Continental Margin

Formation of the Continental Margins

The Water Column

Geography of Continental Margins

Plant Life

Animal Life

Human Life

The Food Web

Spotlight on Continental Margins

For More Information

The continental margin is that part of the ocean floor at the edges of the continents and major islands where, just beyond the shoreline, it tapers gently into the deep sea. The continental margin is made up of the continental shelf, the continental slope, and the continental rise.

The continental shelf begins at the shoreline. It is flat and its width varies. For example, off the Arctic coast of Siberia it is 800 miles (1,280 kilometers) wide. Rich sediment (particles of soil and decaying matter) from rivers that flow to the sea filters down to the shelf. Over time, deposits of these sediments may become many thousands of feet (meters) thick. At its deepest points, the continental shelf is usually less than 660 feet (200 meters) below sea level (the level surface of the sea). Although the continental shelf is easier to explore than deeper areas of the ocean, there is still much to learn.

At the end of the continental shelf is a steep dip that marks the edge of the continent. This is called the continental slope, which descends to depths of 10,000 to 13,000 feet (3,048 to 3,962 meters) and ranges in width from 12 to 60 miles (20 to 100 kilometers). The continental slope usually resembles the edge of a mountain range, and in some places the drop is spectacular. Along the coast of Chile in South America, where the Andes Mountains meet the sea, the drop from the highest mountain peak on land—Aconcagua—to the bottom of the continental slope is more than 9 miles (14 kilometers).

Beyond the continental slope is the continental rise, where sediments drifting down from the continental shelf have collected. These deposits may extend as far as 600 miles (1,000 kilometers) out into the ocean floor, where the deep-sea basin begins.

Formation of the Continental Margins

The World Ocean, which is all the oceans taken together, covers a total of 139,782,000 square miles (363,433,200 square kilometers)—about 71 percent of Earth’s surface. Over 200 million years ago, the World Ocean

was one body of water that surrounded one large continent. As time passed, this land mass began to pull apart. As a result, the continents and islands were formed.

The breakup of that one large continent was caused by heat forces welling up deep within Earth. As earthquakes split the ocean floor, molten rock from below Earth’s crust flowed into the fracture and became solid. For millions of years this process was repeated until the upper parts of Earth’s crust, on which the continents sit, were pushed even farther apart. About 50 million years ago, the continents took their present shapes and positions.

About 20 million years ago, when the sea level was at its lowest, the area that now makes up the continental shelves was above water. Forests may have grown there and it may have been home to many animals. Over millions of years, rain, wind, and wave action eroded (wore away) the shelf surface, and rivers and glaciers flowed across it. Gradually, sediments from the shelves were washed into the water. Later, as the glaciers melted, the sea level rose and covered the shelves so that the entire continental margin was under water.

The Water Column

All of the waters of the ocean, exclusive of the sea bed or other landforms, is referred to as the water column.

Every element known on Earth can be found in ocean water. It is 3.5 percent dissolved salts by weight. The percentage of these salts determines the ocean’s salinity (level of salts). These salts also make seawater heavier than fresh water. The ocean water closest to the surface is usually less salty because of rainfall and fresh water flowing in from rivers.

The temperature of the oceans varies. Temperature changes are greatest near the surface where the heat of the sun can be absorbed. In the warmest regions of the world, this heat absorption occurs to depths of 330 to 1,650 feet (100 to 500 meters).

Zones in the ocean

Different parts of the ocean have different features and different kinds of creatures live in them. These different parts are called zones. Some zones are determined by the amount of light that reaches them.

Over the continental shelves and in the open ocean there is enough light to support photosynthesis, the process by which plants use the energy from sunlight to change water and carbon dioxide into the sugars and starches they use for food. These surface waters, called the sunlit zone, reach down as far as 660 feet (200 meters) below the surface. The sunlit zone supports more plant and animal life than any other zone.

Below the sunlit zone and extending about halfway down the continental slope is the twilight zone, which ranges from 650 to 3,300 feet (198 to 1,006 meters) in depth. Only blue light can filter down to this level. It is too dark for plant life here, but animals can live at this depth.

Beginning about halfway down the continental slope and extending into the deepest region of the oceans is the dark zone. Like the twilight zone, the dark zone is unable to support plant life, but a variety of animals are able to live in its depths.

Circulation

The oceans are constantly, restlessly moving. This movement takes the form of tides, waves, and currents, all of which affect the continental margins.

Tides

Tides are rhythmic movements of the oceans that cause a change in the surface level of the water. They are created by a combination of the gravitational pull of the sun and moon and Earth’s rotation.

High tide occurs when the water level rises. When the level lowers, it is called low tide.

Sea level refers to the average height of the ocean when it is halfway between high and low tides and all wave motion is smoothed out. Sea level changes over time.

Waves

Waves are rhythmic rising and falling movements on the surface of the water. Most surface waves are caused by wind. Their size is due to the speed of the wind, the length of time it has been blowing, and the distance over which it has traveled. Breakers are waves that collapse on a shoreline because the water at the bottom of the wave is slowed by friction as it rolls along the shore. The top of the wave then outruns the bottom and topples over in a heap of bubbling foam.

One type of wave called a tsunami (soo-NAH-mee) is mainly caused by undersea earthquakes. When the ocean floor moves during the quake its vibrations create a powerful wave that travels to the surface. Tsunamis that strike inhabited coastal areas can destroy entire towns and kill many people.

Currents

Currents are flows of water in a certain direction. They can be both large and strong. Most currents are caused by the wind, the rotation of Earth, and the position of continental landmasses. In the North Pacific, for example, currents moving west are pushed northward by Asia and southward by Australia. The same currents then move east until they reach North and South America, which send them back toward the equator. Longshore currents are those that move along a shoreline.

Upward and downward movement of water also occurs in the ocean. Vertical currents are mainly caused by differences in water temperature and salinity. In some coastal areas strong wind-driven currents carry warm surface water away. An upwelling (rising) of cold water from the deep ocean occurs to fill the space. This is more common along the western sides of the continents. These upwellings bring many nutrients from the ocean floor to the surface waters.

At the continental margins, large quantities of sediments enter the ocean and move out along the sea floor. The sediments are often sped along by turbidity (tur-BID-ih-tee) currents, which may be caused by earthquakes or sudden slumping of loose sediments. The thick mixture of sediment and water rushes down the continental slope and through any submarine canyons at considerable speeds for long distances, much like an avalanche of snow. They are so strong they have been known to break underwater cables that lay in their path hundreds of miles away. Turbidity currents can cause many changes to the margin floor.

Geography of Continental Margins

The general shape of the continental margin is usually determined by the shape of the coastline from which it extends. If it extends from a plain, then the margin will be broad and level. If it extends from a mountainous coast, then it will be steep and rocky. Steep cliffs that may have been formed by wave action when the level of the sea was lower may now be submerged and form part of the margin.

The present shape of a continental margin may be due to several influences. Movement of Earth’s crust may have given it a folded appearance where the sea floor cracked and was pushed underneath the margin. Huge boulders and rocks may indicate that a glacier once moved across the region. The presence of a river may mean a larger quantity of sediment

where it enters the ocean, and the weight of tons of this sediment may have forced the underlying rock to sink.

A continental margin located in an area prone to many earthquakes or volcanoes is considered an active margin. It is constantly changing due to the continuous earthquake and volcanic activity. Active margins are often found in the Pacific Ocean, such as those along the west coast of South America. They are narrow and often drop sharply into a deep trench (steep valley).

Passive margins are less active and usually free of earthquakes and volcanoes. They are found where the ocean floor is still gradually spreading. In most cases there is a wide rise. Continental margins along the Atlantic and Indian Oceans are passive margins.

Dams

Volcanic action, earthquakes, or a reef (a ridge of rock or other material) may create a dam at the end of the slope or rise. This dam causes sediments to build up and become an extension of the shelf. A new slope

and rise form at its end. Salt domes, huge mounds of salt that have moved upward from beds buried deep in the ocean floor, can form dams and trap sediment. A salt dam has helped form the shelf in the Gulf of Mexico off the U.S. coast.

Subduction zones and trenches

The sea floor is continuously spreading. When it presses against the edges of the continents, they resist its movement. This results in an area of extreme pressure called a subduction zone. The enormous pressure forces the sea floor to crack, pushing it down and causing it to slide under the continental margin, often causing a deep, V-shaped trench to form. The greatest depths in the oceans are found in these trenches, and the deepest trenches are located in the Pacific Ocean. The Mariana Trench is the deepest at 36,201 feet (11,033 meters). Many earthquakes occur in subduction zones.

Underwater canyons

The continental slope is often cut by deep, V-shaped, underwater canyons (deep, narrow depressions in Earth’s crust). These canyons vary in size but are not as deep as trenches. A canyon off the coast of Africa extends 15 miles (25 kilometers) across, but its depth is only 1,480 feet (450 meters).

The origin of canyons is different from that of trenches. Some canyons appear to be related to rivers on land that during prehistoric times may have extended into the area now covered by ocean. The huge Monterey Canyon off the coast of California is an example. The Salinas River once flowed into its bed during the Ice Ages.

The method of formation for some canyons is a mystery. The Bering Canyon north of the Aleutian Islands in the North Pacific Ocean is an example. It cuts through the continental margin for more than 255 miles (410 kilometers) and is the longest canyon in the world. However, there is no evidence it was ever associated with a river and it is unknown why or how it was formed.

