Pacific Ocean

The Pacific Ocean is the world’s largest ocean, covering an area of 182 million square kilometers. It is twice the size of the next largest ocean, the Atlantic. The Pacific Ocean covers approximately one-third of Earth’s surface and is larger than Earth’s entire landmass area.

Location of the Pacific Ocean

The Pacific Ocean, with a mean depth of 4,255 meters and a maximum depth of 10,970 meters, extends from its southern extreme in Antarctica some 15,550 kilometers north to the Bering Strait, which separates North America from Asia. Its east-west area extends almost 19,440 kilometers from the western coast of Colombia in South America to Asia’s Malay Peninsula.

The Pacific and Arctic Oceans meet at the Bering Strait in the north. The Atlantic and Pacific Oceans come together in the far south at Drake Passage, south of Cape Horn. Exactly where the Pacific meets the Indian Ocean is challenging to define because the two bodies of water intermingle along a string of islands extending east from Sumatra through Java to Timor and across the Timor Sea to Australia’s Cape Londonderry. South of Australia, the Pacific runs across the Bass Strait to the Indian Ocean and continues from Tasmania to Antarctica.

The eastern Pacific generally follows the Cordilleran mountain system, which runs North and South America from the Bering Strait to the Drake Passage and includes both the Rocky Mountains and the Andes. The east coast of the Pacific is, comparatively, quite regular except for the Gulf of California and the fjord regions in its northern and southern extremes. The continental shelf of the eastern Pacific is narrow, and the continental slope at times quite steep, as is characteristic of subduction zones.

The Pacific’s western extreme in Asia is, by contrast, quite irregular. As on the eastern coast of the Pacific, the western coast is bordered by mountain systems that run roughly parallel to the coast. The Western Pacific has many dependent seas, notable among them the Bering Sea, the East China Sea, the Sea of Japan, the Sea of Okotsk, the South China Sea, and the Yellow Sea. The eastern extremes of these seas are characterized by peninsulas jutting out toward the south, by island arcs, or both.

Unlike the Atlantic, much of whose water flows into it from the rivers that feed it, only about one-seventh of the Pacific’s water comes from direct river flow. Most of its water comes from the dependent seas nourished by East Asian rivers, such as the Amur, the Xi, the Mekong, the Yangtze, and the Yellow Rivers.

Origins and Divisions

The Pacific Ocean is the present-day remnant of the world ocean called Panthalassa and has been evolving to its present state for some 200 million years since the single landmass called Pangaea began to split apart through tectonic action. Through continental drift over the intervening millions of years, the landmass divided into several smaller fragments of present-day continents. As the spaces between the continental segments became larger, they filled with the water that surrounded them and, through eons, formed the various oceans and seas that encompass the planet's surface.

The Pacific Ocean is so large that it is difficult to discuss it as a single ocean. Various parts of it are substantially different from other parts. For discussion purposes, the Pacific is usually thought of as comprising three distinct regions.

The western region begins in Alaska’s western Aleutian Trench, extends through the Kuril and Japanese Trenches, runs south to the Tonga and Kermadec Trenches, continues on to an area northeast of New Zealand’s North Island. This region is characterized by large strings of islands, the largest constituting Japan, New Guinea, New Zealand, and the Philippines. Some of these strings, notably Japan, New Guinea, and New Zealand, were, through time, sheered off from the continent by tectonic movements. The entire western Pacific is also dotted with volcanic islands, which are the peaks of high underwater mountains. These were formed as molten rock from successive volcanic eruptions cooled and solidified.

The ocean floor of the central area of the Pacific is the largest underwater expanse on Earth. It is the most geologically stable area on the Pacific floor. It contains sprawling underwater plains, most at a depth of about 4,500 meters, that are essentially flat, although they contain some irregularities and some geological formations, called guyots, that resemble the mesas found in above-water plains.

