Encyclopedic Entry

A storm whips up the Pacific Ocean.

Photograph by James P. Blair

Extraterrestrial Oceans
Mars probably had oceans billions of years ago, but ice and dry seabeds are all that remain today.

Europa, one of Jupiter's moons, is probably covered by an ocean of water more than 96 kilometers (60 miles) deep, but it is trapped beneath a layer of ice, which the warmer water below frequently cracks.

One of Saturn's moons, Enceladus, has cryovolcanism, or ice volcanoes. Instead of erupting with lava, ice volcanoes erupt with water, ammonia, or methane. Ice volcanoes may indicate oceanic activity.

No, the Toilet Doesn't Flush Backwards
in Australia

The Coriolis effect, which can be seen in large-scale phenomena like trade winds and ocean currents, cannot be duplicated in small basins like sinks.

The Most Coast
. . . Canada has 202,080 kilometers (125,567 miles) of coastline.

Short But Sweet
. . . Monaco has 4 kilometers (2.5 miles) of coastline.

International Oil Spill
The largest oil spill in history, the Gulf War oil spill, released at least 40 million gallons of oil into the Persian Gulf. Valves at the Sea Island oil terminal in Kuwait were opened on purpose after Iraq invaded Kuwait in 1991. The oil was intended to stop a landing by U.S. Marines, but the oil drifted south to the shores of Saudi Arabia.

A study of the Gulf War oil spill (conducted by the United Nations, several countries in the Middle East and the United States) found that most of the spilled oil evaporated and caused little damage to the environment.

Ocean Seas
The floors of the Caspian Sea and the Black Sea are more like the ocean than other seasthey do not rest on a continent, but directly on the ocean's basalt crust.

Early Ocean Explorers
Polynesian people navigated a region of the Pacific Ocean now known as the Polynesian Triangle by 700 CE. The corners of the Polynesian Triangle are islands: the American state of Hawaii, the country of New Zealand, and the Chilean territory of Easter Island (also known as Rapa Nui). The distance between Easter Island and New Zealand, the longest length of the Polynesian Triangle, is one-quarter of the Earth's circumference, more than 10,000 kilometers (6,200 miles). Polynesians successfully traveled these distances in canoes.

It would be hundreds of years before another culture explored the ocean to this extent.

The ocean covers 70 percent of the Earth's surface. It contains about 1.35 billion cubic kilometers (324 million cubic miles) of water, which is about 97 percent of all the water on Earth. The ocean makes all life on Earth possible, and makes the planet appear blue when viewed from space. Earth is the only planet in our solar system that is definitely known to contain liquid water.

Although the ocean is one continuous body of water, oceanographers have divided it into four principal areas: the Pacific, Atlantic, Indian, and Arctic Oceans. The Atlantic, Indian, and Pacific Oceans merge into icy waters around Antarctica. Some oceanographers define this as a fifth ocean, most commonly called the Southern Ocean.

Climate

The ocean plays a vital role in climate and weather. The sun’s heat causes water to evaporate, adding moisture to the air. The oceans provide most of this evaporated water. The water vapor condenses to form clouds, which release their moisture as rain or other kinds of precipitation. All life on Earth depends on this process, called the water cycle.

The atmosphere receives much of its heat from the ocean. As the sun warms the water, the ocean transfers heat to the atmosphere. In turn, the atmosphere distributes the heat around the globe.

Because water absorbs and loses heat more slowly than land masses, the ocean helps balance global temperatures by absorbing heat in the summer and releasing it in the winter. Without the ocean to help regulate global temperatures, the Earth’s climate would be bitterly cold.

Ocean Formation

After the Earth began to form about 4.6 billion years ago, it gradually separated into layers of lighter and heavier rock. The lighter rock rose and formed the Earth’s crust. The heavier rock sank and formed the Earth’s core and mantle.

The ocean’s water came from rocks inside the newly forming Earth. As the molten rocks cooled, they released water vapor and other gases. Eventually, the water vapor condensed and covered the crust with a primitive ocean. Today, hot gases from the Earth’s interior continue to produce new water at the bottom of the ocean.

Ocean Floor

Scientists began mapping the ocean floor in the 1920s. They used instruments called echo sounders, which measure water depths using sound waves. Echo sounders use sonar technology. Sonar is an acronym for SOund Navigation And Ranging. The sonar showed that the ocean floor has dramatic physical features, including huge mountains, deep canyons, steep cliffs, and wide plains.

The ocean’s crust is a thin layer of volcanic rock called basalt. The ocean floor is divided into several different areas. The first is the continental shelf, the nearly flat, underwater extension of a continent. Continental shelves vary in width. They are usually wide along low-lying land, and narrow along mountainous coasts.

A shelf is covered in sediment from the nearby continent. Some of the sediment is deposited by rivers and trapped by features such as natural dams. Most sediment comes from the last glacial period, or Ice Age, when the oceans receded and exposed the continental shelf. This sediment is called relict sediment.

At the outer edge of the continental shelf, the land drops off sharply in what is called the continental slope. The slope descends almost to the bottom of the ocean. Then it tapers off into a gentler slope known as the continental rise. The continental rise descends to the deep ocean floor, which is called the abyssal plain.