Reefs

A reef is a ridge or wall of material lying close to the surface of the water just offshore. Reefs are made of rock or coral. A coral reef is a wall formed in shallow ocean areas by small, soft, jellylike animals called corals and marine algae that store calcium deposits. Corals attach themselves to hard surfaces and build a shell-like external skeleton. Many corals live together in colonies. Young coral builds its skeleton next to or on top of older skeletons. Gradually, over hundreds, thousands, or millions of years a wall, or reef, of these skeletons is formed. Reef corals cannot live in cold waters. They are found only in warm waters north and south of the equator. The water must be clear and free of sediment. Coral reefs do not form where rivers flow into the sea.

When a coral reef grows tall enough to break the surface of the water, it may begin to collect sand, gravel, weeds, and other matter. Gradually, an island, called a cay (KEE) or key, begins to form. The wind or visiting animals bring seeds, and plants begin to grow, especially small trees called mangroves. As the plants die and decay, soil forms. After a time, the island may be able to support certain larger species of trees.

Three main types of coral reefs are found on the continental margins: shelf reefs, fringing reefs, and barrier reefs. Shelf reefs form on a continental shelf having a hard, rocky bottom. A shallow body of water called a lagoon may be located between the shelf reef and the shore. Fringing reefs develop close to the land and no lagoon separates them from the shore. The longest is found in the Red Sea, where it stretches for 2,000 miles (3,200 kilometers). Barrier reefs line the edge of the continental shelf and separate it from deep ocean water. A barrier reef may enclose a lagoon and even small islands.

World’s Biggest Ice Cubes

Thousands of icebergs form each year and float freely in the ocean. Those in the Arctic are usually produced by large glaciers moving across Greenland. As the glacier reaches the ocean, huge portions break off. Arctic icebergs often contain soil and other evidence of their origin on land. Antarctic icebergs are gigantic chunks of pack ice that form on open water. Unlike glacier ice, they do not originate on land, and therefore, contain no sediments.

The exposed portion of an iceberg may be more than 200 feet (61 meters) high, but this is only a hint of its actual size. About 90 percent of an iceberg remains underwater. That means its submerged portion could be 1,800 feet (549 meters) deep!

Estuaries and deltas

When a river traveling through lowlands meets the ocean in a semi-enclosed channel (stream bed) or bay, the area is called an estuary. The water in an estuary is brackish—a mixture of fresh and salt water. In these gently sloping areas, river sediments collect, creating muddy shores. When the muddy sediments form a triangular shape over the continental shelf, the area is called a delta. The Mississippi Delta is one of the largest, spanning 60 miles (100 kilometers) across.

Glacial moraines

A glacial moraine is a pile of rocks, gravel, and sand created when a glacier moves across the surface of the land. Glacial moraines are found on some continental shelves that were crossed by glaciers during the Ice Ages more than 10,000 years ago. When the glaciers shrank, the level of the sea rose and covered the area with water. The tops of some glacial moraines remain above the water and form islands. The Aland Islands in the Baltic Sea are an example.

Bars and shoals

Bars and shoals can form where tides and currents move large quantities of sediment, such as sand or gravel. A bar is a ridge of sand that accumulates across a channel. Shoals are areas where enough sediment has accumulated that the water is very shallow and dangerous for

Tiny, But Deadly

Pfiesteria piscidia, a species of tiny, one-celled dinoflagellate algae, has suddenly become more dangerous than its size would indicate. In the late 1980s this normally nontoxic organism turned poisonous and began killing fish in North Carolina waters. Over a billion fish have been destroyed since 1991.

Pfiesteria release strong poisons into the water. The poisons cause fish to gasp for oxygen and develop bleeding sores on their bodies. Pfiesteria then feed on the dying tissues. The poisons are harmful to shellfish and even mammals, including humans, who develop impaired memory and learning abilities.

Most dinoflagellates obtain food by means of photosynthesis, but Pfiesteria passes through many different life stages, and in some stages, it appears to dine on other organisms. Experts suspect that the changes in Pfiesteria may have been caused by fertilizer and sewage runoff from hog and chicken farms in the area.

navigation. The shoals around Nantucket Island, off Massachusetts, for example, have sunk at least 2,100 ships.

Plant Life

Most ocean plants live in waters above the continental shelf. They include tiny, one-celled organisms and many kinds of seaweed and seagrasses. Some scientists estimate that plants over the continental shelf produce more oxygen for Earth than all the forests on land.

Ocean plants are surrounded by salt water at all times. For this reason most have not developed the special tissues and organs needed by land plants for conserving water. Seaweeds, for example, use their “roots” only to anchor them in one spot, not to draw water from the soil.

Water also offers support to ocean plants. Trees on land require a tough, woody stem to hold it erect, but giant underwater plants do not require woody portions because the water helps hold them upright. Their stems are soft and flexible, allowing them to move with the current without breaking.

Marine plants can be classified as either plankton or benthos. Plankton (a Greek word meaning “wanderers”) are plants that float freely on the water’s surface. Benthic (a Greek word meaning “depth”) plants anchor themselves in the sea floor.

Ocean plants can be divided into two main groups: algae (AL-jee) and green plants.

Algae

Algae are single- or multi-celled organisms that do not fit neatly into the plant category. They can range from microscopic blue-green algae, which are really bacteria, to giant brown algae kelp nearly 200 feet (61 meters) long. Other algae are green or red. Green algae are the ancestors of land plants. Like plants, nearly all algae have the ability to make their own food by means of photosynthesis (foh-toh-SIHN-thuh-sihs). They use energy from sunlight to change water and carbon dioxide into the sugars and starches they need for food. Other algae absorb nutrients from their surroundings.

Phytoplankton are a form of algae so tiny they cannot be seen with the naked eye. They float freely in the water, allowing it to carry them from place to place. Other species of algae are massive and live in vast underwater forests anchored to the seafloor. These are the benthic species.

Common algae

Two types of algae commonly found along the continental margins include phytoplankton and kelp.

Phytoplankton Phytoplankton are microscopic single-celled algae that float on the surface of waters within the sunlit zone. They are responsible for about 90 percent of the photosynthesis carried out in the oceans. During photosynthesis, phytoplankton release oxygen into the atmosphere. Two forms of phytoplankton, diatoms and dinoflagellates (dee-noh-FLAJ-uh-lates), are the most common.

Diatoms have simple, geometric shapes and hard, glasslike cell walls. They live in colder regions and even within arctic ice. Dinoflagellates have two whiplike attachments that make a swirling motion. They live in tropical regions around the equator.

Kelp Kelps are usually brown benthic algae that grow on rocks in temperate waters (41° to 72°F[5°C to 22°C]). In northern regions, benthic plants grow below 200 feet (61 meters). In tropical regions, where the sun’s rays are more vertical and can penetrate farther into the water, kelps grow as deep as 400 feet (122 meters). There is evidence that some can grow as deep as 1,000 feet (305 meters), where no light penetrates, because they may obtain food from decomposing matter that filters down from above instead of by means of photosynthesis.

There are several different species of kelp but only two basic forms. One type has a simple trunk between 20 inches and 8 feet (50 centimeters and 2.5 meters) long and a leaflike frond (branches) on top. The second type may grow to more than 250 feet (76 meters) in length and has fronds all along its trunk. Some kelp have gas-filled chambers at the bases of their fronds that help them remain upright. Kelp resemble green plants, but they have no true leaves, stems, or roots. Instead they have a rootlike structure called a holdfast that anchors them to the ocean floor.

Kelp reproduce sexually; cells from a male and female plant unite to form another plant.

Kelp grow in huge, floating groups, or forests, that cover hundreds of square miles (square kilometers) and provide shelter and food for animals such as fish, crabs, and sea otters. On the water’s surface, kelp can be so thick that sea otters can lie down on them as if in a hammock. Sea urchins, which also feed on kelp, can grow so numerous they eventually destroy the kelp beds unless checked by natural predators.

Growing season

Algae contain chlorophyll, a green pigment used to turn energy from the sun into food. As long as light is available, algae can grow. In some species, the green chlorophyll is masked by orange-colored pigments, giving the algae a red or brown color.

Growth of ocean plants is often seasonal. In some areas, such as the Arctic, most growth occurs during the summer when the sun is more nearly overhead. In temperate (moderate) zones, growth peaks in the spring but continues throughout the summer. In regions near the equator, no growth peaks occur since growth is steady throughout the year.

Food

Most algae grow in the sunlit zone where light is available for photosynthesis. Algae require other nutrients found in the water, such as nitrogen, phosphorus, and silicon. In certain regions, upwelling of deep ocean waters during different seasons brings more of these nutrients to the surface. This results in algal blooms, sudden increases in the number of algae. Algal blooms also occur when nitrogen and phosphorus are added to a body of water by sewage or by runoffs from farmland.

Reproduction

Algae reproduce in one of three ways. Some split into two or more parts, with each part becoming a new, separate plant. Others form spores (single cells that have the ability to grow into a new organism). A few reproduce sexually, during which cells from two different plants unite to create a new plant.

Green plants

The true green plants found in the ocean are seagrasses, such as turtle grass, and are similar to land plants. Green algae are often considered green plants. Unlike algae, green plants have roots and bloom underwater. Beds of seagrass occur in sandy bottoms in areas protected from currents, such as in lagoons or behind reefs. Large beds slow the movement of water and help prevent erosion of the shelf. Some marine animals use seagrasses for food and for hiding places.

