Maarg Manthan · Topic 10.1

Major Ocean Relief Features

Geography › Ocean Relief, Oceans and Seas · Topic 10.1

The ocean floor has four major divisions: the continental shelf, the continental slope, the deep sea plain or abyssal plain, and the ocean deeps or trenches. The shelf is the gently sloping extension of the continent to about 200 m depth, the slope drops steeply to the ocean floor, the abyssal plains are the flat floor at 3,000 to 6,000 m, and trenches are the deepest parts.

6 min read · UPSC Prelims · Geography · Ocean Relief, Oceans and Seas

At a Glance

Continental shelf Up to about 200 m; average width about 80 km
Continental slope Steep drop from about 200 m to 3,000 m
Abyssal plain Flat floor at 3,000 to 6,000 m
Trench Deepest; Mariana Trench about 10,900 m
Average ocean depth About 3,700 m
Exam link UPSC Prelims, Geography: Ocean Relief, Oceans and Seas

Where Does It Fit?

This is the first topic of the category on ocean relief, oceans and seas. The previous macro ended with the climate zones on land, and this macro turns to the oceans, starting with the shape of the sea floor. The next topic covers the smaller relief features such as ridges, seamounts and canyons. Previous topic: Cold Snow-Forest and Polar Climates (Groups D and E). Next topic: Minor Ocean Relief Features.

What Are the Major Divisions of the Ocean Floor?

In short: from the coast outward, the sea floor steps down from the shelf to the slope, the rise, the abyssal plain and finally the trenches.

The oceans cover about 71 per cent of the Earth’s surface, and their floor is as varied as the land, with mountains, plains, valleys and canyons. The average depth is about 3,700 m, compared with an average land height of about 840 m. The relief is studied by echo sounding, sonar and satellite measurement of the sea surface, and a graph that shows the share of the Earth’s surface at each height or depth is called the hypsographic curve. It shows two peaks: one near sea level, which is the continents, and one at around 4,000 to 5,000 m depth, which is the abyssal plains. Plate tectonics, as in Plate Tectonics and Types of Plate Boundaries, shapes the larger features, since new floor is created at ridges and destroyed at trenches.

Feature	Depth and width	Description
Continental shelf	Extends to about 100 to 200 m depth; widths from a few km to over 1,000 km; average about 80 km	The submerged margin of the continent, with a gentle slope of about 1 degree; ends at the shelf break
Continental slope	Drops from the shelf break at about 200 m to around 3,000 m; slope of 2 to 5 degrees or more	Steep marginal edge of the continental crust; often cut by submarine canyons
Continental rise	At the foot of the slope; 3,000 to 4,000 m	Gently sloping apron of sediment carried down the slope by turbidity currents
Deep sea plain (abyssal plain)	3,000 to 6,000 m	Flattest and smoothest part of the Earth, covered by fine clay and silt, oozes from the remains of plankton
Ocean deeps or trenches	Deeper than 6,000 m; relatively narrow and long	Deepest parts of the ocean, mainly at the edges of continents and island arcs, where an oceanic plate subducts

Why Is the Continental Shelf Important?

Fisheries: sunlight reaches the bottom and nutrients are brought by rivers and upwelling, so plankton thrive. The shallow shelves, such as the Grand Banks, the North Sea and the Sunda Shelf, are among the richest fishing grounds, as in Ocean Currents – Causes and Types.
Minerals and energy: the shelf holds most of the world’s offshore oil and gas, as in the North Sea, the Gulf of Mexico, Persian Gulf and Bombay High off the Indian coast, and sand, gravel and placer minerals.
Legal importance: under the United Nations Convention on the Law of the Sea, a coastal state has sovereign rights over the natural resources of its continental shelf, which can extend up to 200 nautical miles and, where the geological margin goes further, up to 350 nautical miles; India has made a submission for an extended shelf.
Origin: the shelves are made by the rise of sea level after the ice age, which flooded the edge of the land, by deposition of sediments by rivers and by tectonic movement of the coast. The widest shelf, of more than 1,000 km, lies off Siberia in the Arctic Ocean.

The shelf is wide on the trailing margins of the Atlantic, with the shelf near the Atlantic coast of North America, and narrow on the Pacific, where the coast is a plate boundary, as along the Andes. India’s western shelf is wide, up to around 340 km off Mumbai, while the east coast shelf is narrow, around 20 to 30 km in places.

What Are the Abyssal Plains and Trenches?

