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.
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.
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.
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.
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.
to wear away.
feeding ground Noun
region where organisms go to eat.
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.
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.
composed of material that is not living, and never was, such as rock.
type of seaweed.
land bridge Noun
thin strip of land that connects two land masses and may be submerged by water periodically.
one of many extinct species of large animals related to elephants. The last mastodons became extinct about 11,000 years ago.
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.
to push through.
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.
powdery material produced by plants, each grain of which contains a male gamete capable of fertilizing a female ovule.
materials left from a dead or absent organism.
natural substance composed of solid mineral matter.
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.
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.
beach, or where a body of water meets land.
region of land stretching across Russia from the Ural Mountains to the Pacific Ocean.
top layer of the Earth's surface where plants can grow.
coniferous, or cone-bearing, tree.
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.
depression in the Earth between hills.