Animal Life

The oceans are the largest animal habitat on Earth, and most species are found along the continental margins. The continental shelves underlie only about 8 percent of the total area of the oceans, but their shallow waters support more forms of life than any other area in the ocean—perhaps any other place on Earth. Scientists estimate as many as 30 million species of sea life may still be undiscovered. The types of animals found along the upper continental slope depend on whether the floor is sandy, muddy, or rocky. The lower slope and rise are home to fewer animals.

Animals that live in the sea have developed ways to cope with its high salinity. Naked (covered with thin “skins” or shells) animals maintain high levels of salt in their blood normally and do not need to expel any excess. Others, such as most fish, have special organs that remove extra salt from their system and release it into the water.

Sea-Going Dinosaurs

About 63,000,000 years ago, a number of dinosaur species lived in the oceans. The Tylosaurus was 25 feet (7.6 meters) long and resembled a chubby crocodile with flippers rather than feet. Plesiosaurs, which could have been as long as 50 feet (15 meters), had flippers and long, giraffelike necks. Flying reptiles, called pteranodons, glided through the air over the ocean on leathery wings looking for fish. They returned to land only to lay their eggs.

Water offers support to marine (ocean) animals as it does to plants. Many marine animals have special chambers in their bodies that allow them to adjust their buoyancy (BOY-un-see; ability to float) so they can float in either shallow or deep water. Some, such as seals, have flippers to make swimming easier. Others, such as octopi, forcefully eject water in a kind of jet stream to help them move.

Marine animals are classified as microorganisms, invertebrates, or vertebrates. Like plants, they can be classified according to their range and style of movement. Plankton drift in currents and include such animals as jellyfish. Many plankton move up and down the water column by regulating the amount of gas, oil, or salt within their bodies. (Production of gas or oil and removal of salt causes the organism to rise; the reverse causes them to sink.) Many larger animals spend part of their young lives as plankton. Crabs, which move about on legs as adults, but float with the current in their larval form, are an example. Larger animals that swim on their own, such as fish and dolphins, are called nekton. Benthos are the animals that live on the seafloor. These include snails and clams.

Microorganisms

Most microorganisms are zooplankton (tiny animals that drift with the current). They include foraminiferans, radiolarians, acantharians, and ciliates, as well as the larvae or hatchlings of animals that will grow much larger in their adult form. Some zooplankton eat phytoplankton and are preyed upon by other carnivorous (meat-eating) zooplankton, such as arrow worms.

Bacteria

Bacteria are another type of microorganism found throughout the ocean. They make up much of the dissolved matter in the water column and provide food for microscopic animals. Their numbers increase

along the continental shelf where most animal life is found because they help decompose the dead bodies of larger organisms.

Invertebrates

Animals without backbones are called invertebrates. They compose 95 percent of marine animals. Many invertebrate species, such as worms and squids, are found in the ocean. Perhaps the most numerous and diverse group of invertebrates are crustaceans, which have hard outer shells for protection from potential predators. Crustaceans include animals such as lobsters and crabs.

Common invertebrates

Invertebrates found along the continental margins range from planktonic jellyfish to nektonic octopi to benthic starfish and corals.

Jellyfish are commonly found close to shore where they float on the surface. All jellyfish move by squeezing their bodies to push out water that forms a kind of jet stream behind them. Most of the time they live as plankton and simply float with the current. Jellyfish eat small prey, such as shrimp, tiny fish, and other plankton, which they catch by stinging them with their tentacles.

Another common invertebrate that lives along the continental margins is the octopus. Octopi may crawl around the ocean bottom using their tentacles, or move through the water by means of jet propulsion. Different species are found throughout the ocean, and they vary in size. Those found in shallow water are generally the smallest. Giant octopi with a tentacle spread of 32 feet (9.7 meters) have been found off the coast of Alaska.

Food

Invertebrates may eat phytoplankton, zooplankton, or both. Some also eat plants or larger animals. The cone shell uses a harpoonlike tooth to spear its prey and inject it with poison. Others, including lobster, roam along the bottom to feed on dead organisms by grasping them with their large pincher front claws.

The Clockwork Worm

Some animals seem to have built-in “clocks,” like one species of marine worm called the palolo worm. This little creature, which lives in the waters of the central Pacific, develops eggs and sperm in the rear half of its body. This rear section detaches from the rest of its body and swims to the surface like clockwork on the day of the last quarter of the October-November moon. On this day, the surface waters grow thick with the rising egg sections from millions of worms.

Reproduction

Marine invertebrates reproduce in one of three ways. Two ways use external fertilization (outside the female’s body) of eggs. The first way involves a parent watching over the young in the early stages and offspring number in the hundreds. The second way does not require parental care and offspring number in the millions. Survival depends upon the absence of predators and the direction of currents. The third method of reproduction is internal fertilization (inside the female’s body). A parent cares for the young in the early stages and offspring number in the thousands.

Reptiles

Reptiles are cold-blooded vertebrates, which means their body temperature changes with the temperature of their surroundings. In cold temperatures they become sluggish but can still function. This means they do not have to use energy keeping their body temperatures up as most do mammals and birds.

Only one species of lizard has adapted to life at sea, the marine iguana. There are only two species of crocodile, the Pacific saltwater crocodile and the American saltwater crocodile, that spend a large amount of time off shore. The reptiles most commonly found in the oceans are the sea turtle and the sea snake.

Common marine reptiles

Sea turtles can be distinguished from land turtles by their paddlelike limbs called flippers, which enable them to swim. Sea turtles have glands around their eyes that remove excess salt from their bodies, a process that makes them appear to cry. At least one species of turtle hibernates on the seabed during winter. Green turtles are

migratory and travel as far as 1,250 miles (2,000 kilometers) to return to a particular breeding area where they lay their eggs.

At least fifteen species of sea snakes live in tropical oceans (those around the equator). Half of these are found in regions around Australia and New Guinea. They have long bodies like land snakes, some of which attain 9 feet (2.7 meters) in length. Special salt glands help them maintain a body fluid balance. Their tails are paddle shaped to move them through the water. All sea snakes are venomous. The yellow-bellied sea snake may be the most abundant reptile on Earth.

Food

Sea turtles eat soft plant foods as well as small invertebrates, such as snails and worms. Turtles have no teeth. Instead, the sharp, horny edges of their jaws are used to shred the food so they can swallow it. All seven species of sea turtle are either threatened or endangered.

Sea snakes are carnivorous, feeding primarily on fish and eels they find along the ocean bottom in rocky crevices. They first bite their prey, injecting it with venom so that it cannot escape.

Reproduction

Both snakes and turtles lay eggs. Turtles dig a hole with their flippers on a sandy shore and lay their eggs. They cover the eggs with sand and abandon them, taking no interest in the offspring. Six weeks later the eggs hatch and the young turtles make a run for the water and immediately begin swimming. Sea snakes either come to shore to breed and lay their eggs on land or bear live young at sea, depending upon the species.

Fish

Fish are primarily cold-blooded vertebrates that have gills and fins. Gills are used to draw in water from which oxygen is extracted, and fins help propel the fish through the water. Most fish are long and sleek in design, but shapes vary greatly. Manta rays, for example, are flat and round, while seahorses are narrow and swim in a vertical position.

A Dentist’s Dream

Sharks may have as many as 3,000 teeth in their mouths at one time. These teeth are arranged in rows, and only the first few rows are used. If a tooth is lost during feeding, the next tooth in line moves up to take its place.

Most fish species live over the continental shelf in the sunlit zone. In the twilight and dark zones species such as the hatchetfish, the swallower, and the black dragon-fish can be found.

Common fish

Mackerels and rays are commonly found over the continental margins. Mackerels live over the continental shelves in temperate and tropical oceans. They gather in schools (groups) in the upper waters during the warm months and descend to deeper waters in the winter. Their primary food is zooplankton, such as fish eggs. Mackerels lay their eggs in mid-water where they drift with the currents. Some species are popular as game fish, and many species are important commercially in the fishing industry.

Rays are relatives of the shark, but they have flat, broad bodies. Their eyes are on top and their mouths are on the underside. They feed by flapping and gliding over the seabed in search of clams and similar prey. Some species have the ability to give an electric shock for defense or to kill prey. The largest species, the devil or manta ray, can grow as much as 20 feet (6 meters) in diameter. Most species bear live young.

Food

Plant-eating fish, such as anchovies, have a diet primarily of phytoplankton, benthic algae, or sea grasses. Fish that must swim in search of prey, such as swordfish, have more streamlined bodies than those, like the sea robin, that glide close to the bottom sediments in search of a meal. Many dark zone species eat carrion (decaying flesh).

Reproduction

Most fish lay eggs. Some fish, like the Atlantic herring, abandon eggs once they are laid. Other fish build nests and care for the new offspring. Still others carry the eggs with them, usually in a special body cavity or in their mouths, until the eggs hatch. A few fish, such as certain species of sharks, bear live young.

Sturgeons are one type of fish that travel thousands of miles to return to a particular breeding area where they lay their eggs and then die. By some inherited means of guidance, their young will make the same journey when their turn comes to breed, and the cycle is repeated.

Seabirds

Only 4 percent of bird species have adapted to sea life. Most seabirds remain near land where they can nest during breeding season. Many have adapted to marine environments by means of webbed feet and special glands for removing excess salt from their blood.