The abyssal plains cover around half of the ocean floor, and are the largest habitat on the planet. They are flat because sediment fills the irregularities, and they are widest in the Atlantic, where few trenches trap the sediment. An ocean trench is a narrow, steep-sided depression, usually 3 to 5 km deeper than the surrounding sea floor. Most of them are in the Pacific, around the Ring of Fire, where plates are subducted, as in Distribution of Volcanoes and Earthquakes – Ring of Fire.

Ocean	Deepest trench	Approximate depth
Pacific	Mariana Trench, Challenger Deep	About 10,900 m, the deepest point on Earth (measurements vary from about 10,900 to 11,000 m)
Atlantic	Puerto Rico Trench	About 8,400 m
Indian	Java (Sunda) Trench	About 7,200 to 7,300 m

Challenger Deep is deeper below sea level than Mount Everest is high above it. It was first reached in 1960 by the Trieste with Jacques Piccard and Don Walsh, and since then a few crewed dives have visited it. As an analytical point, the trenches are not only curiosities: they are the sites of the largest earthquakes and tsunamis, as in Tsunami – Mechanism and Properties, and recent studies find life, even in the deepest trenches, and also plastic and pollutants, showing that no part of the ocean is beyond human reach.

Why Is This Topic Important?

Basic facts Divisions and depths asked
Resources Shelf oil, gas and fish
Law Continental shelf rights

What Came Next?

The next topic describes the minor relief features of the ocean floor. Next topic: Minor Ocean Relief Features.

Key People and Terms

Continental shelf Submerged edge of the continent
Continental slope Steep descent from the shelf
Abyssal plain Flat deep sea floor
Trench Deep narrow depression
Shelf break Edge where the shelf steepens
Hypsographic curve Graph of land height and ocean depth

Exam Corner

Points to Remember

The shelf ends at about 200 m.
Abyssal plains lie at 3,000 to 6,000 m.
Challenger Deep is the deepest point.
Most trenches are in the Pacific.
The widest shelf is off Siberia.

Do Not Confuse With

Shelf and slope Gentle versus steep
Slope and rise Steep margin versus sediment apron
Trench and abyssal plain Narrow deeps versus wide flat floor

Memory Hook

Shelf is shallow and rich, slope plunges, rise collects, plain lies flat, trench is the deepest.

Mains Angle

Questions ask for the importance of the shelf. These points add depth.

Shelf Fish, oil and sovereignty
Trenches Subduction and tsunami
India Wide west and narrow east shelf

A Question You May Face

An original practice question, not a past paper question.

“Describe the major relief features of the ocean floor and explain the economic and strategic importance of the continental shelf.”

How to Answer

Divisions Shelf to trench.
Features Depth and gradient.
Importance Resources and law.
India Extended shelf claim.

GS Relevance

Prelims: divisions of the ocean floor, depth, importance of the shelf, deepest trenches. GS Paper 1: oceanography; GS Paper 3: marine resources.

Frequently Asked Questions

What are the major relief features of the ocean floor?

The major relief features are the continental shelf, the continental slope with the continental rise, the deep sea or abyssal plains, and the ocean deeps or trenches. They form a series of steps from the shallow coastal margin to the deepest parts of the ocean.

What is the continental shelf?

The continental shelf is the gently sloping, submerged extension of a continent, extending to a depth of about 100 to 200 metres. It averages about 80 kilometres in width, is rich in fish and offshore oil and gas, and is subject to the sovereign rights of the coastal state.

What is the continental slope?

The continental slope is the steep descent from the edge of the continental shelf at about 200 metres to the deep ocean floor at about 3,000 metres or more. It marks the true edge of the continental crust and is often cut by submarine canyons.

What is an abyssal plain?

An abyssal plain is a very flat area of the deep ocean floor, usually at depths of 3,000 to 6,000 metres. It is covered by fine clay and the remains of tiny organisms, and covers about half of the ocean floor, making it one of the largest habitats on Earth.

Which is the deepest part of the ocean?

The deepest part of the ocean is the Challenger Deep in the Mariana Trench of the western Pacific, at a depth of about 10,900 metres or nearly 11 kilometres. Different surveys give slightly different values. The deepest points of the Atlantic and Indian oceans are the Puerto Rico and Java trenches.

PYQ Practice — Statement Analysis

1 The continental shelf is the steepest part of the ocean floor.

False

The slope is steeper.

2 The abyssal plains are among the flattest areas of the Earth.

True

Sediment fills the irregularities.

3 Most ocean trenches are found in the Pacific Ocean.