Abyssal plains are broad, flat areas that lie at depths of about 4,000 meters to 6,000 meters (13,123 feet to 19,680 feet). Abyssal plains cover 30 percent of the ocean floor and are the flattest feature on Earth. They are covered by fine-grained sediment like clay and silt. Pelagic sediments, the remains of small ocean organisms, also drift down from upper layers of the ocean. Scattered across abyssal plains are abyssal hills and underwater volcanic peaks called seamounts.

Rising from the abyssal plains in each major ocean is a huge chain of mostly undersea mountains. Called the mid-ocean ridge, the chain circles the Earth, stretching more than 64,000 kilometers (40,000 miles). Much of the mid-ocean ridge is split by a deep central rift, or crack. Mid-ocean ridges mark the boundaries between tectonic plates. Molten rock from the Earth’s interior wells up from the rift, building new seafloor in a process called seafloor spreading. A major portion of the ridge runs down the middle of the Atlantic Ocean and is known as the Mid-Atlantic Ridge. It was not directly seen or explored until 1973.

Some areas of the ocean floor have deep, narrow depressions called ocean trenches. They are the deepest parts of the ocean. The deepest spot of all is the Challenger Deep, which lies in the Mariana Trench in the Pacific Ocean near the island of Guam. Its true depth is not known, but the most accurate measurements put the Challenger Deep at 11,000 meters (36,198 feet) below the ocean’s surface—that’s more than 2,000 meters (6,000 feet) taller than Mount Everest, the Earth’s highest point. The pressure in the Challenger Deep is about 8 tons per square inch.

Ocean Life Zones

From the shoreline to the deepest seafloor, the ocean teems with life. The hundreds of thousands of marine species range from microscopic algae to the largest creature to have ever lived on Earth, the blue whale.

The ocean has five major life zones, each with organisms uniquely adapted to their specific marine ecosystem.

The epipelagic zone (1) is the sunlit upper layer of the ocean. It reaches from the surface to about 200 meters (660 feet) deep. The epipelagic zone is also known as the photic or euphotic zone, and can exist in lakes as well as the ocean.

The sunlight in the epipelagic zone allows photosynthesis to occur. Photosynthesis is the process by which some organisms convert sunlight and carbon dioxide into energy and oxygen. In the ocean, photosynthesis takes place in plants and algae. Plants such as seagrass are similar to land plants—they have roots, stems, and leaves. Algae is a type of aquatic organism that can photosynthesize sunlight. Large algae such as kelp are called seaweed.

Phytoplankton also live in the epipelagic zone. Phytoplankton are microscopic organisms that include plants, algae, and bacteria. They are only visible when billions of them form algal blooms, and appear as green or blue splotches in the ocean.

Phytoplankton are a basis of the ocean food web. Through photosynthesis, phytoplankton are responsible for almost half the oxygen released into the Earth’s atmosphere. Animals such as krill (a type of shrimp), fish, and microscopic organisms called zooplankton all eat phytoplankton. In turn, these animals are eaten by whales, bigger fish, ocean birds, and human beings.

The next zone down, stretching to about 1,000 meters (3,300 feet) deep, is the mesopelagic zone (2). This zone is also known as the twilight zone because the light there is very dim. The lack of sunlight means there are no plants in the mesopelagic zone, but large fish and whales dive there to hunt prey. Fish in this zone are small and luminous. One of the most common is the lanternfish, which has organs along its side that produce light.

Sometimes, animals from the mesopelagic zone (such as sperm whales and squid) dive into the bathypelagic zone (3), which reaches to about 4,000 meters (13,100 feet) deep. The bathypelagic zone is also known as the midnight zone because no light reaches it.

Animals that live in the bathypelagic zone are small, but they often have huge mouths, sharp teeth, and expandable stomachs that let them eat any food that comes along. Most of this food comes from the remains of plants and animals drifting down from upper pelagic zones. Many bathypelagic animals do not have eyes because they are unneeded in the dark. Because the pressure is so great and it is so difficult to find nutrients, fish in the bathypelagic zone move slowly and have strong gills to extract oxygen from the water.

The water at the bottom of the ocean, the abyssopelagic zone (4), is very salty and cold (2 degrees Celsius, or 35 degrees Fahrenheit). At depths up to 6,000 meters (19,700 feet), the pressure is very strong—11,000 pounds per square inch. This makes it impossible for most animals to live. Animals in this zone have bizarre adaptations to cope with their ecosystem. Many fish have jaws that look unhinged. The jaws allow them to drag their open mouth along the seafloor to find food, such as mussels, shrimp, and microscopic organisms.

Many of the animals in this zone, including squid and fish, are bioluminescent. Bioluminescent organisms produce light through chemical reactions in their bodies. A type of angler fish, for example, has a glowing growth extending in front of its huge, toothy mouth. When smaller fish are attracted to the light, the angler fish simply snaps its jaws to eat its prey.

The deepest ocean zone, found in trenches and canyons, is called the hadalpelagic zone (5). Few organisms live here. They include tiny isopods, a type of crustacean related to crabs and shrimp.

