Shape and Size:

The Pacific Ocean along with its marginal seas covers about 33% of the total area of the earth’s surface. It is triangular in shape with its apex in the Bering Strait. It is bounded by Asia and Australia on the west, and North America and South America to the east.

Antarctica lies to the south of this vast ocean whose length from north to south is 14,900 km, whereas its width along the equator is a little more than 16,000 km. Bottom Relief

Continental Shelf:


The continental shelf surrounding the margin of the Pacific Ocean is controlled by the shape and structure of its coastline. On the eastern margin of this ocean the width of the shelf is rather narrow.

Due to the presence of the Rockies and the Andes Mountains parallel to the west coasts of North and South Americas respectively the continental shelves have become very narrow. Their width is limited to only 80 km.

However, on the western margin of this ocean, due to the absence of mountain chains or plateaus, the continental shelf is broad. The continental shelves adjoining the coasts of Australia,

East Indies and East Asia are relatively much broader. Along these coasts the width of the shelves varies from 160 to 1600 kilometers with their average depth hardly exceeding 1000 meters. On these shelves are situated most of the islands and marginal seas.


Ridges and basins:

Major part of the floor of this ocean is made up of the abyssal plains. The bottom relief of the Pacific Ocean is characterized by the; absence of any mid-oceanic ridges. The major characteristic of the deep-sea plains is that they are deeper than those in other oceans.

Beyond the continental slopes there is a sudden increase in the depth of the deep-sea plains. However, there are a few submarine ridges of medium heights, and extensive and curvilinear submarine plateaus on the ocean floor.

In the Pacific Ocean the bottom relief to the east of 150° W longi­tude is less conspicuous than that to the west of it. In the eastern Pacific, the East Pacific Ridge or the Alba­tross Plateau is the most important one, and it is very extensive. The depth of water covering this plateau is between 3000 and 4000 meters.


The Cocos Ridge, in fact, represents the north-east projection of the East Pacific Ridge. It extends from the isthmus of the Middle America to the Galpagos Island. To the south of this island, the South-east Pacific Plateau separates the Pacific- Antarctic Basin from the South-east Pacific Basin.

The Pacific-Antarctic Ridge acts as a divide between the Pacific-Antarctic Basin and South­west Pacific Basin. The Pacific-Antarctic Ridge rising from the South-east Pacific Plateau extends upto Antarctica near the longitude of 180°.

Tasmania Basin is located between New Zealand and Australia. Macquarie Ridge rising from this basin forms a boundary line between the Pacific and the Indian Oceans. Hawaii Ridge extends from the Hawaii Islands to 180° in the west.

In the western part of the Pacific Ocean several islands are formed on the submarine ridges. The Aleutian Ridge is situated in the North Pacific Ocean. Other ridges extend towards the south passing through Kurile Islands, Bonin, Marianas, Yap and Patu.


Bismark, Solomon, and Santacruz ridges are the most important ones extending eastwards from Patu. Solomon, Tonga, Karmadi, Chatham and Macquarie Islands are situated on the ridges extending southwards in the Pacific Ocean.

Oceanic Deeps:

As stated earlier in this chapter, most oceanic trenches and deeps-and the deepest ones are found around the rim of the Pacific Ocean. The Challenger Deep, in the South Pacific’s Marianas Trench, is the deepest known part of the ocean. Its bottom lies 10,900 meters
below the sea level.

Most of the trenches are found in the Western Pacific whose total number is 32. Some of these are elongated in shape. In the vicinity of the Aleutian Islands, there is the Aleutian Trench which is 7679 m deep. Kurile and Japan Trenches extend from 28°N to 50° latitude parallel to the Japanese Islands.


In these trenches there are two oceanic deeps which are 10377 and 10375 m deep. The Emden Trough is found near the Philippines Islands and its depth is 5902 fathoms. In the Kurile Trench there is one Tascrora Deep whose depth is 4655 fathoms.

Similarly, the depth of Rampo Deep in the Japan Trench is 5771 fathoms. Mantsu Deep in the Marianas Trench is 5395 fathoms deep. The depth of Aldrich Deep in the Tonga-Karmadec Trench is 5155 fathoms.

There is absence of oceanic deeps and trenches in the middle part of the Pacific Ocean. Off the coast of South America there is a trench parallel to the Andes Mountains which is known as the Atacama Trench whose depth is 4175 fathoms. Besides the afore-mentioned trenches and deeps, there are several others which have not been mentioned here due to certain limitations of space.

Islands of the Pacific Ocean:


The estimated number of islands in the Pacific Ocean is about 20,000, but their total area is relatively less. Almost all the big islands of the Western Pacific are structurally parts of the main land that have been submerged under sea water. Between the main land and these islands there are submerged basins.

