Pacific Ocean

I

Introduction

Pacific Ocean, largest and deepest of the world's five oceans, covering more than a third of the Earth's surface and containing more than half of its free water. It is sometimes divided into two nominal sections: the part north of the equator is called the North Pacific; the part south of the equator, the South Pacific. The name Pacific, which means peaceful, was given to it by the Portuguese navigator Ferdinand Magellan in 1520.

II

Boundaries and Size

The Pacific Ocean is bounded on the east by the North and South American continents; on the north by the Bering Strait; on the west by Asia, the Malay Archipelago, and Australia; and on the south by Antarctica. In the south-east it is arbitrarily divided from the Atlantic Ocean by the Drake Passage along 68° west longitude; in the south-west, its separation from the Indian Ocean is not officially designated. Some of the major marginal seas of the Pacific are the Bering Sea, the Sea of Okhotsk, the Sea of Japan (East Sea), the East China Sea, the South China Sea (see China Sea), the Coral Sea, the Tasman Sea, the Ross Sea, and the Gulf of California. Apart from the marginal seas along its irregular western rim, it has an area of about 165 million sq km (64 million sq mi), substantially larger than the entire land surface of the globe. Its maximum length is about 15,500 km (9,600 mi) from the Bering Strait to Antarctica, and its greatest width is about 17,700 km (11,000 mi) from Panama to the Malay Peninsula. Its average depth is 4,282 m (14,049 ft). The Pacific has the greatest known depth of any of the world's oceans at 11,033 m (36,198 ft) in the Mariana Trench off Guam.

III

Geological Formation and Structural Features

The Pacific is the oldest of the existing ocean basins, its oldest rocks having been dated at about 200 million years. The major features of the basin and rim have been shaped by the phenomena associated with plate tectonics. The coastal shelf, which extends to depths of about 180 m (600 ft), is narrow along North and South America but is relatively wide along Asia and Australia. The East Pacific Rise, a midocean ridge system, extends about 8,700 km (5,400 mi) from the Gulf of California to a point about 3,600 km (2,235 mi) west of the southern tip of South America, and rises an average of about 2,130 m (7,000 ft) above the ocean floor. Along the East Pacific Rise molten rock upwells from the Earth's mantle, adding crust to the plates on both sides of the rise. These plates, which are huge segments of the Earth's surface, are thus forced apart, causing them to collide with the continental plates adjacent to their outer edges. Under this tremendous pressure, the continental plates fold into mountains, and the oceanic plates buckle downwards, forming deep trenches, called subduction zones, from which crust is carried back into the mantle. The stresses at the areas of folding and subduction are responsible for the earthquakes and volcanoes that give the rim of the Pacific basin the name “ring of fire”. In fact, the Pacific contains more than 80 per cent of the Earth’s active volcanoes and most of its earthquake activity.

IV

Islands

The largest islands, in the western region, form volcanic island arcs that rise from the broad continental shelf along the eastern edge of the Eurasian Plate. They include Japan, Taiwan, the Philippines, Indonesia, New Guinea, and New Zealand. The oceanic islands, collectively called Oceania, are the tops of mountains built up from the ocean basin by extruding molten rock. The Pacific Ocean contains more than 30,000 islands of this type; their total land area, however, amounts to only one-quarter of one per cent of the ocean's surface area. The mountains that remain submerged are called seamounts. In many areas, particularly the South Pacific, the land features above the sea surface are accretions of coral reef. Along the eastern edge of the Pacific, the continental shelf is narrow and steep, with few island areas. The major groups are the Galápagos at the equator, which rise from the Nazca Plate, the Aleutians in the north, which are part of the North American continental shelf, and the islands of Hawaii, which rise some 5,550 m (more than 18,000 ft) from the seafloor of the central Pacific, reaching in Mauna Kea a height of 4,205 m (13,796 ft) above sea level.