Invertebrates such as sponges and sea cucumbers thrive in the abyssopelagic and hadalpelagic zones. Like many sea stars and jellyfish, these animals are almost entirely dependent on falling parts of dead or decaying plants and animals, called marine detritus.

Not all bottom dwellers, however, depend on marine detritus. In 1977, oceanographers discovered a community of creatures on the ocean floor that feed on bacteria around openings called hydrothermal vents. These vents discharge superheated water enriched with minerals from the Earth’s interior. The minerals nourish unique bacteria, which in turn nourish creatures such as crabs, clams, and tube worms.

Ocean Currents

Currents are streams of water running through a larger body of water. Oceans, rivers, and streams have currents. The ocean’s salinity and temperature and the coast’s geographic features determine an ocean current’s behavior. The Earth’s rotation and wind also influence ocean currents. Currents flowing near the surface transport heat from the tropics to the poles and move cooler water back toward the Equator. This keeps the ocean from becoming extremely hot or cold.

Deep, cold currents transport oxygen to organisms throughout the ocean. They also carry rich supplies of nutrients that all living things need. The nutrients come from plankton and the remains of other organisms that drift down and decay on the ocean floor.

Along some coasts, winds and currents produce a phenomenon called upwelling. As winds push surface water away from shore, deep currents of cold water rise to take its place. This upwelling of deep water brings up nutrients that nourish new growth of plankton, providing food for fish. Ocean food chains constantly recycle food and energy this way.

 

Some ocean currents are enormous and extremely powerful. One of the most powerful is the Gulf Stream, a warm surface current that originates in the tropical Caribbean Sea and flows northeast along the eastern coast of the United States. The Gulf Stream measures up to 80 kilometers (50 miles) wide and is more than a kilometer (3,281 feet) deep.

Like other ocean currents, the Gulf Stream plays a major role in climate. As the current travels north, it transfers moisture from its warm tropical waters to the air above. Westerly, or prevailing, winds carry the warm, moist air to the British Isles and to Scandinavia, causing them to have milder winters than they otherwise would experience at their northern latitudes. Northern parts of Norway are near the Arctic Circle but remain ice-free for most of the year because of the Gulf Stream.

The weather pattern known as El Nino includes a change to the Humboldt Current (also called the Peru Current) off the western coast of South America. In El Niño conditions, a current of warm surface water travels east along the Equator and prevents the normal upwelling of the cold, nutrient-rich Humboldt Current. El Niño, which can devastate the fisheries of Peru and Ecuador, occurs every two to seven years, usually in December.

The paths of ocean currents are partially determined by the Earth’s rotation. This is known as the Coriolis effect. It causes large systems, such as winds and ocean currents that would normally move in a straight line, to veer to the right in the northern hemisphere and to the left in the southern hemisphere.

People and the Ocean

For thousands of years, people have depended on the ocean as a source of food and as a route for trade and exploration. Today, people continue to travel on the ocean and rely on the resources it contains.

Nations continue to negotiate how to determine the extent of their territory beyond the coast. The United Nations’ Law of the Sea treaty established exclusive economic zones (EEZs), extending 200 nautical miles (230 miles) beyond a nation’s coastline. Even though some countries have not signed or ratified the treaty (including the U.S.), it is regarded as standard.

Russia has proposed extending its EEZ beyond 200 nautical miles because two mid-ocean ridges, the Lomonosov and Medeleev Ridges, are extensions of the continental shelf belonging to Russia. This territory includes the North Pole. Russian explorers in a submersible vehicle planted a metal Russian flag on the disputed territory in 2007.

Through the centuries, people have sailed the ocean on trade routes. Today, ships still carry most of the world’s freight, particularly bulky goods such as machinery, grain, and oil.

Ocean ports are areas of commerce and culture. Water and land transportation meet there, and so do people of different professions: businesspeople who import and export goods and services; dockworkers who load and unload cargo; and ships’ crews. Ports also have a high concentration of migrants and immigrants with a wide variety of ethnicities, nationalities, languages, and religions.

Important ports in the U.S. are New York/New Jersey and New Orleans. The busiest ports around the world include the Port of Shanghai in China and the Port of Rotterdam in the Netherlands. Ocean ports are also important for a nation’s armed forces. Some ports are used exclusively for military purposes, although most share space with commercial businesses. “The sun never sets on the British Empire” is a phrase used to explain the scope of the empire of Great Britain, mostly in the 19th century. Although based on the small European island nation of Great Britain, British military sea power extended its empire from Africa to the Americas, Asia and Australia.

Scientists and other experts hope the ocean will be used more widely as a source of renewable energy. Some countries have already harnessed the energy of ocean waves, temperature, currents, or tides to power turbines and generate electricity.

One source of renewable energy are generators that are powered by tidal streams or ocean currents. They convert the movement of currents into energy. Ocean current generators have not been developed on a large scale, but are working in some places in Ireland and Norway. Some conservationists criticize the impact the large constructions have on the marine environment.

Another source of renewable energy is ocean thermal energy conversion (OTEC). It uses the difference in temperature between the warm surface water and cold deep water to run an engine. OTEC facilities exist in places with significant differences in ocean depth: Japan, India and the U.S. state of Hawaii, for instance.