Common seabirds

Birds living along the ocean’s margins are divided into four groups. The first group includes pelicans and their relatives, such as frigate and cormorants. This is a varied group, but all members share the common trait of webbing on all four toes. The second group is called tubenoses and is known to be a group of superb flyers. Petrels, shearwaters, and albatrosses belong to this group. Another group contains a mix of gulls, terns, puffins, and auks. Finally, penguins, which do not fly but can swim in the water, are the last group. There are seventeen species of penguin and all of them live in the Southern Hemisphere (below the Equator).

Food

Seabirds are carnivorous. They eat fish, squid, or zooplankton and live where food is plentiful. Several species, such as sea ducks, dive underwater to feed on shrimps, worms, or crabs. Other species, such as cormorants, spot their prey from high above in the sky and then plunge in just deep enough to catch it. Some, like terns, swoop down on fish swimming close to the surface.

Reproduction

All seabirds nest on land. Some nest in huge colonies on the ground, others dig burrows, and still others prefer ledges on cliffs. Like land birds, seabirds lay eggs and remain on the nest until the young can leave on their own. Some birds live and feed in one area and migrate to another for breeding. Birds that nest on sandy shores tend to lay speckled or blotchy eggs in beige and brown colors that blend in with the sand and pebbles.

Marine mammals

Mammals (warm-blooded vertebrates that bear live young nursed with milk) must come to the surface to breathe, and many, such as seals and polar bears, live part of the time on land. Whales, porpoises, sea cows, and their relatives remain in the water at all times.

Mammals must maintain a high body temperature. Most marine mammals have a special layer of fat below their skin to protect them from the cold water. Some, such as walruses, have very thick skins that make it difficult to keep cool in the sun. Others, such as seals, are covered with fur that helps insulate them against the cold.

The Web of Life: Animal Partners

Many animals of different species form partnerships that help them survive. Certain fish, such as the moray eel, attract parasites that irritate their skin, gills, and mouth. That is when a cleaner fish comes in handy. The cleaner fish swims into the mouth of the eel, picks off the parasites, and eats them. Sometimes the customer forgets why the cleaner fish is there and has it for a snack.

Another partnership is formed between the clownfish and the sea anemone. Sea anemones (ah-NEH-moh-nees) are soft-bodied animals that attach themselves to rocks and reefs. These colorful creatures look like underwater flowers. However, they have long, poisonous tentacles they use to kill their prey. The clownfish cautiously approaches and rubs itself against the anemone. For some reason, this prevents the anemone from stinging the clownfish, which settles in among the tentacles and waits for dinner to swim by. The clownfish feeds on prey that escape the anemone and draws other fish into the anemone’s grasp so it, too, can eat.

Common marine mammals

Whales, dolphins, and porpoises are mammals that resemble fish. The killer whale, a relative of the dolphin, hunts in large groups and feeds on other mammals as well as fish and birds. Another fishlike mammal, the narwhal, or sea unicorn, has a tooth that grows into a spiral as much as 10 feet (3 meters) in length.

Seals are common along the continental shelves where they may plunge as deep as 3,300 feet (1,000 meters) in search of fish. Some species can stay underwater for as long as 30 minutes. Seals usually gather in huge on-shore colonies to breed.

Although polar bears are usually thought of as land animals, they spend much of their time in the water and may swim 20 miles (32 kilometers) or more in search of food. Their primary prey is seal. They may wait for hours along a shore for a seal to surface. If they catch it, they drag it ashore to eat it.

Food

Sea cows, such as the dugong, are the only plant-eating mammals that truly live in the sea. Marine mammals are typically carnivorous. Seals and walruses, for example, feed on fish and squid

The Brainiest Mammal

The mammal with the largest brain is the sperm whale, a species found in all oceans. The whale’s head makes up one-third of its body. Since an average-sized male whale is about 63 feet (19 meters) long, its head would be about 20 feet (6 meters) long.

that live on the continental shelf. Other mammals, like killer whales, hunt for seals and other mammals.

Reproduction

Marine mammals usually have only one offspring at a time. The young are nursed on the mother’s milk until they are able to find food on their own. This is true whether the mammal spends all its time in the water or part of the time on shore.

Endangered species

Sea turtles lay their eggs on beaches, which makes the eggs easy to hunt and/or destroy. Turtle eggs are a popular food among humans in many parts of the world, as are the turtles themselves. Turtles must come to the surface to breathe air where many get caught in fishing nets that pull them below the water, and they drown. Over 70 conservation laws have been passed to protect the sea turtles.

Many fish used as food have been caught in such large numbers that they are disappearing. Since the 1970s, the number of bluefin tuna has decreased 90 percent and Atlantic swordfish have decreased by 30 percent.

In certain areas, other animals are threatened. They include the starlet sea anemone, the giant clam, the Olive Ridley turtle, the loggerhead turtle, the coelacanth, the Dalmatian pelican, the West Indian manatee, the marine otter, the monk seal, and the polar bear.

About 11 percent of coral reefs were destroyed by human actions before 1998. Another 16 percent were lost during the El Niño event in 1997–98. El Niños are extreme weather conditions caused by the slackening of the southeast Pacific trade winds. Due to a resulting shift in temperature, the corals eject their color, which is created by the colorful dinoflagellate algae that live on them. This is called coral bleaching because the corals become pale. It is estimated that by the year 2020 another 30 percent of coral reefs will be lost. As a result, all of the animals that depend upon the reef for food and shelter will be affected.

Human Life

The waters over the continental shelves have been explored by humans more than any other area of the ocean because the water is fairly shallow

and reachable by divers. Most commercial fishing areas are located over the shelves, as are those areas from which oil and natural gas are extracted.

The best-known shelves are those off the coasts of the United States, eastern Canada, western Europe, and Japan. In these places, scientific studies are routinely conducted and the information is made available to everyone. Oil companies that work in other areas, such as the Persian Gulf, obtain knowledge about them. Oceanographic organizations have studied the Red Sea, the Yellow Sea, and the shelf off the coasts of Argentina and northwestern Africa. A few developing countries have closed their waters to foreign scientists; as a result, little is known about those shelves.

Impact of the continental margin on human life

The continental margin is the ocean area most easily reached by humans and so it has an important effect on human life.

Food

About 90 percent of the world’s marine food resources come from the waters over the continental shelves. Most of those resources consist of fish. Different methods and equipment are used to catch fish, depending upon the type of fish desired and where they are found. Some fish live in the water column and may be caught on the surface or in mid-water, while others

live on the ocean floor. These two categories may be divided further into fish caught at the shoreline, above the continental shelf, or in mid-ocean.

Sport fishing is done for recreation. The catch is taken home and may be eaten, but the family does not depend upon it for food. Subsistence fishing is done to obtain food for a family or even an entire community. Extra fish may be sold to neighboring communities. Native peoples who live on islands often do subsistence fishing.

Commercial fishing is done to earn money. When carried out by small owner-operated companies, much of the work is done by hand. The equipment is usually simple and the number of fishing boats small. Industrial, large-scale fishing usually involves modern, high-powered equipment. Huge factory ships may be as long as 330 feet (100 meters) and are equipped with automatic machinery needed to catch, handle, store, and process huge amounts of fish.

Energy

Millions of years ago, sediments from dead animal and plant life collected on the ocean bottom. Over time, these sediments fossilized (turned to stone). More time, heat, and pressure from overlying rock worked to liquefy these sediments and turn them into fossil fuels, primarily gas and oil. To obtain fossil fuels, oil companies build large rigs—platforms high above the ocean surface but anchored to the sea bed. From these platforms, drilling is done into the rocky ocean floor, releasing the gas or oil, which is then pumped to shore through pipelines. Most gas and oil deposits have been obtained from offshore rigs. More than $1 billion worth of gas and oil is pumped from the continental shelves of the United States each year.

Ocean surface waters absorb large quantities of solar energy (energy from the sun). A process known as ocean thermal energy conversion is used to capture some of that energy for human use. Conversion plants are located in Hawaii and other tropical islands. Energy from ocean currents and waves is also being explored as a source of power. One day it may be possible to anchor a turbine (energy-producing engine) in a fast-moving current, such as the Gulf Stream, and use it to produce power.

The energy in the tides has already been harnessed in different ways. The tides are channeled into salt ponds, where salt from the water is collected for sale. The tides also lift ships in and out of drydock where repairs are made. The first tidal power station was developed in an estuary in France where turbines were built into a dam that spans the estuary. As the tides flow in and out of the estuary, they turn the turbine blades, which produce electricity.

Minerals and metals

Minerals and metals are other important oceanic resources. Rocks, sand, and gravel dredged from the sea bottom, especially in the North Sea and the Sea of Japan, are used in the construction of roads and buildings. Along the Namibian coast of southwest Africa, diamonds are mined from the sea floor. Some minerals, such as sulfur, are pumped from the ocean beds as liquids.

Impact of human life on the continental margin

The area of the ocean most affected by human action is over the continental margin.

Use of plants and animals

After World War II (1939–45) the technology of commercial fishing improved, and a growing population increased the demand for fish as a food source. By the 1970s major food species, such as herring and cod, had been greatly reduced. By 1995, 22 percent of marine fishing areas had been overused or depleted, and in 44 percent of fishing areas the maximum numbers of fish allowed by

Wells in the Sea

The first offshore oil well was drilled off the coast of California by 1897. This well, and those that followed, was drilled from a pier extending from shore. The first free-standing drilling structures were not built until the 1940s.

