Bottom Reliefs of the Pacific Ocean

• The Pacific Ocean, the largest ocean of the world having one-third area of the globe, extends from east to west for 16,000 km from the east coast of Asia in the west to the west coasts of Americas in the east and for 14,880 km from north to south between Bering Strait in the north to Cape Adre (Antarctica) in the south.
• The overall shape of the ocean is triangular if its extent in both the hemispheres is considered sepa­rately.
• Average depth of the ocean in 4,572m.
• Both the coasts (east and west) of the Pacific are paralleled by the chains of folded mountains and therefore the de­scent from the coast to the abyssal plains is very steep.
• More or less uniform broad and extensive ocean floor is characterized by several swells, rises, sea mounts and depressions (trenches and deeps).
• The Ocean has the largest number of islands (more than 2,000). It may be pointed out that the western coast is studded with islands, island arcs and festoons while the eastern coast has only a few islands.
• The islands of the Pacific are grouped in 3 categories e.g.:
  • The continental islands (Aleutian Islands, islands off British Columbia of Canada, and Chilean island)
  • Island arcs and fes­toons (Kuriles, Japanese Archipilago, Philippines and Indonesian islands)
  • Scattered smaller islands which are further subdivided into two major sub-categories e.g.:

1. 1. Islands based on racial grouping such as:
   2. Melanesia (Solomons, New Hebrides and Fizi)
   3. Micronesia (Marshalls, Carolines, Gilbert and Ellice)
   4. Polynesia (Society, Cook, and Tuamotu)
   5. Islands formed of volcanic materials and coral reefs (Hawaii island-volcanic island, Fizi, Faunafuti, Ellice etc. coral islands).

The Pacific Ocean has been divided into four sub-regions:

(1) The Northern Pacific represents the deepest part of the whole Pacific where average depth ranges between 5000m and 6000m. This region makes contact with the Arctic Sea through Bering Strait.

(2) The Central Pacific is characterized by largest number of islands most of which are of volcanic and coral origin. H.H. Hess has identified 160 flat-topped sea mounts in this region. There are a few sub-parallel island chains which have been named by E. Suess as Oceanides.

(3) The South-West Pacific carries a large number of islands, marginal seas, extensive continen­tal shelves and oceanic trenches.

(4) The South-East Pacific has the most striking relief of the Pacific Ocean as the East Pacific Rise or Ridge but there is absence of marginal seas.

Continental Shelf:

• There is significant difference in the extent and characteristics of continental shelves on the eastern and western coasts of the Pacific.
• The shelves are quite broad and extensive along the eastern coasts of Aus­tralia and Asia where the width varies from 160 km to 1600 km and the depth ranges between 1000 m and 2000m.
• Several islands are seated on these broad continental shelves (viz. Kuriles, Japanese islands, Philippines, Indonesia, New Zealand etc.).
• These con­tinental shelves also carry numerous marginal seas like Bering Sea, Okhotsk Sea, Japan Sea, Yellow Sea, China Sea, Java Sea, Coral Sea, Tasmania Sea, Arafura Sea etc.
• The continental shelves are less extensive along the western coasts of Americas because of nearness of cordillerean chains of folded mountains to the coastal lands.
• The average width is 80 km.

East Pacific Rise:

• The Pacific Ocean does not have central or mid- oceanic ridge like the Atlantic and the Indian Oceans, albeit there are a few scattered ridges having local importance.
• The East Pacific Rise or Ridge known as Albatross Plateau is 1600 km wide and it extends from north of New Zealand to the Californian coast.
• It sends off two branches between 23°S-35°S.
• The eastern branch merges with Chilean coast while the other branch moves southward in the name of Eastern Island Rise.

Galapagos Ridge

A minor ridge known as Galapagos Ridge runs parallel to the East Pacific Ridge in the east between the Eastern Island Fracture Zone and Galapagos is­lands from where in moves in two branches viz.:

(i) Carnegie Ridge, and

(ii) Cocos Ridge in north-east direction.

The New Zealand Ridge: The New Zealand Ridge is about 200m to 2000 m below sea level and widens near Fiji island to form Fiji Plateau which is 2000 m below sea level.

The Hawaiian Rise extends from north-west to south-east direction between 35°N -17°N for a distance of 960 km. This is the most extensive ridge (2640 km wide) of the Pacific Ocean.

The other minor ridges are

• Nazca Ridge off Peru coast, Lord Howe Rise off eastern coast of Australia between 20°S and 40° S latitude.
• Norfolk Island Ridge between New Caledonia and New Zea­land (23°S-35°S)
• Eauripik-New Guinea Rise north of New Guinea and parallel to 140°E longitude
• Caroline- Soloman Ridge north of Soloman Islands etc.

Besides, there are a few fracture zones running from west to east e.g., (from north to south) Mendocino Fracture Zone (40°N), Murray Fracture Zone (30°N), Molokai Fracture Zone (25°N), Clarion Fracture Zone (20°N), Clipperton Fracture Zone (10°N), Eastern Is­land Fracture Zone (30°S), Challenger Fracture Zone (40°S) etc.

Ocean Basins:

There are different basins of different shapes and sizes. These basins are separated by ridges and ‘rises’.

The following are a few important basins of the Pacific Ocean:

(1) Philippine basin is located to the east of Philippines and extends from south of Japan to 5°N latitude. Kyushu – Paian Ridge runs through the middle of the basins. Average depth ranges from 5000m to 6000m.

(2)Fiji basin is located to the south of Fiji Island between 10°S and 32°S latitudes and the average depth is 4000m. The basin to the north of 20°S is known as North Fiji Basin whereas the South Fiji Basin between 20°S and 32°S is bordered by Norkolk Island Ridge in the west and Karmadec – Tonga Trenches in the east.

(3) East Australian basin is situated between the east coast of Australia and New Zealand Ridge with average depth of more than 5000m.

(4) South Australian Basin also known as Jeffreys Basin is located to the south-east of Australia having average depth of 5000m.

(5)Peru basin is located to the west of Peru coast between 5°S and 24°S latitudes and extends upto 110°W longitude. The average depth of the basin is 4000m.

(6) South-Western Pacific basin is an elongated basin stretching between 20°S and 50°S latitudes and 180-129°W longitudes. Karmadec Trench with the depth of 10,047 m is located to the west of this basin.

(7)Pacific-Antarctic Basin is located to the south­west of Chilean coast between 40°S and 60°S latitudes and extends up to 130°W longitude.

Oceans Deeps:

There are several trenches and deeps in the Pacific Ocean. These depressions are located either along the island arcs or mountain chains. It may be pointed out that the trenches are found mainly in the western Pacific Ocean.

The following are the signifi­cant trenches:

The genesis of oceanic trenches and deeps is related mainly to tectonic activities caused by plate motions.