The eastern part of the Pacific, which abuts the United States’ west coast, has a narrow continental shelf and a steep continental slope so close to shore, there are deep areas, such as the Monterey Canyon, which is more than three kilometers below the water’s surface. The two most significant trenches in this part of the ocean are the Middle America Trench in the North Pacific and the Peru-Chile Trench in the South Pacific.

Volcanic activity has historically been more pronounced along the western edge of the Pacific than along its eastern edge, although the East Pacific Rise, a chain of underwater mountains that runs from Southern California almost to the tip of South America, is also quite volcanically active. In the central and western Pacific, however, there is a profusion of islands that are the tops of underwater volcanic mountains.

Pacific Currents

Like the Atlantic, the Pacific has two large current systems called gyres. The Northern Gyre moves in a westerly direction from near the equator and is a surface ocean current that carries warm surface water into the Kuroshio Current, giving Japan moderately warm temperatures. The Kuroshio Current then moves northeast away from the Kamchatka Current, which brings cold water from the area around the Bering Strait. Part of it divides, moving north into the North Pacific Drift. The other part heads southeast.

Off the west coast of North America, part of the North American Drift moves north and becomes the Alaska Current. It brings to the Alaskan coastline waters warm enough to keep its shoreline and harbors from freezing in winter. Another part of the North American Drift veers south and forms the California Current, whose waters are cooled as they flow south to join the North Equatorial Current, which again flows west, completing its clockwise motion.

The South Equatorial Current also moves west toward Australia and New Guinea, but it then veers south to begin its counterclockwise course. The East Australian Current skirts Australia’s east coast and passes between Australia and New Zealand, south of which it feeds into the West Wind Drift, which moves east toward the coast of South America. Just north of South America’s tip, it feeds into the Peru or Humboldt Current, which runs north along South America until it connects with the South Equatorial Current, which moves west to complete the South Pacific Gyre.

In both the northern and southern areas of the Pacific, winds from the west move the water in an easterly direction. Closer to the equator, however, trade winds move the water toward the west. This balance permitted early sailors to head east by riding the westerly winds and to return to the east by riding the trade winds. There are virtually no winds on the equator itself. Sailors can founder there, in the so-called doldrums, for days with none of the propulsive forces that nature provides both north and south of the Earth’s dividing line.

The Pacific's currents are important both for transportation and for bringing warm and cold waters to parts of the ocean, thereby giving them and the land adjoining them more moderate temperatures than might be expected at their latitudes. Occasionally, nature runs afoul of this balance as the southern trade winds diminish and fail to push the cool Peru Current north. Instead, the Peru Current is replaced by the Pacific Equatorial Countercurrent. The Peruvians first dubbed this phenomenon El Niño, which means “the Christ child,” because it usually comes during the Christmas season. In some years, El Niño is followed by its opposite sibling, La Niña, resulting in two successive years of hardship. As waters reach unaccustomed temperatures, much sea life fails to appear where it would otherwise be, bringing catastrophe to people dependent upon fish and seaweed for their diets or for income.

Formation of Pacific Islands

The Pacific Ocean has more islands than any other ocean. In the west, most islands are volcanic, although some, such as Japan, New Guinea, and New Zealand, were formed when the corresponding chunks of land became separated from the continental landmass in prehistoric times through tectonic action. The volcanic islands in the western Pacific often rise just barely above sea level. Any change in the ocean’s level or tidal range can inundate them. Global warming threatens to melt ice in the polar regions, which will, in turn, cause sea levels to rise and, in some cases, cause islands to disappear. Indeed, some small, politically independent island nations face total inundation. It is estimated that by the year 2050, the world's oceans might rise as much as fifty centimeters, which could wipe out currently habitable regions occupied by millions of people in the South Pacific.

Another major structure in the South Pacific is the atoll, a tropical island on which a massive coral reef, often ringlike, generally rests on a volcanic base. Kwajalein is the largest atoll in the world, with a circumference of nearly 324 kilometers. Other coral formations, such as ridges or reefs composed of coral, can grow to enormous lengths, such as the Great Barrier Reef east of Australia. Coral formations are composed of living organisms that require light and a stable environment to survive. They are, therefore, always close to the ocean's surface and often protrude above the surface as tidal sea levels change. Most of them teem with aquatic life. In some cases, when they form around low-lying islands, the island disappears as the ocean surrounding it rises. The coral formation, however, often remains, filled with entrapped sand and rising above the water’s surface as an atoll.

