areas have more moderate climates than continental interiors.

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Ocean Salinity

Salinity is the total amount of dissolved salts in seawater, measured in parts per thousand (ppt or ‰). Average ocean salinity is about 35 ppt (35 grams of salt per kilogram of seawater).

The major dissolved salt is sodium chloride (NaCl): about 77% of total dissolved salt. Other salts include magnesium chloride, magnesium sulphate, and calcium sulphate.

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Factors Affecting Salinity

Temperature: warmer water evaporates more, removing water but leaving salts: increasing salinity.

Evaporation: high evaporation concentrates salts.

Precipitation: rain dilutes salts: decreasing salinity near heavy rainfall zones.

River inflow: rivers bring fresh water: decreasing salinity near river mouths and in enclosed seas.

Freezing: when seawater freezes, salt is expelled from the ice into the surrounding water: making the surrounding water MORE saline: NOT less.

Why swimming is easier in sea water: Sea water is denser than fresh water because of dissolved salts. Denser water provides more buoyancy. Both statements are correct and the second explains the first.

Ocean Zone Definitions

Halocline: the zone of sharp salinity change with depth: where salinity increases rapidly as you go deeper. [NDA 2018-II]

Thermocline: the zone of sharp temperature change with depth.

Pycnocline: the zone of sharp density change with depth.

Photic zone: the upper zone where enough sunlight penetrates for photosynthesis, typically the top 200 metres.

Zone	What Changes Sharply	NDA Tag
Halocline	Salinity	[NDA 2018-II]
Thermocline	Temperature	—
Pycnocline	Density	—
Photic zone	(Sunlight; photosynthesis possible)	—

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Global Salinity Pattern

Highest salinities: in the subtropical oceans: where evaporation is high (subtropical high pressure = warm, dry air) and precipitation is low.

Lowest salinities: near river mouths (fresh water input) | polar regions (ice melting in summer) | equatorial regions (heavy rainfall) | enclosed seas with large rivers (Baltic Sea).

The Red Sea has the highest salinity among the major seas: because of very hot, dry climate (extreme evaporation), very little fresh water input (almost no rivers), and largely enclosed geography. [NDA 2016-I]

The Baltic Sea has the lowest salinity among major seas: because it receives large amounts of fresh water from rivers, has cold climate (low evaporation), and is a semi-enclosed body.

Ascending salinity order: Baltic Sea → Arctic Ocean → Gulf of California → Red Sea

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Coral Reefs

Coral reefs are underwater structures built by tiny marine organisms called coral polyps: small animals related to jellyfish. Each polyp secretes a hard calcium carbonate skeleton. Over thousands of years, billions of these skeletons accumulate to form massive reef structures.

Conditions for coral reef formation: Warm water (>20°C; optimal 23–25°C) | Shallow water (<60 m; sunlight needed) | Clear water (turbid water blocks sunlight) | Saltwater (corals cannot survive in fresh water) | Well-oxygenated water.

This is why coral reefs are found only in tropical and subtropical regions, and never near river mouths.

Types of Coral Reefs

Fringing Reef: the simplest and most common type. Grows directly from the shore: attached to the coastline or extending as a narrow band just offshore. Very little open water between reef and shore.

Barrier Reef: a reef separated from the mainland coast by a wide, deep lagoon. Runs parallel to the shore but at some distance. The Great Barrier Reef in Australia is the world’s largest barrier reef, stretching about 2,300 km along the northeastern coast of Australia.

Atoll: an almost circular or ring-shaped coral reef enclosing a central lagoon. Atolls form when a volcanic island slowly subsides (sinks) beneath the sea. The coral reef that originally formed around the island keeps growing upward as the island sinks, eventually forming a ring. The island disappears but the coral ring remains. [NDA 2008-I]

The Maldives and Lakshadweep are examples of atoll islands.

Coral Bleaching: when corals are stressed: primarily by warming ocean temperatures: they expel the coloured algae (zooxanthellae) living in their tissues, turning the coral white. Without the algae, corals lose their food source and may die.

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