An emerging source of renewable energy is salinity gradient power, also known as osmotic power. It is an energy source that uses the power of fresh water entering into salt water. This technology is still being developed, but it has potential in delta areas where fresh river water is constantly interacting with the ocean.

Fishing

Fishers catch more than 90 million tons of seafood each year, including more than 100 species of fish and shellfish. Millions of people, from professional fishers to business owners like restaurant owners and boat builders, depend on fisheries for their livelihood. Fishing can be classified in two ways. In subsistence fishing, fishers use their catch to help meet the nutritional needs of their families or communities. In commercial fishing, fishers sell their catch for money, goods or services. Popular subsistence and commercial fish are tuna, cod, and shrimp.

Ocean fishing is also a popular recreational sport. Sport fishing can be competitive or noncompetitive. In sport fishing tournaments, individuals or teams compete for prizes based on the size of a particular species caught in a specific time period. Both competitive and noncompetitive sport fishers need licenses to fish, and may or may not keep the caught fish. Increasingly, sport fishers practice catch-and-release fishing, where a fish is caught, measured, weighed, and often recorded on film before being released back to the ocean. Popular game fish (fish caught for sport) are tuna and marlin.

Whaling is a type of fishing that involves the harvesting of whales and dolphins. It has declined in popularity since the 19th century but is still a way of life for many cultures, such as those in Scandinavia, Japan, Canada, and the Caribbean.

The ocean offers a wealth of fishing and whaling resources, but these resources are threatened. People have harvested so much fish and marine life for food and other products that some species have disappeared.

During the 1800s and early 1900s, whalers killed thousands of whales for whale oil (wax made from boiled blubber) and ivory (whales’ teeth). Some species, including the blue whale and the right whale, were hunted nearly to extinction. Many species are still endangered today.

In the 1960s and 1970s, catches of important food fish, such as herring in the North Sea and anchovies in the Pacific, began to drop off dramatically. Governments took notice of overfishing—harvesting more fish than the ecosystem can replenish. Fishers were forced to go farther out to sea to find fish, putting them at risk. (Deep-sea fishing is one of the most dangerous jobs in the world.) Now, they use advanced equipment, such as electronic fish finders and large gill nets or trawling nets, to catch more fish. This means there are far fewer fish to reproduce and replenish the supply.

In 1992, the collapse, or disappearance, of cod in Canada’s Newfoundland Grand Banks put 40,000 fishers out of work. A ban was placed on cod fishing, and to this day, neither the cod nor the fisheries have recovered.

To catch the dwindling numbers of fish, most fishers use trawl nets. They drag the nets along the seabed and across acres of ocean. These nets accidentally catch many small, young fish and mammals. Animals caught in fishing nets meant for other species are called bycatch. The fishing industry and fisheries management agencies argue about how to address the problem of bycatch and overfishing. Those involved in the fishing industry do not want to lose their jobs, while conservationists want to maintain healthy levels of fish in the ocean.

A number of consumers are choosing to purchase sustainable seafood. Sustainable seafood is harvested from sources (either wild or farmed) that do not deplete the natural ecosystem.

Mining and Drilling

Many minerals come from the ocean. Sea salt is a mineral that has been used as a flavoring and preservative since ancient times. Sea salt has many additional minerals, such as calcium, that ordinary table salt lacks.

Hydrothermal vents often form seafloor massive sulfide (SMS) deposits, which contain precious metals. These SMS deposits sit on the ocean floor, sometimes in the deep ocean and sometimes closer to the surface. New techniques are being developed to mine the seafloor for valuable minerals such as copper, lead, nickel, gold, and silver. Mining companies employ thousands of people and provide goods and services for millions more.

Critics of undersea mining maintain that it disrupts the local ecology. Organisms—corals, shrimp, mussels—that live on the seabed have their habitat disturbed, upsetting the food chain. In addition, destruction of habitat threatens the viability of species that have a narrow niche. Maui’s dolphin, for instance, is a critically endangered species native to the waters of New Zealand’s North Island. The numbers of Maui’s dolphin are already reduced because of bycatch. Seabed mining threatens its habitat, putting it at further risk of extinction.

Oil is one of the most valuable resources taken from the ocean today. Offshore oil rigs pump petroleum from wells drilled into the continental shelf. About one-quarter of all oil and natural gas supplies now comes from offshore oil deposits around the world.

Offshore drilling requires complex engineering. An oil platform can be constructed directly onto the ocean floor, or it can “float” above an anchor. Depending on how far out on the continental shelf an oil platform is located, workers may have to be flown in. Underwater, or subsea, facilities are complicated groups of drilling equipment connected to each other and a single oil rig. Subsea production often requires remotely operated underwater vehicles (ROVs).

Some countries invest in offshore drilling for profit and to prevent reliance on oil from other regions. The Gulf of Mexico near the U.S. states of Texas and Louisiana is heavily drilled. Several European countries, including the United Kingdom, Denmark, and the Netherlands, drill in the North Sea. Offshore drilling is a complicated and expensive program, however. There are a limited number of companies that have the knowledge and resources to work with local governments to set up offshore oil rigs. Most of these companies are based in Europe and North America, although they do business all over the world.

