Atlantic Ocean

The Atlantic Ocean separates the North and South American continents from Europe and Africa. With an area, excluding its dependent seas, of about 85.133 million square kilometers, it is second in size only to the Pacific Ocean. The dependent seas add another 23 million square kilometers to the Atlantic’s total size.

Location and Origin

The Atlantic Ocean, with an average depth of 3,300 meters and a maximum depth of 8,380 meters, touches the eastern coastlines of North and South America and the western coastlines of Europe and Africa. The Atlantic Ocean is often divided into the North Atlantic and the South Atlantic, although the entire oceanic mass may properly be considered a single ocean. The North Atlantic extends from the equator north to the Arctic Ocean. The South Atlantic, that part of the ocean south of the equator that extends as far as Antarctica, meets the Pacific Ocean at Cape Horn, the southernmost tip of South America, and the Indian Ocean at the Cape of Good Hope, the southernmost tip of Africa.

More than 200 million years ago, Earth’s land surface consisted of a single supercontinent, referred to as Pangaea, surrounded by an enormous sea, called Panthalassa. A large, shallow bay known as the Tethys Sea also protruded into the supercontinent from Panthalassa. The landmass, like contemporary landmasses, was a large, solid rock plate that floated on the mantle layer of molten rock. Over the intervening millions of years, convection currents in the mantle drove Pangaea to break apart into several smaller segments and become the present-day continents. The magmatic currents have driven, and are still driving, the continental masses and tectonic plates across the face of the planet. Looking at a map of the modern world and regarding it as a huge jigsaw puzzle, it is easy to visualize how the western coast of Africa fits into the eastern coast of the Americas and how the coastlines of other geographical areas fit neatly into those facing them.

As Pangaea split apart, the Tethys Sea essentially became enclosed by land as continental drift continued over millions of years, forming part of the present-day Arctic Ocean and Mediterranean Sea. The Pacific Ocean is the remnant of Panthalassa that exists today. The main rift in Pangaea separated large tectonic plates that have since formed North and South America, Asia, Africa, Europe, India, and Australia. The ever-widening space between the separating continents filled with water to form what is now the Atlantic Ocean. Current data regarding the movement of the continents and tectonic plates reveal that the North and South American continental structure and the Europe-Africa-Asia-Australia chain continue to move away from each other as though pivoting about a point in the North Atlantic Ocean approximately at the location of Iceland, such that the greater the distance from that location, the faster the tectonic plate is moving. Thus Europe and the easternmost coast of Canada are separating at the slowest rate, while Australia continues to move in a northeasterly direction at a relatively fast rate.

Two major tectonic plates, each a segment of the Atlantic Ocean floor, are moving apart annually at the average rate of 1.3 centimeters from a volcanically active line that runs approximately down the middle of the Atlantic Ocean, called the Mid-Atlantic Ridge. Where volcanic activity has been intense, molten rock has cooled into volcanic cones that sometimes protrude above the water’s surface to create islands. However, in the Atlantic, unlike in the Pacific (where islands are scattered throughout the ocean), more islands are found close to shore than toward the middle of the ocean. The rate of movement of the Atlantic Ocean seafloor away from the Mid-Atlantic Ridge is such that the entire seafloor is replaced about every 100,000 years, precluding the formation of mid-Atlantic islands. The few exceptions are actually the exposed peaks of underwater volcanic mountains, some of which, such as Mount Tiede in the Canary Islands, extend more than 3,600 meters above sea level.

Ancient people named the Atlantic Ocean after the mythological giant Atlas, who is said to have carried the world on his shoulders. Many ancients, believing that earth was flat, feared that anyone sailing far enough into the ocean would eventually fall over the edge of the earth into an abyss. This belief inhibited early explorers, although even in ancient times some skeptical adventurers sailed far into the ocean and returned safely to shore.