Since that time, the design of drilling rigs has changed. To stand in deep water and withstand storms at sea, they require long legs and a sturdy platform. One of the tallest platforms currently in operation is off the coast of Santa Barbara, California. The distance from its base anchored in the ocean floor to the top of a derrick on its platform high above the water is 1,165 feet (355 meters). This makes the rig only about 100 feet (30 meters) shorter than the Empire State Building in New York.

Exploration for wells is often done by mobile rigs positioned on drilling ships. The ships maintain their positions over the well using special propellers, and drilling in deeper waters is possible. During the 1980s, test wells were drilled from these ships in waters more than 6,500 feet (2,000 meters) deep.

regulations were being taken. Commercial fishing is done primarily with large nets that may also catch other unwanted creatures, many of which die. For every 1 pound (2.2 kilograms) of shrimp caught in the ocean, 5 pounds (11 kilograms) of other species are thrown away.

It has been said that human greed and carelessness have threatened many species of fish. However, not all scientists agree that the problems are serious or that they lack solutions. For example, between 1983 and 1993, the total number of fish available increased slightly. Conservation ideas include monitoring the numbers of fish in certain species to prevent overfishing and extending human consumption to more types of fish.

Fish farms are another means of helping maintain certain species of commercially popular fish. Also called aquaculture, fish farming involves raising fish species under the best growing conditions in farms built along waterways. The fish most commonly farmed are shellfish, such as oysters, mussels, scallops, clams, and shrimp. Crabs, lobsters, salmon, trout, and tilapia are also farm raised, but to a lesser extent. The output of fish farms has tripled since 1984, and it is estimated that more than 30 percent of all the seafood eaten comes from fish farms.

Other sea plants and animals are endangered because they are collected as souvenirs or art objects. Seashells left behind by dead animals can usually be taken with no harm. However, many shells available commercially are taken from living animals and, as a result, the animals are left to die.

In response to these problems, marine parks and reserves have been set up all over the world to protect endangered species. They include the Shiprock Aquatic Reserve in Australia and the Hervey Bay Marine Park in California.

Natural resources

Large quantities of natural resources, such as oil and minerals, can be found in the ocean water or beneath the ocean floor. These resources have not been used up because they are still too difficult or too expensive to obtain. As methods improve that may change. The first areas likely to be depleted are those around the continental margins. Already, some sections of the sea floor have slumped because underlying oil and gas have been removed.

The Law of the Sea

After World War II, many countries began to expand their use of the oceans. Some countries started using ocean areas other countries had already claimed for themselves, and arguments resulted. In 1994, the United Nations approved a treaty among all nations called the Law of the Sea.

The Sneaky Vegetable

Although seaweed is popular as a food in some countries, particularly Japan, many people in the United States associate it with those slimy green things lying draped over the rocks at the beach. Most people have eaten seaweed and never realized it. As a product called carra-geenan, seaweed is added to toothpaste, ice cream, gelatin, peanut butter, marshmallows, and even some meat products. It acts as a glue to help hold the other ingredients together.

The Law of the Sea Treaty maintains that a country’s territory extends about 14 miles (22.2 kilometers) from its coast. That country has a right to defend that territory. The same country also has the right to control the natural resources on its continental shelf, or an area extending about 230 miles (370 kilometers) from its coast. This includes resources obtained from fishing and drilling for oil and natural gas. The rest of the ocean is international territory and all countries can share in its resources.

Quality of the environment

In 1996, 14 billion pounds (31 billion kilograms) of waste were dumped into the oceans. Most oceanic pollution caused by humans is concentrated along the continental margins. Sewage and industrial wastes are contributed from coastal cities, adding dangerous metals and chemicals to the water. Discarded items, such as plastic bags and old fishing nets, pose a hazard for animals that get caught in them. Medical wastes, such as needles and tubing, are also a danger.

Insecticides (insect poisons) and herbicides (weed poisons) reach the oceans when rain washes them from fields into rivers that carry them to the sea. These poisons often enter the food chain, become concentrated in the bodies of some fish and other organisms, and are consumed by humans who eat the fish. Fertilizers and human sewage create another problem. They cause phytoplankton to reproduce rapidly. When the plants die, their decaying bodies feed bacteria. The bacteria rapidly multiply and use up the oxygen in the water, and other organisms, such as fish, soon die.

Oil spills from tanker ships and leaks from pipelines and offshore oil wells are major pollutants to oceans. Power plants and some industries often dump warm water into the oceans, causing thermal (heat) pollution. Organisms that require cooler water are killed by the heat.

Agriculture, construction, and removal of trees on land dig up the soil. Often the rain washes this loose soil into streams and rivers. Eventually, it enters the oceans and collects as sediment in coastal areas. This kills some organisms, such as clams, that cannot survive in heavy sediment.

The Food Web

The transfer of energy from organism to organism forms a series called a food chain. All the possible feeding relationships that exist in a biome make up its food web. In the ocean, as elsewhere, the food web consists of producers, consumers, and decomposers. These types of organisms all transfer energy within the ocean environment.

Phytoplankton are the major primary producers in the oceans. They produce organic materials from inorganic chemicals and outside sources of energy, primarily the sun. Producers are sometimes called autotrophs, meaning “self-feeders.” Green plants are an example of producers because they manufacture the compounds they need through photosynthesis.

Zooplankton and other animals are consumers. Zooplankton that eat only plants are primary consumers in the oceanic food web. Secondary consumers eat the plant-eaters and include zooplankton that eat other zooplankton. Tertiary consumers are predators, such as tunas and sharks. Humans fall into this category. Humans are omnivores, which means they eat both plants and animals.

Decomposers feed on dead organic matter. These organisms convert dead organisms to simpler substances. Decomposers include lobsters and large petrels, as well as microscopic bacteria.

Dangerous to the oceanic food web are the concentration of pollutants and harmful organisms that become trapped in sediments where life forms feed. These life forms are fed upon by other life forms, and at each step in the food chain the pollutant becomes more concentrated. Finally, when humans eat these sea animals, they are in danger of serious illness. Diseases such as cholera, hepatitis, and typhoid, can survive and accumulate in certain sea animals. These diseases can be transmitted to humans who eat the infected animals.

Spotlight on Continental Margins

Queensland, Australia and The Great Barrier Reef

The Great Barrier Reef, the longest structure in the world created by living organisms, consists of slightly fewer than 3,000 smaller reefs joined together. It covers approximately 132,974 square miles (344,400 square kilometers), which is almost the size of the state of Kansas. At least 500,000 years old, the reef can be seen from space and was first mapped by the Apollo 7 astronauts in 1968.

Queensland, Australia and the Great Barrier Reef

Location: The Coral Sea off the northeast coast of Australia

Area: 132,974 square miles (344,400 square kilometers)

Located at the edge of Australia’s continental shelf, the Great Barrier Reef stands in water from 325 to 650 feet (100 to 200 meters) deep. On the landward side is a lagoon. On the seaward side, the continental slope plunges thousands of feet into the deep-ocean basin.

Many of the algae found on the reef live within the bodies of the corals, absorbing sunlight. Red algae, green algae, and kelp grow among the coral skeletons. Grasses and shrubs can be found on many cays, as well as banyan and breadfruit trees. The tree most commonly found is the mangrove.

The reef is home to millions of living creatures, including corals, sea urchins, sea slugs, oysters, and clams. The largest type of clam in the world, the giant clam, is found on the Great Barrier Reef. It weighs up to 1,000 pounds (454 kilograms) and has a diameter of up to 4 feet (1.2 meters). Many species of sea turtles come to the reef to lay their eggs. Parrot fish, squirrelfish, trumpet fish, lionfish, coral trout, and moray eels make the reef their home.

Seabirds nest on the cays and islands. More than 100,000 terns flock together annually on Raine Island. Frigate birds, gannets, and sea eagles can be seen skimming over the waters in search of food.

The sands that line the beaches are valuable to industry, and some people believe that oil lies beneath the shelf. To prevent the reef’s destruction, the Australian government established the Great Barrier Reef Marine Park in 1975. The park is the world’s largest protected marine area. Visitors are controlled, many areas are reserved only for study, and bird and turtle breeding areas found in the park are closed during breeding season so people do not disturb the animals.

Northwestern Australia and Southeast Asia

On the continental shelf off the coast of northwestern Australia are the islands of Sumatra, Java, Borneo, and the Malaysian Peninsula, the most volcanic islands in the world. Other islands, including New Guinea, New Zealand, and the Philippines, are formed from part of the shelf. South of Java, the slope suddenly plunges into the Indonesian Trench, a deep, steep-sided valley in the ocean floor.

Northwestern Australia and Southeast Asia

Location: Indian and Pacific Oceans

Area: Approximately 140,000 square miles (300,000 square kilometers)

This continental shelf is approximately 140,000 square miles (300,000 square kilometers) and holds the most extensive beds of seagrass in the world, including such species as wireweed and paddleweed. Anchored by the grass, sandbanks over 33 feet (10 meters) thick and many miles (kilo-meters) long have built up over time.

Blue swimmer-crabs, pen shells, fan mussels, green turtles, and sea snakes are just a few of the marine animals that live over the shelf. Coastal waters yield herring, salmon, sardines, snapper, swordfish, and tuna.

A large colony of dugongs, plant-eating relatives of the manatee, are attracted to the ample supply of seagrass and make this area their home. The dugong population is declining because they are hunted by native peoples for food and are often killed by boat propellers.