Some governments have banned offshore oil drilling. They cite safety and environmental concerns. There have been several accidents where the platform itself has exploded, at the cost of many lives. Offshore drilling also poses threats to the ocean ecosystem. Spills and leaks from oil rigs and oil tankers that transport the material seriously harm marine mammals and birds. Oil coats feathers, impairing birds’ ability to maintain their body temperature and remain buoyant in the water. The fur of otters and seals are also coated, and oil entering the digestive tract of animals may damage their organs.

Offshore oil rigs also release metal cuttings, minute amounts of oil, and drilling fluid into the ocean every day. Drilling fluid is the liquid used with machinery to drill holes deep in the Earth. This liquid can contain pollutants such as toxic chemicals and heavy metals.

Pollution

Most oil pollution does not come from oil spills, however. It comes from the runoff of pollutants into streams and rivers that flow into the ocean. Most runoff comes from individual consumers. Cars, buses, motorcycles, and even lawn mowers spill oil and grease on roads, streets, and highways. (Runoff is what makes busy roads shiny and sometimes slippery.) Storm drains or creeks wash the runoff into local waterways, which eventually flow into the ocean.

The largest U.S. oil spill in the ocean took place in Alaska in 1989, by the tanker Exxon Valdez. The Exxon Valdez spilled at least 10 million gallons of oil into Prince William Sound. In comparison, American and Canadian consumers spill about 16 million gallons of oil runoff into the Atlantic and Pacific Oceans every year.

For centuries, people have used the ocean as a dumping ground for sewage and other wastes.

In the 21st century, the wastes include not only oil, but also chemical runoff from factories and agriculture. These chemicals include nitrates and phosphates, which are often used as fertilizers. These chemicals encourage algae blooms. An algae bloom is an increase in algae and bacteria that threatens plants and other marine life. Algae blooms limit the amount of oxygen in a marine environment, leading to what are known as dead zones, where little life exists beneath the ocean’s surface. Algae blooms can spread across hundreds or even thousands of miles.

Another source of pollution is plastics. Most ocean debris, or garbage, is plastic thrown out by consumers. Plastics such as water bottles, bags, six-pack rings, and packing material put marine life at risk. Sea animals are harmed by the plastic either by getting tangled in it or by eating it.

An example of marine pollution consisting mainly of plastics is the Great Pacific Garbage Patch. The Great Pacific Garbage Patch is a floating dump in the North Pacific Ocean. It’s about twice the size of Texas and probably contains about 100 million tons of debris. Most of this debris comes from the western coast of North America (the U.S. and Canada) and the eastern coast of Asia (Japan, China, Russia, North Korea, and South Korea). Because of ocean currents and weather patterns, the patch is a relatively stable formation and contains new and disintegrating debris. The smaller pieces of plastic debris are eaten by jellyfish or other organisms, and are then consumed by larger predators in the food web. These plastic chemicals may then enter a human’s diet through fish or shellfish.

Another source of pollution is carbon dioxide. The ocean absorbs most carbon dioxide from the atmosphere. Carbon dioxide, which is necessary for life, is known as a greenhouse gas and traps radiation in the Earth’s atmosphere. Carbon dioxide forms many acids, called carbonic acids, in the ocean. Ocean ecosystems have adapted to the presence of certain levels of carbonic acids, but the increase in carbon dioxide has led to an increase in ocean acids. This ocean acidification erodes the shells of animals such as clams, crabs, and corals.

Global Warming

Global warming contributes to rising ocean temperatures and sea levels.

Warmer oceans radically alter the ecosystem. Global warming causes cold-water habitats to shrink, meaning there is less room for animals such as penguins, seals, or whales. Plankton, the base of the ocean food chain, thrives in cold water. Warming water means there will be less plankton available for marine life to eat.

Melting glaciers and ice sheets contribute to sea level rise. Rising sea levels threaten coastal ecosystems and property. River deltas and estuaries are put at risk for flooding. Coasts are more likely to suffer erosion. Seawater more often contaminates sources of fresh water. All these consequences—flooding, erosion, water contamination—put low-lying island nations, such as the Maldives in the Indian Ocean, at high risk for disaster.

To find ways to protect the ocean from pollution and the effects of climate change, scientists from all over the world are cooperating in studies of ocean waters and marine life. They are also working together to control pollution and limit global warming. Many countries are working to reach agreements on how to manage and harvest ocean resources.

Although the ocean is vast, it is more easily polluted and damaged than people once thought. It requires care and protection as well as expert management. Only then can it continue to provide the many resources that living things—including people—need.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

absorb

Verb

to soak up.

abyssal plain

Noun

extensive, featureless region of the deep ocean floor.

abyssopelagic zone

Noun

zone of the open ocean, starting at 3,962 meters (13,000 feet) below sea level.

adaptation

Noun

a modification of an organism or its parts that makes it more fit for existence. An adaptation is passed from generation to generation.

Encyclopedic Entry: adaptation

agriculture

Noun

the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

Encyclopedic Entry: agriculture

algae

Plural Noun

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

algal bloom

Noun

the rapid increase of algae in an aquatic environment.

atmosphere

Noun

layers of gases surrounding a planet or other celestial body.