Continental Shelf and Coastline

Close to shore, the Atlantic Ocean is generally shallow, seldom exceeding a depth of 545 meters. Waters cover a shelf of rock that, in most of the North Atlantic, extends between 65 and 80 kilometers from the coastline into the ocean. At the end of this shelf, the ocean drops precipitously to a depth of about 3,940 meters, gradually leveling off into what is designated the abyssal plain. The continental shelf off the coast of Africa is much narrower than the continental shelf off the coast of North America.

The coastlines that border the Atlantic Ocean have been forming for millions of years, shaped largely by the motion and activity of the water that laps the shore. The rockiest areas are found in the northern parts of the North Atlantic and in the South Atlantic. The level of the ocean has risen considerably over the past 10,000 years as glaciers have melted. With the end of the Ice Age, huge quantities of water poured down rivers into the oceans. The point at which these rivers meet the ocean is called an estuary. Among the deepwater estuaries are the Chesapeake Bay in the United States and the Falmouth Estuary in Great Britain, both of which are sufficiently deep to accommodate large ships.

Less rocky portions of coastline are often characterized by long stretches of sandy beaches that have been formed over the years as the moving waters of the ocean have pulverized rocks and shells, transforming them into sand. Sometimes sandbars form near the coast. These may be long expanses of sand, often permanently covered by water, while others are below the water’s surface only at high tide. Some sandbars that are permanently above water may be broad enough to be inhabited, as are the Outer Banks of North Carolina, although at times devastating floods occur in communities built on sandbars. Sandbars endanger ships that venture too close to them, sometimes causing them to founder. Such ships may float off a sandbar as the water rises at high tide, but in many cases this does not happen, and the ship, trapped on the sandbar, must be hauled off. Most seaports employ pilots whose duty it is to guide incoming ships through deep channels away from dangerous sandbars or rock formations beneath the surface of the water. Lagoons sometimes form between the coast and sandbars that are not permanently covered with water. These are salty bodies of water in which marsh grasses and other vegetation grow. Many of them are rich in the microorganisms that fish need for their survival. Extensive fish and bird populations cluster around lagoons.

No ocean is fed by more rivers than the Atlantic. These rivers have, over hundreds of thousands of years, carried silt and sand toward the ocean they feed. These deposits have, in many places, built up to form deltas, roughly triangular areas whose broad base fronts the ocean. Some deltas, particularly those of the Amazon and Niger Rivers, are hundreds of miles wide with channels running through them. Often in tropical areas, mangrove swamps flourish in the deltas, the roots of the mangrove trees being covered with saltwater at high tide to provide them with the nutrients they require for their growth. Fish and birds flourish in mangrove swamps.

Tides and Trade Winds

Ocean tides are generally predictable. High tide and low tide each occur twice in any twenty-four-hour period. Tidal ranges—the depth variations between high tide and low tide—are most pronounced on continental shelves and in deepwater bays. In some parts of the world, the tidal range is dramatic. In the Canadian province of Nova Scotia, the Bay of Fundy has a tidal range of more than 12 meters, one of the largest in the world.

Gentler tides are observed as one approaches the equatorial regions, although there can be dramatic tidal activity in such areas during hurricanes and tropical storms, which have traditionally occurred in August through October in the Northern Hemisphere and February through April in the Southern Hemisphere. There is considerable concern at present that global climate change is causing more powerful hurricanes and extending the hurricane season. Hurricanes usually form in tropical areas but can move rapidly into more temperate zones where, if they strike land, they can result in injury, death, and substantial destruction. In some cases, such storms erode entire beaches and completely inundate waterside property. The Atlantic coastline in many places is shrinking rapidly as the result of water erosion from such storms.

Predictable wind currents, much like jet streams in the upper atmosphere, blow across the Atlantic Ocean. The trade winds near the equatorial areas blow in a westward direction. Such winds made it possible for early explorers to sail from European ports to the Americas. For their return trips, explorers depended upon winds to the north of the areas where the trade winds blow called the “westerlies,” which blow from west to east. Along the equator is a narrow area where waters are calm and where there is virtually no wind. In this area, referred to as the doldrums, sailors can drift languidly for long periods of time with no wind to propel them.