In the shallow waters over the shelf, petroleum and natural gas are being extracted. Pearls are harvested here, although in sharply declining volume.

In 1986 the member nations of the South Pacific Forum declared the area a nuclear-free zone in an attempt to halt nuclear testing and prevent the dumping of nuclear waste.

Western United States

The continental margin off the northwestern coast of the United States was partly formed by a dam of rock thrust up by earthquake action about 25 million years ago. Where the dam breaks the surface of the water, it forms the Farallon Islands off San Francisco. The shelf is so narrow here—only about 1 mile (1.6 kilometers) wide—that the heads of many submarine canyons reach almost to the shoreline. Large quantities of sand from the beaches are carried by currents down the canyon walls.

Western United States

Location: Pacific Ocean

Area: Approximately 173,700 square miles (450,000 square kilometers)

Rainfall and runoff from rivers occur only during the winter months. The California Current travels south in the summer, and in winter the Davidson Current appears and moves north.

The North Pacific waters are rich in marine life, including sponges, flying fish, sharks, manta rays, seals, and many species of whales.

Commercial fishing is done all along the coast but primarily in the waters around Alaska. Oil and gas are also obtained from the area.

Northeastern United States

Between 270,000,000 and 60,000,000 years ago, a large dam created by earthquake action was formed off the northeastern coast of the United States. A trench on its landward side was gradually filled in with sediment over time, making it now part of the continental shelf, which measures 71,410 square miles (185,000 square kilometers). Sediment that spilled over the top formed a new continental slope that is unstable and subject to landslides. Few submarine canyons are found here, although the Hudson Canyon, associated with the Hudson River, extends from the river’s mouth into the ocean.

Northeastern United States

Location: Atlantic Ocean

Area: 71,410 square miles (185,000 square kilometers)

Changes in sea level have altered the appearance of the coastline and shelf. Fifteen thousand years ago sea level was much lower and much of the continental shelf in the northeastern United States was exposed. Gradually, as sea levels rose, the shelf was covered by water.

Numerous rivers empty into the Atlantic along this shelf, and the presence of fresh water reduces the ocean’s salinity. The warm Gulf Stream current affects water temperature and circulation.

Waters here support a variety of marine life, including sea slugs, starfish, mussels, many species of crabs and lobsters, poisonous toadfish, and sperm whales. Commercially important fish found along this shelf include ocean perch, cod, and haddock. Oil and gas are obtained from the shelf off the coast of Newfoundland and farther north.

The North Sea and Western Europe

The continental shelf off the coast of Europe holds the entire United Kingdom (England and its islands). It measures 212,300 square miles (550,000 square kilometers). Waters over this shelf include the North Sea, the Skagerrak (an arm of the North Sea), and the English Channel. Under the North Sea the shelf slopes gently downward from south to north. Although the southernmost areas are shallow, a deep canyon lies off the mouth of the Humber River, an estuary on the east coast of England. In the northeast is the Norway Deep, or Trough, an underwater canyon running parallel to the Norwegian coast

The North Sea and Western Europe

Location: The North Atlantic

Area: 212,300 square miles (550,000 square kilometers)

and into the Skagerrak. Other troughs lie to the west of Ireland. In the north, the coastline is marked by steep cliffs.

Tides are important in the North Sea because they influence ocean traffic. Along the coast of Norway, the tidal range (difference between high tide and low tide) is often less than 3 feet (1 meter). On the French side of the English Channel, tidal ranges of more than 27 feet (8 meters) are common. Tidal currents deposit sand in areas along the coast, causing problems in shipping routes. Several large rivers, such as the Rhine and the Elbe, lower the salinity of the water and create currents where they flow into the sea.

Sediments from the rivers and upwelling from deep, cold waters makes the North Sea rich in nutrients. The rocky shelf supports much algae and plant life, such as kelp, and eel grass. Invertebrates like cockles, mussels, scallops, sponges, and snails thrive in the rocky areas.

More than 170 species of fish live in the North Sea, including sharks, rays, herring, mackerel, haddock, cod, and sole. The fish attract large numbers of sea birds, including puffins, gannets, terns, gulls, and ducks. Among mammals living here are gray seals, harbor seals, dolphins, porpoises, and killer whales.

Fishing has been done in the area since 500 AD. Herring, haddock, plaice, cod, and whiting are commercially important. Atlantic salmon are farmed in some areas.

The North Sea is also the site of offshore oil and gas drilling. By 1989, 149 oil platforms were operated in the North Sea by British, Dutch, Norwegian, Danish, and German companies.

More sand and gravel is removed from the North Sea—24.5 million tons (22 metric tons) annually—than anywhere else in the world. These operations occur close to shore in waters less than 115 feet (35 meters) deep. Lime, another mineral, is mined from the seabed (sea floor). All of these materials are used as construction materials.

The Persian Gulf

The Persian Gulf lies between the Arabian Peninsula and Iran. Millions of years ago, it was much larger, but the floor beneath the Persian Gulf is shrinking as the floor beneath its neighbor, the Red Sea, on the opposite side of the peninsula, expands. It is estimated that after another 50,000 years, the Persian Gulf will be completely closed as the peninsula is pushed toward Iran. At one time the floor of the gulf was above sea level. Now it is part of the continental shelf, which measures 92,640 square miles (240,000 square kilometers). No earthquakes or volcanoes are found here, and the seafloor has both muddy and sandy areas.

The Persian Gulf

Location: The Arabian Sea, between Saudi Arabia and Iran

Area: 92,640 square miles (240,000 square kilometers)

Surrounded by desert, the water in the Persian Gulf is warm, salty, and only about 330 feet (100 meters) deep. A small amount of fresh water flows in from the Tigris and Euphrates Rivers, but the climate is so hot that more water is lost from evaporation than is gained.

Large stands of mangrove trees and beds of seagrass are found in the gulf. The mud in which they grow is low in oxygen, so the thickets are not as well developed as those found in other areas.

Few coral reefs are found in the Persian Gulf because the water is too warm. The gulf is home to many kinds of shellfish, such as mussels, shrimp, and oysters. Fish like sardines, anchovies, mackerel, and barracuda are also found here. The largest fish living in the area is the whale shark, which can reach 40 feet (12 meters) in length. Common birds of the gulf include terns, ospreys, and the fish-eating eagle, which travels several thousand miles to Scotland to nest. Porpoises are common, and the narwhal is seen occasionally.

The Persian Gulf has been important for oil production since 1935, and oil platforms dot its surface. Other commercially important minerals and metals found here include salt, copper, and zinc.

Sea cucumbers, shellfish, sardines, and anchovies are all important to the fishing industry. Bahrain, a tiny island in the gulf, has been the source of pearl oysters for 2,000 years. The pearl industry has declined since it became possible to artificially stimulate pearl growth.

Hudson Bay

The continental shelf underlying Hudson Bay extends from the Canadian Provinces of Quebec, Manitoba, Ontario, and the Northwest Territories. It measures 480,000 square miles (768,000 square kilometers) and its depth ranges from 120 to 600 feet (36 to 182 meters). A deep canyon cuts through the bay and extends toward Hudson Strait.

Hudson Bay

Location: Northeastern Canada

Area: 480,000 square miles (768,000) square kilometers

The cold waters in Hudson Bay originate in the Arctic. The waters are low in salt because many rivers feed into the bay. From January until May, the bay is covered with floating ice, which in northern regions is slow to melt and makes navigation difficult.

Many fish live in the bay, including cod and salmon. Seabirds found here include ducks, geese, loons, and ptarmigans. Whales frequent the bay, and were at one time hunted. Native peoples still hunt and fish in the area.

During the last half of the twentieth century, the bay and strait became a popular shipping route for goods going to England. Oil and natural gas are being pumped from beneath the shelf in the northern regions, making the shelf important commercially.

For More Information

BOOKS

Carson, Rachel L. The Sea Around Us, Rev. ed. New York: Chelsea House, 2006.

Cox, Donald D. A Naturalist’s Guide to Seashore Plants: An Ecology for Eastern North America. Syracuse, NY: Syracuse University Press, 2003.

Haris, Vernon. Sessile Animals of the Sea Shore. New York: Chapman and Hall, 2007.

Peschak, Thomas P. Wild Seas, Secret Shores of Africa. Cape Town: Struik Publishers, 2008.

Preston-Mafham, Ken, and Rod Preston-Mafham. Seashore. New Edition. New York: HarperCollins Publishers, 2004.

Scientific American, ed. Oceans: A Scientific American Reader. Chicago: University Of Chicago Press: 2007.

Worldwatch Institute, ed. Vital Signs 2003: The Trends That Are Shaping Our Future. New York: W.W. Norton, 2003.

PERIODICALS

Maynard, Barbara. “Fire in Ice: Natural Gas Locked Up in Methane Hydrates Could Be the World’s Next Great Energy Source—If Engineers Can Figure Out How to Extract it Safely.” Popular Mechanics. 183. 4 April 2006: 40.

ORGANIZATIONS

American Cetacean Society, PO Box 1391, San Pedro, CA 90733, Internet: http://www.acsonline.org.

American Littoral Society, Sandy Hook, Highlands, NJ 07732, Phone: 732-291-0055, Internet: http://www.alsnyc.org.

Center for Marine Conservation, 1725 DeSales Street, NW, Suite 600, Washington, DC 20036, Phone: 202-429-5609; Fax: 202-872-0619, Internet: http://www.cmc-ocean.org.

