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Earth^41

The ocean floor has landscapes as dramatic and varied as those of

the Earth’s continents. Some parts have deep chasms, or towering

cliffs, or volcanoes. Other places are vast, featureless plains. Many

features of the ocean floor are caused by movements of the

tectonic plates that form the Earth’s crust.

WHAT IS A TSUNAMI?

Tsunamis are giant waves usually caused by undersea

earthquakes or volcanic eruptions. Far out to sea,

tsunamis are not so noticeable. However, as they

reach land, they can tower up to 75 m (250 ft) high.

Giant tsunamis have smashed ports and even drowned

whole islands. Tsunamis are sometimes wrongly called

“tidal waves”, but they are not caused by tides.

The surface of the sea is never completely still, even in calm

weather. Winds ruffle the surface to form ripples. If the wind keeps

blowing strongly, the ripples grow into waves. As the waves

approach land, their size and strength increases until they break

onto the shore, to build up beaches or wear away coasts.

ARE THE OCEANS EXPANDING?

Some of the oceans are expanding, as molten rock wells

up at the edges of tectonic plates to make new crust

(a process known as sea spreading). In the Atlantic

Ocean, new crust is forming along the Mid-Atlantic

Ridge, which runs down the ocean’s centre. The Atlantic

is growing about 2.5 cm (1 in) wider each year.

2 FEATURES OF THE OCEAN FLOOR

The vast, flat plain at the bottom

of the oceans is dotted with

isolated mountains called

seamounts, which were probably

once volcanic islands. Ridges form

where magma (hot, molten rock)

rises from below the crust, then

cools and hardens.

2 BREAKING WAVES

Waves may travel great distances

across the ocean, but the water

in each wave stays in the same

place, moving round in circles.

As a wave reaches the shore,

the circulation of water at the

bottom of the wave is blocked by

the seabed and the top spills over.

WAVES

OCEAN FLOOR

Spilling breaker – a

type of tall, tumbling

wave that breaks on a

shallow beach

Spray thrown up

as crest tumbles over

and breaks onto

the shore

Continental

shelf (a ledge

of land around

each continent)

Guyot

(flat-topped

mountain,

eroded by waves)

Abyssal plain

(the flat expanse

of ocean floor)

Crest

(top of a wave)

Mid-ocean

ridge

Sediment

fan

Continental

slope

Submarine

canyon

Rising

magma

Ocean

trench

Seamount

FIND OUT MORE. Coasts 59 • Continents 39 • Fish 112–114 • Fishing 67