Atlantic Currents

Ocean water is never still. Much ocean water moves in one direction in predictable flows, in essence forming rivers through the ocean. Such rivers are called ocean currents. The warm tropical currents near the equator push the waters of the Atlantic Ocean westward toward North and South America, moving clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

The northern equatorial current runs from just north of the equator along the northeastern coast of South America and on toward the southern coast of North America. There, in the ocean east of the United States, the Gulf Stream runs through the Atlantic, flowing in a northerly direction. The Labrador Current that flows from the Arctic Ocean meets the Gulf Stream off Newfoundland, causing temperature variations that result in high humidity and dense fogs. North of Labrador, the Gulf Stream divides into the North Atlantic Drift, which flows north toward Greenland, and the Canaries Current, which flows southeast through the Atlantic off the west coast of Africa. These clockwise currents flow in what is termed the North Atlantic Gyre.

South of the equator, the southern equatorial current moves counterclockwise off South America’s east coast, proceeds south to the Brazilian Current, which then veers southeast to meet the Benguela Current that flows from just north of Antarctica to the Tropic of Capricorn. These counterclockwise currents are referred to as the South Atlantic Gyre.

The oceanic rivers that form the currents of the North and South American Gyres have a profound effect upon climate. The Gulf Stream, which brings warm, tropical waters north at the rate of some 130 kilometers per day, makes far northern areas such as Greenland and Iceland warm enough for human habitation. Its southeasterly branch gives Great Britain a more temperate climate than one would expect at such latitudes. The northern regions of the United States are considerably warmer than they would be if the Gulf Stream did not flow along their coasts. Even areas within the Arctic Circle feel the effects of the Gulf Stream, which keeps Russia’s Arctic port of Murmansk free of ice throughout the year.

The South Atlantic Ocean’s Benguela Current propels cold waters from the Antarctic Ocean north along Africa’s southwestern coast, keeping its temperatures much more temperate than would be expected in such latitudes. Cold air, however, does not hold moisture well, so when warm, humid winds from Africa’s southwestern coast strike the cold air over the Benguela Current, they cause rain to fall over the ocean. The result is that the whole southwestern coast of Africa is arid. The South Atlantic Ocean there is bordered by desert.

Food Chain

The world’s oceans are teeming with life. In a somewhat hierarchical arrangement called the food chain, the sea’s larger creatures feed on the smaller ones. At the base of the food chain are microscopic organisms called plankton, which form the basis for life in oceans. Plankton, which exist in both animal and vegetable form, are the basic diet of many aquatic creatures and hence of all ocean life. Plankton cannot live without sunlight, so they are found close to the water’s surface. Plankton depend upon nutrients borne in the water for sustenance, and such nutrients are usually found in coastal areas that are not very deep, such as mangrove swamps, where plankton often flourish. Some of the minerals the plankton require are washed into the ocean through estuaries that carry silt from inland areas. Other minerals come from deep in the sea, carried to the surface by ocean currents. In the Atlantic, the most abundant plankton are found in the extreme northern and southern reaches of the ocean, as well as off the west coast of Africa. These areas have a high concentration of the nutrients—abundant in deep, cool waters—on which plankton feed.

The presence of large amounts of plankton in these areas results in a wealth of other aquatic creatures, most of which live near the surface of the ocean where plankton are found, although exploration of the very deep oceans has revealed a remarkable amount of sea life in parts of the ocean so deep that it was previously thought that nothing could survive there.

The Atlantic has an intriguing population of creatures that range from among the smallest on Earth, such as plankton, to the largest, such as whales. Most of the fish caught by commercial fishermen for human consumption—notably sardines, smelt, cod, halibut, mackerel, hake, sole, and anchovies—swim in large schools near the ocean’s surface to harvest the plankton and smaller fish that constitute their diets. Larger fish, such as sharks, whales, swordfish, and sailfish, are fast-moving and live on a diet of smaller fish. They are often found in the cooler waters of the Atlantic, where plankton are abundant enough to nourish the small fish on which larger fish feed.

