Childrens Illustrated Encyclopedia

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Oceans and seas

388

OceanOgraphy

Our knowledge of the oceans comes from

oceanographers, who study the oceans. They

sail in special ships with instruments that take

samples of the water and mud on the seabed,

chart ocean currents, and map the ridges and

trenches in the ocean floor. The scientists also

dive in submersibles and use underwater

robots to see the strange creatures that live in

the depths. Satellites look down from space

and send back information about the oceans.

The continental shelf

ends in a cliff called

the continental slope.

Under The OceanS

a strange landscape lies hidden

beneath the oceans. There are huge

cliffs, great ranges of mountains, and deep

chasms, all far larger than any on land. Much of

the ocean floor is a vast flat plain that lies up to

4 miles (6 km) below the surface. Trenches descend

as deep as 7 miles (11 km), more than the height of

the highest mountain on land. Undersea mountains and

volcanoes rise from the plain, many poking their summits

above the waves to form islands. The seas around the shores

of most continents are not very deep. Most offshore islands

are high land rising from the shallow seabed. coral reefs

and atolls grow up from the seabed in warm seas.

Undersea

mountains rise

from the seabed.

WaveS

The surface of the sea is restless, even

on the calmest day. Waves ceaselessly rise

and fall, eventually reaching the land to lap

or crash on the shore. Waves are caused by winds

blowing over the ocean. The energy from

waves can be used to power generators

and produce electricity. however,

tsunamis, huge waves that can

reach heights of 100 ft (30 m),

are very powerful and

destructive. Sometimes

wrongly referred to as

tidal waves, tsunamis

are in fact caused by

earthquakes and

volcanic eruptions.

hOW WaveS MOve

The water in a wave does not move forward.

It moves in a circle, so the water only goes up and

down as a wave passes. The approaching shore holds

back the base of the wave, making the top of the wave

move faster to break on the shore.

Long, wide ocean

ridges run through

most oceans.

Crest topples over

to break on shore.

Long, deep trenches

lie near the edges

of some oceans.

Many

continents

extend out

into the ocean

and have a wide

undersea continental

shelf which is about

400 ft (130 m) deep.

Water reaches base of

circle in trough of wave.

Water reaches top of

circle in crest of wave.

ShOreS and cOaSTS

high land at the shore ends in cliffs, and low land

slopes gently to form beaches. The waves hurl

stones at the base of cliffs, causing rocks to fall

and form coasts with bays and headlands. Strange

rock formations and caves may result. The waves

batter the rocks and break them up into pebbles

and then into sand. Beaches form at the base of

cliffs, and the sea also sweeps pebbles and sand

along the shore to

form beaches

elsewhere.

Large offshore islands

rise from the ocean floor

or continental shelf.

Some volcanoes rise

from the deep ocean

floor to form islands.

Find out more

continents

deep-sea wildlife

earthquakes

Fishing industry

Indian ocean

Ocean wildlife 389-

Seashore wildlife

US_388_Oceans_Seas_2.indd 388 22/01/16 4:45 pm