Encyclopedic Entry: atmosphere

basalt

Noun

type of dark volcanic rock.

bathypelagic zone

Noun

zone of the open ocean, starting at 914 meters (3,000 feet). Also known as the midnight or aphotic zone.

bioluminescence

Noun

light emitted by living things through chemical reactions in their bodies.

Encyclopedic Entry: bioluminescence

blubber

Noun

thick layer of fat under the skin of marine mammals.

Encyclopedic Entry: blubber

buoyant

Adjective

capable of floating.

bycatch

Noun

fish or any other organisms accidentally caught in fishing gear.

canyon

Noun

deep, narrow valley with steep sides.

Encyclopedic Entry: canyon

carbon dioxide

Noun

greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

carbonic acid

Noun

chemical produced as carbon dioxide dissolves in water.

cargo

Noun

goods carried by a ship, plane, or other vehicle.

catch-and-release

Adjective

method of fishing where the fish is measured and sometimes tagged before being returned to its habitat.

Challenger Deep

Noun

deepest measured point in the ocean (part of the Mariana Trench), about 11,000 meters (36,198 feet), located in the South Pacific Ocean.

cliff

Noun

steep wall of rock, earth, or ice.

Encyclopedic Entry: cliff

climate

Noun

all weather conditions for a given location over a period of time.

Encyclopedic Entry: climate

commerce

Noun

trade, or the exchange of goods and services.

commercial

Adjective

having to do with the buying and selling of goods and services.

commercial fishing

Noun

industry responsible for catching and selling fish.

condense

Verb

to turn from gas to liquid.

conservation

Noun

management of a natural resource to prevent exploitation, destruction, or neglect.

Encyclopedic Entry: conservation

consumer

Noun

person who uses a good or service.

continental shelf

Noun

part of a continent that extends underwater to the deep-ocean floor.

Encyclopedic Entry: continental shelf

continental slope

Noun

the sometimes-steep descent of a continental shelf to the ocean floor.

core

Noun

the extremely hot center of Earth, another planet, or a star.

Encyclopedic Entry: core

Coriolis effect

Noun

the result of Earth's rotation on weather patterns and ocean currents. The Coriolis effect makes storms swirl clockwise in the Southern hemisphere and counterclockwise in the Northern Hemisphere.

Encyclopedic Entry: Coriolis effect

crust

Noun

rocky outermost layer of Earth or other planet.

Encyclopedic Entry: crust

crustacean

Noun

type of animal (an arthropod) with a hard shell and segmented body that usually lives in the water.

current

Noun

steady, predictable flow of fluid within a larger body of that fluid.

Encyclopedic Entry: current

dead zone

Noun

area of low oxygen in a body of water.

Encyclopedic Entry: dead zone

decay

Verb

to rot or decompose.

delta

Noun

the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments.

Encyclopedic Entry: delta

digestive tract

Noun

the stomach and the intestines, organs in the digestive system responsible for breaking down food into nutrients and waste products. Also called the gastrointestinal or GI tract.

disintegrate

Verb

to fall apart and disappear.

distribute

Verb

to divide and spread out materials.

drilling fluid

Noun

chemical liquid used with machinery to make deep holes in the Earth.

Earth

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

Encyclopedic Entry: Earth

echo sounder

Noun

device that measures the depth of water using sound pulses. Also called a sonic depth finder.

ecology

Noun

branch of biology that studies the relationship between living organisms and their environment.

Encyclopedic Entry: ecology

ecosystem

Noun

community and interactions of living and nonliving things in an area.

Encyclopedic Entry: ecosystem

El Nino

Noun

irregular, recurring weather system that features a warm, eastern-flowing ocean current in the eastern Pacific Ocean.

Encyclopedic Entry: El Niño

endanger

Verb

to put at risk.

engineering

Noun

the art and science of building, maintaining, moving, and demolishing structures.

enormous

Adjective

very large.

enrich

Verb

to supply with valuable material.

epipelagic zone

Noun

upper zone of the ocean. This zone goes down to approximately 183 meters (600 feet). Also called the euphotic or sunlit zone.

Equator

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

Encyclopedic Entry: equator

erosion

Noun

act in which earth is worn away, often by water, wind, or ice.

Encyclopedic Entry: erosion

evaporate

Verb

to change from a liquid to a gas or vapor.

exclusive economic zone (EEZ)

Noun

zone extending 200 nautical miles off a country's coast. A country has the right to explore and exploit the living and nonliving things in its EEZ.

expand

Verb

to grow.

exploration

Noun

study and investigation of unknown places.

export

Verb

to transport goods to another place for trade.

extinction

Noun

process of complete disappearance of a species from Earth.

Exxon Valdez

Noun

oil tanker that hit a reef in Alaska's Prince William Sound and spilled 40.9 million liters (10.8 million gallons) of oil into the Pacific Ocean in 1989.

fertilizer

Noun

nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.

fishery

Noun

industry or occupation of harvesting fish, either in the wild or through aquaculture.

food chain

Noun

group of organisms linked in order of the food they eat, from producers to consumers, and from prey, predators, scavengers, and decomposers.