Environmental Defense Fund, 257 Park Ave. South, New York, NY 10010, Phone: 212-505-2100; Fax: 212-505-2375, Internet: http://www.edf.org.

Environmental Network, 4618 Henry Street, Pittsburgh, PA 15213, Internet: http://www.environlink.org.

Environmental Protection Agency, 401 M Street, SW, Washington, DC 20460, Phone: 202-260-2090, Internet: http://www.epa.gov.

Friends of the Earth, 1717 Massachusetts Ave. NW, 300, Washington, DC 20036-2002, Phone: 877-843-8687; Fax: 202-783-0444; Internet: http://www.foe.org.

Greenpeace USA, 702 H Street NW, Washington, DC 20001, Phone: 202-462-1177; Fax: Internet: http://www.greenpeace.org.

Sierra Club, 85 2^nd Street, 2^nd fl., San Francisco, CA 94105, Phone: 415-977-5500; Fax: 415-977-5799 Internet: http://www.sierraclub.org.

World Meteorological Organization, 7bis, avenue de la Paix, Case Postale No. 2300 CH-1211PO Box 2300, Geneva 2, Switzerland, Phone: 41 22 7308111; Fax: 41 22 7308181, Internet: http://www.wmo.ch.

World Wildlife Fund,1250 24th Street NW, Washington, DC 20090-7180, Phone: 202-293-4800; Internet: http://www.wwf.org.

WEB SITES

Discover Magazine. http://www.discovermagazine.com (accessed August 14, 2007).

Journey North Project. http://www.learner.org/jnorth (accessed August 14, 2007).

Monterey Bay Aquarium. http://www.mbayaq.org (accessed August 14, 2007).

National Geographic Society. http://www.nationalgeographic.com (accessed August 14, 2007).

National Oceanic and Atmospheric Administration. http://www.noaa.gov (accessed August 14, 2007).

Oceana. http://www.americanoceans.org (accessed August 14, 2007).

Scientific American Magazine. http://www.sciam.com (accessed August 14, 2007).

Time Magazine. http://time.com (accessed August 14, 2007).

Continental Margin

Continental shelf

The continental slope

Submarine canyons

The continental rise

Resources

The continental margin is that portion of the ocean that separates the continents from the deep ocean floor. For purposes of study, the continental margin is usually subdivided into three major sections: the continental shelf, the continental slope, and the continental rise. In addition to these sections, one of the most important features of the continental margin is the presence of very large submarine canyons that cut their way through the continental slope and, less commonly, the continental shelf.

Continental shelf

The continental shelf is a portion of the continent to which it is adjacent and not actually part of the ocean floor. As a result of continual earth movement, the shelf is continuously exposed and covered by water. Even when covered by water, as it is today, it shows signs of once having been dry land. Fossil riverbeds, for example, are characteristic of some slopes. Remnants of glacial action can also be found in some regions of the continental shelf.

The continental shelf tends to be quite flat, with an average slope of less than 6.5 ft (2 m) for each 0.6 mi (1 km) of distance. It varies in width from a few miles (kilometers) to more than 932 mi (1, 500 km), with a worldwide average of about 43 mi (70 km). Some of the widest continental slopes are to be found along the northern coastline of Russia and along the western edge of the Pacific Ocean, from Alaska to Australia. Very narrow continental slopes are to be found along the western coastline of South America and the comparable coasts of west Africa. The average depth at which the continental shelf begins to fall off toward the ocean floor (the beginning of the continental slope) is about 440 ft (135 m).

Materials washed off the continents by rivers and streams gradually work their way across the continental shelf to the edge of the continental slope. In some instances, the flow of materials can be dramatically abrupt as, for example, following an earthquake. At the outer edge of the continental shelf, eroded materials are dumped, as it were, over the edge of the shelf onto the sea floor below.

The continental shelf is one of the best studied portions of the ocean bottom. One reason for this fact, of course, is that it is more accessible to researchers than are other parts of the sea floor. More than that, however, the waters above the continental shelf are the richest fishing grounds in the world. A number of nations have declared that their national sovereignty extends to the end of the continental shelf around their territory—often a distance of 120 mi (200 km)—to protect their marine resources.

Included among those resources are extensive mineral deposits. Many nations now have offshore wells with which they extract oil and natural gas from beneath the continental shelf.

The continental slope

At the seaward edge of the continental shelf, the ocean floor drops off abruptly along the continental slope. The break point between the shelf and slope is sometimes known as the continental shelf break. The continental slopes are the most dramatic cliffs on the face of the Earth. They may drop from a depth of 656 ft (200 m) to more than 9, 840 ft (3, 000 m) in a distance of about 62 mi (100 km). In the area of ocean trenches, the drop-off may be even more severe, from 656 ft (200 m) to more than 32, 800 ft (10, 000 m).

The average slope of sea floor along the continental slope is about 4°, although that value may range from as little as 1° to as much as 25°. In general, the steepest slopes tend to be found in the Pacific Ocean, and the least steep slopes in the Atlantic and Indian Oceans. Sedimentary materials carried to the continental slope from the continental shelf do not remain along the slope (because of its steep sides), but flow downward into the next region, the continental rise.

Submarine canyons

The most distinctive features of the continental slopes are the submarine canyons. These are V-shaped features, often with tributaries, similar to canyons found on dry land. The deepest of the submarine canyons easily rivals similar landforms on the continents. The Monterrey Canyon off the coast of northern California, for example, drops from a water depth of 354 ft (108 m) below sea level near the coastline to 6, 672 ft (2, 034 m) below sea level. That vertical drop is half again as great as the depth of the Grand Canyon.

There seems little doubt that the submarine canyons, like their continental cousins, have been formed by erosion, but for many years oceanographers were puzzled as to the eroding force that might be responsible for formation of the submarine canyons. Today, scientists agree that canyons are produced by the flow of underwater rivers that travel across the continental slopes (and sometimes the continental shelf), carrying with them sediments that originated on the continents. These rivers are known as turbidity currents.

Evidence for the turbidity current theory of canyon formation was obtained in 1929 when an earthquake struck the Grand Banks region of the Atlantic Ocean off Newfoundland. An enormous turbidity current was set in motion that traveled at a speed ranging from 25–60 mph (40–100 km/h), breaking a sequence of transatlantic telegraph cables along the way. The pattern of cable destruction was what made it possible, in fact, for scientists to track so precisely the movement of the giant turbidity current.

The continental rise

Sediments eroded off continental land, after being carried across the shelf and down the continental slope, are finally deposited at the base of the slope in a region of the ocean known as the continental rise. By some estimates, half of all the sediments laid down on the face of the planet are found in the continental rise.

In many regions, the continental rise looks very much like a river delta such as the one found at the mouth of the Mississippi River. In fact, these

KEY TERMS

Continental rise— A region at the base of the continental slope in which eroded sediments are deposited.

Continental shelf— A relatively shallow, gently sloping, submarine area at the edges of continents and large islands, extending from the shoreline to the continental slope.

Continental shelf break— The outer edge of the continental shelf, at which the ocean floor drops off quite sharply in the continental slope. Continental slope—A steeply-sloping stretch of the ocean that reaches from the outer edge of the continental shelf to the continental rise and deep ocean bottom.

Submarine canyon— A steep V-shaped feature cut out of the continental slope by underwater rivers known as turbidity currents.

Turbidity currents— Local, rapid-moving currents that result from water heavy with suspended sediment mixing with lighter, clearer water. Causes of turbidity currents are earthquakes or when too much sediment piles up on a steep underwater slope. They can move like avalanches.

underwater deltas may also include a network of channels and natural levees similar to those found in the area of New Orleans. One of the most thoroughly studied sections of the continental rise is the Amazon Cone, located northeast of the coast of Brazil. The Amazon Cone has a total width of about 30 mi (50 km) and a depth of about 1, 000 ft (300 m). It is bisected by a primary channel that is 800 ft (250 m) deep and as much as 2 mi (3 km) wide.

See also Ocean zones.

Resources

BOOKS

Hancock, Paul L., Brian J. Skinner, and David L. Dineley, ed. Oxford Companion to the Earth Oxford: Oxford University Press, 2001.

Garrison, Tom. Oceanography: An Invitation to Marine Science. 5th ed. Stamford, CT: Thompson/Brooks Cole, 2004.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 4th ed. John Wiley & Sons, 2000.

Thurman, Harold V., and Alan P. Trujillo. Essentials of Oceanography. 7th ed. Englewood Cliffs, NJ: Prentice Hall, 2001.

David E. Newton

Continental margin

The continental margin is that portion of the ocean that separates the continents from the deep ocean floor. For purposes of study, the continental margin is usually subdivided into three major sections: the continental shelf , the continental slope, and the continental rise. In addition to these sections, one of the most important features of the continental margin is the presence of very large submarine canyons that cut their way through the continental slope and, less commonly, the continental shelf.

Continental shelf

The continental shelf is a portion of the continent to which it is adjacent, and not actually part of the ocean floor. As a result of continual earth movement, the shelf is continuously exposed and covered by water . Even when covered by water, as it is today, it shows signs of once having been dry land. Fossil river beds, for example, are characteristic of some slopes. Remnants of glacial action can also be found in some regions of the continental shelf.