Among the various island arcs which are very extensive the Kurile Islands, the Japan Islands, Philippines, East Indies and New Zealand are the most important. In the eastern part of the North Pacific Ocean the important islands are the following; Aleutian Islands, Islands off the coast of British Colombia, and the Chilean Islands.

It may be stated that most of these island arcs represent folded mountain chains with numerous volcanic peaks. Since they are located in weak crustal zones, they are subjected to frequent earthquakes.

In the south-western part of the Pacific Ocean there are several smaller scattered groups of islands. All these smaller island groups are classified under three names on the basis of their racial groupings: Melanesia, Micronesia, and Polynesia.

Melanesia includes the islands of Solomons, New Hebrides and Fiji. All the islands of the Melanesia group are situated to the south of Micronesia group of islands. Most of these are coral islands. New Guinea Island is also included in this group.

Micronesia group includes Carolines, Marshalls, Gilbert and Ellice Islands. Most of these islands are small-sized and are situated north of the equator to the west of 180° longitude. Almost all of these are coral islands. Polynesia group of islands are situated in a triangular area.

This group of islands is bounded in the north by Hawaii Islands, in the south -east by Easter Island and in the south-west by New Zealand. Line Islands, Samoa, Tonga, Cook, Society and Tuamotu Islands are some of the important ones that form this group. Marquises Island is also considered a part of Polynesia group.

The northern most islands of the Pacific islands are Hawaii Islands which is connected with the Hawaii Submarine Ridge. But a major part of the north-eastern and eastern Pacific is conspicuous by the absence of islands.

However, there are a few isolated island groups namely, Clipperton, the Galapagos Archipelago, and Juan Fernandez etc. The islands of the Pacific Ocean are classified into three categories:

(i) The continental islands of the fold ranges, (ii) the high volcanic islands and (iii) the low coral islands. The volcanic cones of Mauna Kea and Mauna Loa belong to the Hawaii group of islands. Most of the Pacific islands are located between the Tropic of Cancer and Tropic of Capricorn. They are all unequally distributed. However, the maximum numbers of islands are found in the Western Pacific Ocean.

Pacific Ocean.

Deep-sea basins

Deep-sea trenches

(i) Central Pacific Basin

(i) Aleutian Trench

(ii) Philippines Basin

(ii) Kurile Trench

(iii) Caroline Basin

(iii) Japan Trench

(iv) Coral Basin

(iv) Bonin Trench

(v) Fiji Basin

(v) Mariana Trench

(vi) Tasman Basin

(vi) Japan Trench

(vii) South Pacific Basin

(vii) Philippines Trench

(viii) Berlinghausen Basin

(viii) Riukiu Trench

(ix) Peru-Chile Basin

(ix) Bougainville-New Britain Trench

(x) California Basin

(x) New Hebrides Trench

(xi) Banda Sea

(ix) Tonga Trench

(xii) Celebes sea

(xii) Kermadec Trench

(xiii) North China Sea

(xiii) Chile Trench

Ridges and rises

(xiv) Peru (Atacama) Trench

(i) Bonin Ridge

(xv) Californian Trench

(ii) Eastern Pacific Ridge

(iii) South Pacific Ridge

(iv) Macquarie Ridge

(v) Fanning Ridge

(vi) Hawaii Ridge

(vii) Fiji Ridge

(viii) New Hebrides Ridge

Marginal Seas :

Almost all the marginal seas are found on the western margin of the Pacific Ocean. Due to the peculiarity of the American coast there is dearth of marginal seas along the eastern coast of this ocean. Gulf of California is the only marginal sea on this side of the Pacific.

On the contrary, along the west coast there are a large number of marginal seas between the main land and nearby island groups. In the North Pacific are located the following marginal seas: the Bering.

Sea surrounded by the Aleutian Islands, the Sea of Okhotsk within the Kamchatka Peninsula, the Sea of Japan between Korea and the Japanese Islands, the Yellow Sea between Korea and the Chinese mainland, the East China Sea between China and the Ryuku Islands, and the South China Sea, enclosed by the Philippines, Borneo, Malaysia, Indo-China and South China.

Excepting the Yellow Sea, the average depth of the remaining marginal seas exceeds 1500 fathoms. The maximum depths sounded in the Celebes and Japan Seas are 2795 and 1955 fathoms respectively. Besides, Banda Sea and Sula Sea are also located in the East Indies, but they are very small in size.

The Gulf of Carpentaria, the Arafura Sea, and Bass Straits are all situated on the continental shelf, and are relatively shallow. The following table brings out clearly the details of the deep-sea basins, trenches and submarine ridges of the Pacific Ocean.