Encyclopedic Entry: food chain

food web

Noun

all related food chains in an ecosystem. Also called a food cycle.

Encyclopedic Entry: food web

freight

Noun

goods transported by air, land, or sea for profit.

game fish

Noun

species of fish caught for competition or recreation.

gill net

Noun

effective type of fishing net that is suspended vertically in water by weights and floats. Fish are caught as they try to swim through.

glacial period

Noun

time of long-term lowering of temperatures on Earth. Also known as an ice age.

glacier

Noun

mass of ice that moves slowly over land.

Encyclopedic Entry: glacier

global warming

Noun

increase in the average temperature of the Earth's air and oceans.

Encyclopedic Entry: global warming

Great Britain

Noun

large island in Western Europe consisting of the countries of England, Scotland, and Wales.

Great Pacific Garbage Patch

Noun

area of the North Pacific Ocean where currents have trapped huge amounts of debris, mostly plastics.

Encyclopedic Entry: Great Pacific Garbage Patch

greenhouse gas

Noun

gas in the atmosphere, such as carbon dioxide, methane, water vapor, and ozone, that absorbs solar heat reflected by the surface of the Earth, warming the atmosphere.

Gulf Stream

Noun

warm current that starts in the Gulf of Mexico and travels along the eastern coast of the U.S. and Canada before crossing the North Atlantic Ocean.

habitat

Noun

environment where an organism lives throughout the year or for shorter periods of time.

Encyclopedic Entry: habitat

hadalpelagic zone

Noun

deepest zone of the open ocean, starting at around 6,000 meters (20,000 feet).

harness

Verb

to control or guide for a specific purpose.

harvest

Noun

the gathering and collection of crops, including both plants and animals.

heavy metal

Noun

chemical substance with a specific gravity of at least 5.0.

hydrothermal

Adjective

related to hot water, especially water heated by the Earth's internal temperature.

ice sheet

Noun

thick layer of glacial ice that covers a large area of land.

Encyclopedic Entry: ice sheet

import

Noun

good traded from another area.

invertebrate

Noun

animal without a spine.

isopod

Noun

type of bug (crustacean).

ivory

Noun

hard, white substance that forms the teeth or tusks of some animals.

krill

Noun

small marine crustacean, similar to shrimp.

latitude

Noun

distance north or south of the Equator, measured in degrees.

Encyclopedic Entry: latitude

liquid

Noun

state of matter with no fixed shape and molecules that remain loosely bound with each other.

livelihood

Noun

ability to economically support oneself.

luminous

Adjective

glowing or radiant.

mantle

Noun

middle layer of the Earth, made of mostly solid rock.

Encyclopedic Entry: mantle

Mariana Trench

Noun

deepest place on Earth, located in the South Pacific Ocean at 11,000 meters (36,198 feet) at its deepest.

marine

Adjective

having to do with the ocean.

marine detritus

Noun

parts of dead or decaying plants or animals that drift to the seafloor.

mesopelagic zone

Noun

zone of the open ocean, extending from about 183 to 914 meters (600 to 3,000 feet). Also known as the twilight or dysphotic zone.

microscopic

Adjective

very small.

midnight zone

Noun

zone of the open ocean, starting at about 914 meters (3,000 feet) deep. Also known as the bathypelagic or aphotic zone.

mid-ocean ridge

Noun

underwater mountain range.

military

Noun

armed forces.

mineral

Noun

inorganic material that has a characteristic chemical composition and specific crystal structure.

molten

Adjective

solid material turned to liquid by heat.

nautical mile

Noun

unit of distance for sea or air travel, equal to 1,852 meters (6,076 feet).

niche

Noun

role and space of a species within an ecosystem.

nitrate

Noun

type of salt used as fertilizer. Excess nitrates can choke freshwater ecosystems.

Northern Hemisphere

Noun

half of the Earth between the North Pole and the Equator.

nutrient

Noun

substance an organism needs for energy, growth, and life.

Encyclopedic Entry: nutrient

ocean acidification

Noun

decrease in the ocean's pH levels, caused primarily by increased carbon dioxide. Ocean acidification threatens corals and shellfish.

oceanographer

Noun

person who studies the ocean.

ocean thermal energy conversion

Noun

method of generating power by using the difference in temperature between the ocean's deep (cold) and the surface (warm) water.

offshore

Adjective

having to do with facilities or resources located underwater, usually miles from the coast.

oil

Noun

fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.

oil rig

Noun

complex series of machinery and systems used to drill for oil on land.

overfish

Verb

to harvest aquatic life to the point where species become rare in the area.

oxygen

Noun

chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

pelagic

Adjective

having to do with the open ocean.

petroleum

Noun

fossil fuel formed from the remains of ancient organisms. Also called crude oil.

phosphate

Noun

type of salt used as fertilizer. Excess phosphates can choke freshwater ecosystems.

photosynthesis

Noun

process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

phytoplankton

Noun

microscopic organism that lives in the ocean and can produce its own food through photosynthesis.

plain

Noun

flat, smooth area at a low elevation.

