Encyclopedic Entry

The legal definition of a continental shelf is different than the geographic one. According to the UN Convention on the Law of the Sea, every nation has a continental shelf extending no more than 200 nautical miles from the nation's coastline.

Map by Sean O'Connor

Oil on the Shelf
A lot of fuel we use is collected from beneath the continental shelves. For example, 30 percent of all the oil and 20 percent of the natural gas produced in the U.S. comes from offshore drilling. Most of these sites are on the North American continental shelf off of Alaska and the Gulf of Mexico.

A continental shelf is the edge of a continent that lies under the ocean. Continents are the seven main divisions of land on Earth. A continental shelf extends from the coastline of a continent to a drop-off point called the shelf break. From the break, the shelf descends toward the deep ocean floor in what is called the continental slope.

Even though they are underwater, continental shelves are part of the continent. The actual boundary of a continent is not its coastline, but the edge of the continental shelf. The widths of the continental shelves vary. Along parts of the U.S. state of California, for example, the continental shelf extends less than a kilometer (.62 miles). But along the northern coast of Siberia, the shelf extends about 1,290 kilometers (800 miles). The average width of a continental shelf is 65 kilometers (40 miles).

Most continental shelves are broad, gently sloping plains covered by relatively shallow water. Water depth over the continental shelves averages about 60 meters (200 feet). Sunlight penetrates the shallow waters, and many kinds of organisms flourish—from microscopic shrimp to giant seaweed called kelp. Ocean currents and runoff from rivers bring nutrients to organisms that live on continental shelves.

Plants and algae make continental shelves rich feeding grounds for sea creatures. The shelves make up less than 10 percent of the total area of the oceans. Yet all of the ocean’s plants and many types of algae live in the sunny waters.

In some places, deep canyons and channels cut through the continental shelves. Little light penetrates these submarine canyons, and they are sometimes the least-explored areas of continents. Often, submarine canyons are formed near the mouths of rivers. Strong river currents cut deeply into the soft material of the continental shelf, just like they erode rocks above ground. The Congo Canyon, extending from the mouth of the Congo River, is 800 kilometers (497 miles) long and 1,200 meters (3,900 feet) deep. The Congo Canyon is part of Africa.

Formation of a Continental Shelf

Over many millions of years, organic and inorganic materials formed continental shelves. Inorganic material built up as rivers carried sediment—bits of rock, soil, and gravel—to the edges of the continents and into the ocean. These sediments gradually accumulated in layers at the edges of continents. Organic material, such as the remains of plants and animals, also accumulated.

Many continental shelves were once dry land. Some 18,000 years ago, at the peak of the most recent ice age, much of the Earth’s water was frozen into huge masses of ice called glaciers. The sea level dropped, exposing continental shelves. During this glacial period, scientists say that sea levels were perhaps 100 meters (330 feet) lower than they are today.


The continental shelves between North America and Asia were probably exposed during the Ice Age. Some scientists say that the shelves provided a “land bridge” between the two continents. People may have used this land bridge—now the Bering Strait—to migrate from Siberia to what is now Alaska, becoming the first human beings in North America.

Biologists have also found the remains of land-based plants and animals on shelves that are now underwater. For example, scientists have discovered 11,000-year-old mastodon teeth and spruce pollen off the coast of the northeastern United States. Scientific instruments can show that the mastodon and pollen lived during the time of the last ice age.

When the shelves were above water, glaciers moved over them and changed their surfaces. As huge alpine glaciers moved quickly downhill, they gouged deep, narrow valleys. Now, the valleys are filled with seawater. These narrow, flooded valleys that descend into the continental shelf are known as fjords.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

accumulate

Verb

to gather or collect.

algae

Plural Noun

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

alpine glacier

Noun

mass of ice that moves downward from a mountain.

boundary

Noun

line separating geographical areas.

Encyclopedic Entry: boundary

canyon

Noun

deep, narrow valley with steep sides.

Encyclopedic Entry: canyon

channel

Noun

deepest part of a shallow body of water, often a passageway for ships.

continent

Noun

one of the seven main land masses on Earth.

Encyclopedic Entry: continent

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.

current

Noun

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

Encyclopedic Entry: current

descend

Verb

to go from a higher to a lower place.

erode

Verb

to wear away.

feeding ground

Noun

region where organisms go to eat.

fjord

Noun

long, narrow ocean inlet between steep slopes.

Encyclopedic Entry: fjord

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

gravel

Noun

small stones or pebbles.

ice age

Noun

long period of cold climate where glaciers cover large parts of the Earth. The last ice age peaked about 20,000 years ago. Also called glacial age.

inorganic

Adjective

composed of material that is not living, and never was, such as rock.

kelp

Noun

type of seaweed.

land bridge

Noun

thin strip of land that connects two land masses and may be submerged by water periodically.

mastodon

Noun

one of many extinct species of large animals related to elephants. The last mastodons became extinct about 11,000 years ago.

microscopic

Adjective

very small.

mouth

Noun

place where a river empties its water. Usually rivers enter another body of water at their mouths.

Encyclopedic Entry: mouth

nutrient

Noun

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

Encyclopedic Entry: nutrient

ocean

Noun

large body of salt water that covers most of the Earth.

Encyclopedic Entry: ocean

organic

Adjective

composed of living or once-living material.

penetrate

Verb

to push through.

plain

Noun

flat, smooth area at a low elevation.

Encyclopedic Entry: plain

plant

Noun

organism that produces its own food through photosynthesis and whose cells have walls.

pollen

Noun

powdery material produced by plants.

remains

Noun

materials left from a dead or absent organism.

rock

Noun

natural substance composed of solid mineral matter.

runoff

Noun

overflow of fluid from a farm or industrial factory.

Encyclopedic Entry: runoff

sea level

Noun

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

Encyclopedic Entry: sea level

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

shelf break

Noun

underwater edge of a continental shelf, where it begins a rapid slope to the deep ocean floor.

shoreline

Noun

beach, or where a body of water meets land.

Siberia

Noun

region of land stretching across Russia from the Ural Mountains to the Pacific Ocean.

soil

Noun

top layer of the Earth's surface where plants can grow.

spruce

Noun

coniferous, or cone-bearing, tree.

strait

Noun

narrow passage of water that connects two larger bodies of water.

Encyclopedic Entry: strait

submarine canyon

Noun

underwater valley formed by eroding streams of muddy water through which sediment ultimately reaches and spreads across the flat abyssal plains of the ocean floor.

valley

Noun

depression in the Earth between hills.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writers

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrators

Tim Gunther
Mary Crooks, National Geographic Society

Editors

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

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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