Oceans: Floor, Temperature & Salinity – NDA Geography Notes

Introduction

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 allowed scientists to map the ocean floor and discover a landscape as varied and dramatic as anything on land — with towering underwater mountain ranges, vast flat plains, deep trenches, and active volcanoes.

This chapter covers the structure of the ocean floor, how ocean temperature and salinity vary, and the formation and types of coral reefs. These are moderately tested in NDA — questions tend to be direct identification questions about which trench belongs to which ocean, what separates the continental shelf from the deep ocean, and what determines salinity patterns.

Understand the Topic

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; sometimes counted as part of the other three oceans.

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.

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1. 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 and photosynthesis
• Major fishing grounds of the world are on continental shelves — plankton and fish thrive in shallow, sunlit water
• Most offshore oil and gas reserves are found here
• It is the area most affected by tides and waves

Grand Banks fishing grounds:

The Grand Banks off Newfoundland, Canada are one of the world’s most productive fishing grounds. The reason: a vast continental shelf where sunlight reaches the bottom, allowing plankton to grow in enormous quantities. Fish thrive on the plankton. [NDA tested via CDS]

Both of these statements are true and the second explains the first:

1. Grand Banks is a major fishing ground — because of the vast continental shelf
2. Planktons grow in shallow waters — TRUE, and this explains why the shelf is productive

2. 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 ocean basin. Depth increases rapidly from 200 m to several thousand metres.

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3. Continental Rise

At the base of the continental slope, the gradient lessens and sediment accumulates — this is the continental rise. It slopes gently toward the abyssal plain.

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4. Abyssal Plain

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

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5. 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 roughly down the middle of the Atlantic Ocean from Iceland to the South Atlantic. It has a central rift valley where the plates pull apart and active volcanic activity is common.

Iceland is actually the exposed surface portion of the Mid-Atlantic Ridge — built up above sea level by exceptional volcanic activity.

The 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]

6. 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	Notes
Mariana Trench (Challenger Deep)	Pacific Ocean (western)	~11,034 m	Deepest point on Earth
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
South Sandwich Trench	Southern Ocean	~7,235 m	—

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

The Sunda (Java) Trench is in the Indian Ocean — south of Java, Indonesia. [NDA 2008-II]

Milwaukee Deep = deepest point in the Atlantic. Java Trench = deepest in Indian Ocean. Challenger Deep = deepest in Pacific and all oceans. [NDA tested via CDS]

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 on the west
• North Atlantic Current on the north
• Canary Current on the east
• North Equatorial Current on the south

It is characterised by:

• Warm, calm, clear blue water
• Sargassum — thick mats of floating seaweed that give the sea its name
• Anticyclonic circulation (clockwise gyre)
• High salinity (because evaporation exceeds precipitation) but NOT the highest salinity in the Atlantic Ocean [NDA tested via CDS]

The Sargasso Sea is characterised by typical marine vegetation (sargassum seaweed). [NDA tested via CDS]

The Sargasso Sea is the only sea with no coastline — bounded entirely by ocean currents. [NDA tested via CDS]

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. [NDA tested via CDS]

Ocean Temperature

Ocean temperature varies with latitude, depth, and season.

Variation with latitude:

The ocean surface is warmest near the equator (about 27–28°C) and coolest near the poles (near 0°C or slightly below for salt water). This is because 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. This sharp transition zone where temperature drops quickly with increasing depth is called the thermocline. [NDA tested via CDS]

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

Annual range of temperature:

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

What is salinity?

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

The major dissolved salts are sodium chloride (NaCl — about 77% of total), followed by magnesium chloride, magnesium sulphate, and calcium sulphate.

Correct ascending order of salts in seawater:
Calcium Carbonate → Calcium Sulphate → Magnesium Chloride (Sodium Chloride is actually most abundant but among these three, the ascending order is CaCO₃ → CaSO₄ → MgCl₂). [NDA tested via CDS]

Factors Affecting Salinity

Temperature: Warmer water evaporates more — removing water but leaving salts — increasing salinity.

Evaporation: High evaporation removes water and 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 with large rivers.

Freezing: When seawater freezes, salt is expelled from the ice into the surrounding water — making the surrounding water MORE saline. [NDA tested via CDS]

When seawater freezes, salinity of the surrounding unfrozen water increases — not decreases. Students often get this wrong. [NDA tested via CDS]

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 — it is easier to swim in sea water AND the density of sea water is higher — are correct and the second explains the first. [NDA tested via CDS]

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Halocline

The halocline is the zone of sharp salinity change with depth — where salinity increases rapidly as you go deeper in some ocean areas. Similar to the thermocline for temperature. [NDA 2018-II]

Pycnocline is the zone of rapid density change with depth — density depends on both temperature and salinity.

Photic zone is the upper zone of the ocean where enough sunlight penetrates for photosynthesis to occur — typically the top 200 metres.

Global Salinity Pattern

The highest salinities are found in the subtropical oceans — where evaporation is high (subtropical high pressure = warm, dry, sinking air) and precipitation is low.

The lowest salinities are found near:

• River mouths (fresh water input)
• Polar regions (ice melting adds fresh water in summer)
• Equatorial regions (heavy rainfall dilutes salt)
• Enclosed seas with large rivers (Baltic Sea)

The Red Sea has the highest salinity among the major seas:

• Very hot, dry climate → extreme evaporation
• Very little fresh water input (almost no rivers)
• Enclosed (nearly landlocked) → salts concentrate
• High salinity is explained by high rate of evaporation [NDA 2016-I]

The Baltic Sea has the lowest salinity among major seas:

• Cold climate → low evaporation
• Many large rivers discharge into it (Vistula, Oder, Neva)
• Limited connection to open ocean

Salinity ascending sequence:
Baltic Sea → Arctic Sea → Gulf of California → Red Sea [NDA tested via CDS]

Also tested as: Baltic Sea → North Sea → Gulf of California → Red Sea [NDA tested via CDS]

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 — temperature must be above 20°C; optimal 23–25°C
• Shallow water — corals need sunlight for the algae (zooxanthellae) living inside them; depth generally less than 60 m
• 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. [NDA tested via CDS]

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 surface. The coral reef that originally formed around the island keeps growing upward as the island sinks, eventually forming a ring. [NDA 2008-I]

An almost circular coral reef with a central lagoon is called an atoll. [NDA 2008-I]

The Maldives and Lakshadweep are examples of atoll islands.

Coral Bleaching:

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