Encyclopedic Entry: plain

plastic

Noun

chemical material that can be easily shaped when heated to a high temperature.

plate tectonics

Noun

movement and interaction of the Earth's plates.

pole

Noun

extreme north or south point of the Earth's axis.

pollutant

Noun

chemical or other substance that harms a natural resource.

pollution

Noun

introduction of harmful materials into the environment.

Encyclopedic Entry: pollution

port

Noun

place on a body of water where ships can tie up or dock and load and unload cargo.

Encyclopedic Entry: port

precipitation

Noun

all forms in which water falls to Earth from the atmosphere.

Encyclopedic Entry: precipitation

predator

Noun

animal that hunts other animals for food.

pressure

Noun

force pressed on an object by another object or condition, such as gravity.

prey

Noun

animal that is hunted and eaten by other animals.

primitive

Adjective

simple or crude.

radiation

Noun

energy, emitted as waves or particles, radiating outward from a source.

recede

Verb

to retreat or withdraw.

recycle

Verb

to clean or process in order to make suitable for reuse.

regulate

Verb

to determine and administer a set of rules for an activity.

relict sediment

Noun

material on the continental shelf deposited during the last glacial period, when sea levels were lower.

renewable energy

Noun

energy obtained from sources that are virtually inexhaustible and replenish naturally over small time scales relative to the human life span.

replenish

Verb

to supply or refill.

rift

Noun

break in the Earth's crust created by it spreading or splitting apart.

rotation

Noun

object's complete turn around its own axis.

Encyclopedic Entry: rotation

route

Noun

path or way.

runoff

Noun

overflow of fluid from a farm or industrial factory.

Encyclopedic Entry: runoff

salinity

Noun

saltiness.

salinity gradient power

Noun

energy produced as freshwater interacts with salty ocean water.

Scandinavia

Noun

region and name for some countries in Northern Europe: Iceland, Norway, Sweden, Finland, and Denmark.

seafloor massive sulfide (SMS) deposit

Noun

minerals that form from underwater hydrothermal vents.

seafloor spreading

Noun

rift in underwater mountain range where new oceanic crust is formed.

Encyclopedic Entry: seafloor spreading

sea level

Noun

base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

Encyclopedic Entry: sea level

sea level rise

Noun

increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year.

sea salt

Noun

salt obtained from the evaporation of seawater.

seaweed

Noun

marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria.

sediment

Noun

solid material transported and deposited by water, ice, and wind.

Encyclopedic Entry: sediment

sewage

Noun

liquid and solid waste material from homes and businesses.

shellfish

Noun

any aquatic animal that has a shell.

solar system

Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

sonar

Noun

method of determining the presence and location of an object using sound waves (echolocation).

sound wave

Noun

wave of air pressure producing sound.

Southern Hemisphere

Noun

half of the Earth between the South Pole and the Equator.

sport fishing

Noun

catching fish for competition or recreation.

storm drain

Noun

system to empty streets of excess rainwater. Storm drains flow into local creeks, rivers, or seas.

submersible

Noun

small submarine used for research and exploration.

sustainable seafood

Noun

fish, shellfish, and other aquatic organisms harvested from fish farms or fisheries that can be maintained without damaging the ecosystem.

tectonic plate

Noun

large, moveable segment of the Earth's crust.

teem

Verb

to overflow or be full of.

temperature

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

Encyclopedic Entry: temperature

territory

Noun

land an animal, human, or government protects from intruders.

tidal stream

Noun

an ocean current produced by the tide.

toxic

Adjective

poisonous.

trade

Noun

buying, selling, or exchanging of goods and services.

trade route

Noun

path followed by merchants or explorers to exchange goods and services.

trawl

Verb

to fish by dragging a large net along the bottom of the body of water.

treaty

Noun

official agreement between groups of people.

trench

Noun

long, deep depression, either natural or man-made.

tropics

Noun

region generally located between the Tropic of Cancer (23 1/2 degrees north of the Equator) and the Tropic of Capricorn (23 1/2 degrees south of the Equator).

Encyclopedic Entry: tropics

twilight zone

Noun

middle zone of the open ocean. On average, this zone extends from about 200 to 1,000 meters (660 to 3,300 feet) deep. Also known as the dysphotic or mesopelagic zone.

unique

Adjective

one of a kind.

upwelling

Noun

process by which currents bring cold, nutrient-rich water to the ocean surface.

Encyclopedic Entry: upwelling

vapor

Noun

visible liquid suspended in the air, such as fog.

water cycle

Noun

movement of water between atmosphere, land, and ocean.

Encyclopedic Entry: water cycle

weather

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

Encyclopedic Entry: weather

whaling

Noun

industry of hunting whales.

zooplankton

noun, plural noun

microscopic organism that lives in the ocean.

Credits

Media Credits

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Writers

Joe Jaszewski
Hilary Costa
Andrew Turgeon
Tara Ramroop
Santani Teng
Audrey Carangelo
Melissa McDaniel
Erin Sprout
Diane Boudreau

Illustrators

Mary Crooks, National Geographic Society
Dinara Sagatova
Tim Gunther, Illustrator

Editors

Kara West
Jeff Hunt
Kim Rutledge
Jeannie Evers

Educator Reviewer

Nancy Wynne

Expert Reviewer

Sarah Wilson, National Geographic Society

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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