Some of the sea’s creatures are extremely mobile. Whales travel from the polar regions to areas thousands of kilometers away for spawning. Dolphins swim fast enough to cover 245 kilometers in a single day. Conversely, some shellfish, such as mussels, attach themselves to formations in the sea and are immobile, protected from predators by their thick shells. Some forms of sea life are protected by camouflage that allows them to blend in with the surrounding area. Others are able to emit noxious jets of fluid into the waters around them when they are threatened.

Considerable bird life is associated with the Atlantic Ocean. Most of the birds that depend upon the Atlantic for their food supply can fly, but some of them, such as the cormorant and the puffin, can swim as well. They dive into the water to catch their prey and can swim to catch it. Penguins, found in the Antarctic, cannot fly and can barely walk, but they swim as well as many fish. Cranes and other long-legged birds flourish in shallow waters, where they eat tiny sea creatures that they strain from the water and mud through their beaks.

In the middle of the North Atlantic is a large, relatively calm area of water known as the Sargasso Sea, whose surface is covered with huge fields of nutritious seaweed. The Sargasso Sea has become a spawning ground for eels that migrate to it during mating season from Europe and North America.

Seaside Settlements

Throughout history, humans have tended to settle beside the sea or along rivers. The transportation opportunities provided by such locations still make them desirable places for settlement. Such areas usually offer a temperate climate as well. The ancient cultures of Rome and Greece grew up on the shores of the Mediterranean and the Aegean, two of the dependent seas of the Atlantic Ocean. The Iberian Peninsula borders on the Atlantic to the west, as do France and Great Britain. Advanced civilizations have flourished on the shores of the Atlantic, particularly in Europe and Africa, since long before recorded history.

The islands of the Atlantic—products of violent volcanic activity that created large, hilly outcroppings—were slow to develop. The exceptions are the Canary Islands and Madeira, both of which became necessary stopover points for explorers sailing from Europe to the Americas. In recent times, considerable development of the Atlantic islands has taken place in such areas as the Caribbean. The Falkland Islands off the southeastern coast of South America have a stable, permanent population, as do such island enclaves as Prince Edward Island and Newfoundland in the north.

Shipping and commerce have been the backbone of the economy in most of the areas that have developed along the Atlantic coastline. Raw materials are brought into Atlantic port cities such as New York, Philadelphia, Rio de Janeiro, and Lisbon to feed the manufacturing industries of those countries. The fishing industry has flourished for many centuries along the Atlantic coastline. Tourism has also become a major economic factor in the more temperate and scenic regions on the Atlantic coast.

Trade patterns were substantially altered with the opening in 1896 of the Suez Canal, which connects the Mediterranean Sea with the Indian Ocean, and with the opening in 1914 of the Panama Canal, which links the Atlantic and Pacific Oceans. These strategic canals opened up a great deal of trade worldwide and overcame the necessity of plying the turbulent and dangerous waters around Cape Horn and the Cape of Good Hope in order to deliver goods to distant markets.

Principal Terms

continental drift: the gradual movement of continental landmasses across the earth’s surface driven by convection processes in the mantle

continental shelf: the part of the seafloor that is generally gently sloping and extends beneath the ocean from adjacent continents

continental slope: the part of the continental shelf that drops off sharply toward the ocean’s floor

equator: an imaginary line, equidistant from the North and South Poles, around the middle of the planet

estuary: an area where the mouth of a river broadens as it approaches the sea, characterized by the mixing of freshwater and saltwater

lagoon: a long, narrow body of saltwater that is separated from the ocean by a bank of sand

tectonic plates: large segments of the earth’s crust, affected by the movement of magma in the underlying mantle layer

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

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