Such island chains as the Hawaiian, Pitcairn-Tuamotu, and Tubai Islands are the result of great plumes of extraordinarily hot magma that have risen through the Earth’s mantle over millions of years and erupted through fissures in the oceanic plate above. The resulting formation occurs when the molten rock cools is an island composed largely of basaltic rock. Over great time spans, the island is slowly carried away to the northwest as the Pacific Ocean tectonic plate continues its normal motion, clearing the way for another such island to be formed above the plume at the same relative location in the oceanic plate. Hundreds of islands lined up almost like pearls in a necklace have resulted from this ongoing volcanic activity, though many have not survived to breach the ocean surface due to erosion by waves and currents in the ocean water.

Pacific Rim

The areas around the Pacific have many volcanic mountains, large ranges that line the coasts and still larger ranges that are submerged. The highest mountains on Earth are the Himalayas, with Mount Everest towering to 8,795 meters. Many of the mountains submerged in the Pacific, however, are higher. From the ocean’s floor at the Mariana Trench, undersea mountains, or seamounts, rise more than 10,600 meters as they approach the ocean’s surface.

Many mountains on the Pacific Rim arose as, over time, the oceanic crust is subducted beneath the continental crust. Intense heat is generated by friction in the subduction zone beneath a continental mass, while the pressure exerted by the inexorable motion of the crustal plates causes fractures to form in the overlying rock, permitting magma to erupt through the continental crust as volcanoes. This process occurs all around the so-called Ring of Fire, from the North Island of New Zealand, through the Philippines, Malaysia, Japan, and Siberia, to the Aleutian Islands and on down the west coast of North and South America to Peru and Chile. Ninety percent of the world's earthquakes occur in this region.

The coast along the Pacific Rim is narrow, giving way to the mountain ranges that run close to the shoreline. An exception to this is in eastern China, where the Yellow and the Yangtze Rivers have, for many centuries, carried silt toward the ocean, creating fertile coastal plains.

Geological activity beneath the Pacific results in thousands of small earthquakes every year along the Pacific Rim as oceanic plates move relative to each other. These earthquakes, most of which are too insignificant to be felt by humans, result in little or no damage. They relieve pressure that builds up between tectonic plates as they collide. When such pressure builds up without relief over many years, however, a major earthquake, usually causing considerable property damage and loss of life, may occur. Severe earthquakes on the sea floor may generate extraordinarily large waves called tsunamis. Tsunamis, known to have achieved heights of more than thirty meters, can engulf large stretches of shoreline, as in Alaska in 1964. They are most common in the Pacific Ocean because of the numerous earthquakes in the Pacific Ocean Basin. The worst tsunamis are usually caused by sudden displacements of crustal material in the subduction process, in turn, causing a massive displacement of seawater that translates to the surface as an energy wave. Such waves travel through the ocean at speeds of up to 800 km per hour and can cross the entire Pacific Ocean in a matter of hours. The tsunami that devastated the coast of Japan in 2011, producing worldwide economic consequences, was produced by a subduction zone earthquake.

Food Resources

The feeding hierarchy that exists among sea creatures is usually referred to as a food chain. One can envision this hierarchy as a triangle, at whose broad base are phytoplankton, microscopic plant organisms that require sunlight for their survival and are plentiful near the surfaces of oceans. Zooplankton, microscopic animal organisms, feed on the phytoplankton. They, in turn, constitute the diet of numerous species of small fish called anchoveta. These fish, in turn, are food for such larger fish as tuna and dolphin, as well as such birds as cormorants and pelicans, which dive into the water to catch fish. Animal waste, decaying plants, and dead fish and birds sink to the ocean floor, where scavengers consume them. Bacteria at those levels cause such droppings to decay, in the process releasing nutrients into the seawater that provide the phytoplankton with nourishment, thereby completing the food chain.