The continental shelf tends to be quite flat, with an average slope of less than 6.5 ft (2 m) for each mile (km) of distance . It varies in width from a few miles (kms) to more than 932 mi (1,500 km) with a worldwide average of about 43 mi (70 km). Some of the widest continental slopes are to be found along the northern coastline of Russia and along the western edge of the Pacific Ocean, from Alaska to Australia . Very narrow continental slopes are to be found along the western coastline of South America and the comparable coasts of west Africa . The average depth at which the continental shelf begins to fall off toward the ocean floor (the beginning of the continental slope) is about 440 ft (135 m).

Materials washed off the continents by rivers and streams gradually work their way across the continental shelf to the edge of the continental slope. In some instances, the flow of materials can be dramatically abrupt as, for example, following an earthquake . At the outer edge of the continental shelf, eroded materials are dumped, as it were, over the edge of the shelf onto the sea floor below.

The continental shelf is one of the best studied portions of the ocean bottom. One reason for this fact, of course, is that it is more accessible to researchers than are other parts of the sea floor. More than that, however, the waters above the continental shelf are the richest fishing grounds in the world. A number of nations have declared that their national sovereignty extends to the end of the continental shelf around their territory—often a distance of 120 mi (200 km)—to protect their marine resources.

Included among those resources are extensive mineral deposits. Many nations now have offshore wells with which they extract oil and natural gas from beneath the continental shelf.

The continental slope

At the seaward edge of the continental shelf, the ocean floor drops off abruptly along the continental slope. The break point between the shelf and slope is sometimes known as the continental shelf break. The continental slopes are the most dramatic cliffs on the face of the Earth. They may drop from a depth of 656 ft (200 m) to more than 9,840 ft (3,000 m) in a distance of about 62 mi (100 km). In the area of ocean trenches, the drop-off may be even more severe, from 656 ft (200 m) to more than 32,800 ft (10,000 m).

The average slope of sea floor along the continental slope is about 4°, although that value may range from as little as 1° to as much as 25°. In general, the steepest slopes tend to be found in the Pacific Ocean, and the least steep slopes in the Atlantic and Indian Oceans. Sedimentary materials carried to the continental slope from the continental shelf do not remain along the slope (because of its steep sides), but flow downward into the next region, the continental rise.

Submarine canyons

The most distinctive features of the continental slopes are the submarine canyons. These are V-shaped features, often with tributaries, similar to canyons found on dry land. The deepest of the submarine canyons easily rivals similar landforms on the continents. The Monterrey Canyon off the coast of northern California, for example, drops from a water depth of 354 ft (108 m) below sea level near the coastline to 6,672 ft (2,034 m) below sea level. That vertical drop is half again as great as the depth of the Grand Canyon.

There seems little doubt that the submarine canyons, like their continental cousins, have been formed by erosion . But, for many years, oceanographers were puzzled as to the eroding force that might be responsible for formation of the submarine canyons. Today, scientists agree that canyons are produced by the flow of underwater rivers that travel across the continental slopes (and sometimes the continental shelf) carrying with them sediments that originated on the continents. These rivers are known as turbidity currents .

Evidence for the turbidity current theory of canyon formation was obtained in 1929 when an earthquake struck the Grand Banks region of the Atlantic Ocean off Newfoundland. An enormous turbidity current was set in motion that traveled at a speed ranging from 25 to 60 mph (40 to 100 km per hour), breaking a sequence of transatlantic telegraph cables along the way. The pattern of cable destruction was what made it possible, in fact, for scientists to track so precisely the movement of the giant turbidity current.

The continental rise

Sediments eroded off continental land, after being carried across the shelf and down the continental slope, are finally deposited at the base of the slope in a region of the ocean known as the continental rise. By some estimates, half of all the sediments laid down on the face of the planet are found in the continental rise.

In many regions, the continental rise looks very much like a river delta such as the one found at the mouth of the Mississippi River. In fact, these underwater deltas may also include a network of channels and natural levees similar to those found in the area of New Orleans. One of the most thoroughly studied sections of the continental rise is the Amazon Cone located northeast of the coast of Brazil. The Amazon Cone has a total width of about 30 mi (50 km) and a depth of about 1,000 ft (300 m). It is bisected by a primary channel that is 800 ft (250 m) deep and as much as 2 mi (3 km) wide.

See also Ocean zones.

Resources

books

Duxbury, Alyn C., and Alison Duxbury. An Introduction to theWorld's Oceans. MA: Addison-Wesley Publishing Company, 1984.

Golden, Fred, Stephen Hart, Gina Maranto, and Bryce Walker. How Things Work: Oceans. Alexandria, VA: Time-Life Books, 1991.

Hancock, Paul L., Brian J. Skinner, and David L. Dineley, eds. Oxford Companion to the Earth. Oxford: Oxford University Press, 2001.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth:An Introduction to Physical Geology. 4th ed. John Wiley & Sons, 2000.

Thurman, Harold V., and Alan P. Trujillo. Essentials of Oceanography. 7th ed. Englewood Cliffs, NJ: Prentice Hall, 2001.

Woodhead, James A. Geology. Boston: Salem Press, 1999.

David E. Newton

KEY TERMS

Continental rise

—A region at the base of the continental slope in which eroded sediments are deposited.

Continental shelf

—A relatively shallow, gently sloping, submarine area at the edges of continents and large islands, extending from the shoreline to the continental slope.

Continental shelf break

—The outer edge of the continental shelf, at which the ocean floor drops off quite sharply in the continental slope.

Continental slope

—A steeply-sloping stretch of the ocean that reaches from the outer edge of the continental shelf to the continental rise and deep ocean bottom.

Submarine canyon

—A steep V-shaped feature cut out of the continental slope by underwater rivers known as turbidity currents.

Turbidity currents

—Local, rapid-moving currents that result from water heavy with suspended sediment mixing with lighter, clearer water. Causes of turbidity currents are earthquakes or when too much sediment piles up on a steep underwater slope. They can move like avalanches.

Continental margin

The continental margin is that underwater plain connected to continents, separating them from the deep ocean floor. The continental margin is usually divided into three major sections: the continental shelf, the continental slope, and the continental rise.

Continental shelf

Continental shelves are the underwater, gradually sloping ledges of continents. They tend to be quite flat, with an average seaward slope of less than 10 feet per mile (about 3 meters per kilometer). They vary in width from almost zero to more than 930 miles (1,500 kilometers), with a worldwide average of about 50 miles (80 kilometers). The widest shelves are in the Arctic Ocean off the northern coasts of Siberia and North America. Narrow shelves are found off the western coasts of North and South America. The average depth at which the continental shelf begins to fall off toward the ocean floor (the beginning of the continental slope) is about 430 feet (130 meters).

Changes in sea level during Earth's history have alternatingly exposed and then covered portions of the continental shelf. During lowered sea level, land plants and animals, including humans and their ancestors, lived on the shelf. Today, their remains are often found there. For example, 12,000-year-old bones of mastodons, extinct relatives of the elephant, have been recovered off the coast of the northeastern United States.

Vast deposits of muds, sands, and gravels compose the continental shelf. Most commercial fishing takes place in the rich waters above the shelf. Many nations around the world claim ownership of the extensive oil, natural gas, mineral, and other natural resource deposits beneath the continental shelf adjacent to their land areas. Many nations also dump much of their waste in the ocean over the continental shelves.

Words to Know

Continental rise: A region at the base of the continental slope in which eroded sediments are deposited.

Continental shelf: A gently sloping, submerged ledge of a continent.

Continental shelf break: The outer edge of the continental shelf, at which the ocean floor drops off quite sharply in the continental slope.

Continental slope: A steeply sloping stretch of the ocean that reaches from the outer edge of the continental shelf to the continental rise and deep ocean bottom.

Submarine canyon: A steep V-shaped feature cut out of the continental slope by underwater rivers known as turbidity currents.

Turbidity current: An underwater movement of water, mud, and other sediments.

Continental slope

At the seaward edge of the continental shelf is an immense drop-off. The steep edge where this occurs is known as the continental slope. The break point between the shelf and slope is sometimes known as the continental shelf break. The continental slopes are the most dramatic cliffs on the face of Earth. They may drop from a depth of 650 feet (200 meters) to more than 10,000 feet (3,000 meters) over a distance of 60 miles (100 kilometers). In the area of ocean trenches, the drop-off may be even more severe, from 650 feet (200 meters) to more than 33,000 feet (10,000 meters). In general, the steepest slopes tend to be found in the Pacific Ocean, and the least steep slopes in the Atlantic and Indian Oceans.

Submarine canyons. The most distinctive features of the continental slopes are submarine canyons. These are V-shaped features, often with tributaries, similar to canyons found on dry land. The deepest of the submarine canyons easily rival the size of the Grand Canyon of the Colorado River. Submarine canyons are created by the eroding flow of underwater rivers that travel across the continental slopes (and sometimes the continental shelf) carrying with them sediments that originated on the continents. These rivers are known as turbidity currents.

Continental rise

Sediments eroded off continental land, after being carried across the shelf and down the continental slope, are finally deposited at the base of the slope in a region of the ocean known as the continental rise. The deep ocean floor begins at the seaward edge of the rise. By some estimates, half of all the sediments laid down on the face of the planet are found in the gently sloping, smooth-surfaced continental rises.

[See also Ocean ]

continental margin Zone that consists of the continental shelf, continental slope, and continental rise. It extends from the shoreline to the deep-ocean floor at a depth of 2000 m. The zone is underlain by continental crust. Continental margins have been divided into active margins or passive margins depending on their coincidence, or otherwise, with plate margins.