The food chain triangle becomes smaller as it approaches the top. Small fish survive by eating plankton. Larger fish eat the smaller fish, then sharks and whales eat the larger fish. A food chain that begins with millions of plankton may end up producing hundreds of thousands of anchoveta. Thousands of cod, hake, and mackerel; and one-half dozen sharks for every whale that is part of this intricate hierarchy.

Although fish are the major food source harvested from the world’s oceans, the Pacific also yields a great deal of kelp and seaweed, which, particularly in Asian countries, constitute a significant part of human diets. Seaweed is often harvested and laid in strips beside the ocean, where the sun dries it, thereby preserving it and making it easy to transport and to store. Even though they are not consumed as readily in the eastern parts of the Pacific Rim as in Asia, kelp and other sea plants are used in many food and pharmaceutical preparations. They have valuable pharmacological properties that are in great demand by drug manufacturers throughout the world.

Mineral Resources

The mineral wealth beneath the Pacific Ocean has barely been tapped. Among the minerals available near the shore are chromite, gold, iron, monazite, phosphorus, tin, titanium, and zircon. The greatest exploitation of the ocean’s treasure trove has been by the petroleum industry. Offshore drilling takes place worldwide for the recovery of crude oil and natural gas. In addition, large quantities of sand and gravel are harvested every year for use in construction and manufacturing.

The deep sea has remained a mysterious place. The historic descent in 1960 of the bathyscaphe Trieste into the Mariana Trench, the deepest area of ocean anywhere in the world, unlocked many mysteries. Subsequent exploration of the deepest areas of the ocean has challenged many long-held beliefs and revolutionized deep-sea research. It was long thought impossible for life to exist under the enormous pressure of the ocean below 760 meters, but the Trieste discovered life at the very bottom of the Mariana Trench, much to the surprise of oceanographers. In early 2012, James Cameron undertook a solo dive mission to the floor of the Mariana Trench in a specially-built one-man submarine, returning with a priceless video record of that environment. In 2019, Victor Vescovo dove in this same trench several times. In the early 2020s, Russia and China deployed Vityaz-D and Striver, respectively, to explore the Mariana Trench.

The deep ocean is richest in its deposits of cobalt, copper, manganese, and nickel. These deposits remain largely unharvested because of the difficulty of getting to them, but such problems will undoubtedly be overcome as deep-sea mining technology is developed.

The Pacific Ocean is home to the Great Pacific Garbage Patch, also known as the Pacific trash vortex, a mass of marine debris. As the garbage patch is largely composed of plastic, the best way to reduce its size is by limiting single-use plastics. The Ocean Voyage Institute began a project in 2009 to remove plastic from oceans, but after removing hundreds of thousands of tons, the patch still measured over 1.6 million square kilometers in the mid-2020s.

Principal Terms

continental drift: the gradual movement of continental landmasses within Earth’s crust, driven by magmatic convection in the underlying mantle layer

continental shelf: part of a continental landmass, usually gently sloping, that extends beneath the ocean from the water’s edge to the continental slope

continental slope: the defining edge of a continental shelf where it drops off sharply toward the ocean’s floor

delta: a triangular area with its longest side abutting the sea where a river deposits silt, sand, and clay as it flows into an ocean, lake, or other body of water

equator: an imaginary line, equidistant from the North and South Poles, around the middle of the planet where day and night are of equal length

guyot: an undersea mountain, or seamount, that has formed by volcanic activity, from which the peak has been eroded through wave action

lagoon: a body of saltwater separated from the ocean by a bank of sand

tectonic plate: a segment of Earth’s crust that is put into movement by magmatic convection in the underlying mantle layer

tidal range: the difference in water depth between high and low tides

trench: an extraordinarily deep region of the sea floor where two tectonic plates meet and the one making up the deep sea floor is subducted beneath the less dense, and, therefore, lighter, continental mass of the adjacent tectonic plate

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