homework encyclopedia

42 Earth

An island is an area of land smaller than a continent

and entirely surrounded by water. Islands range from

single rocks to huge landmasses, such as the island

of Greenland. There are two main types of island –

continental islands and oceanic islands. Islands

are also found in rivers and lakes.

HOW ARE VOLCANIC ISLANDS FORMED?

Volcanic islands are formed in different ways.

One way is by volcanic activity on the sea bed, often

near the boundaries of the tectonic plates that form

the Earth’s crust. Where two plates pull apart, lava

erupts to form an undersea ridge. Layers of lava build

up until a ridge breaks the sea’s surface to form an

island. Sometimes a whole chain of volcanic islands

called an island arc is formed in this way. Some island

arcs contain thousands of islands.

HOW DO CORAL REEFS FORM?

A coral reef is formed from the hard shelly remains

of coral polyps. These tiny creatures live in large

colonies on rocks in shallow, sunlit water, such as the

top of a seamount. When they die, their chalky, tube-

shaped skeletons remain and new, young coral grows

on top. The coral skeletons build up over many years

until they reach the sea’s surface, forming a reef.

WHAT IS A CONTINENTAL ISLAND?

Continental islands are found in shallow seas off

large landmasses. They were formed when rising

seas (for example, at the end of an ice age) cut off

part of the land from a continent. Great Britain is

an example of a continental island.

2 AN ISLAND IS BORN

In November 1963,

sailors saw a plume of

smoke and ash rising from

the sea off Iceland from an

undersea volcanic eruption.

A day later, as the eruption

continued, lava broke the

surface to form land. The

new island was named

Surtsey after

the Norse god

of fire.

FIND OUT MORE. Cnidarians 103 • Oceans 40–41 • Volcanoes 44

1 OCEANIC ISLAND

Oceanic islands are often far

from the mainland. From above,

they look like tiny specks in a

vast glittering ocean. Many

tropical oceanic islands have

coral reefs fringing their bases,

or barrier reefs separated from

the main island by a lagoon.

WORLD’S LARGEST ISLAND 2

Greenland, in the Arctic Ocean, is the world’s largest island, at

2.2 million sq km (0.85 million sq miles). Although huge, few people

live there because it is almost permanently covered in snow and ice.

Islands

ATLANTIC

OCEAN

PACIFIC

OCEAN

ARCTIC

OCEAN

Greenland

A fringing coral reef forms

in the shallows around the

base of a seamount, or

volcanic island, in warm

tropical waters.

A barrier, or offshore reef,

is formed as the coral slowly

builds up, while the cone is

worn away or covered by

rising sea levels.

A ring-shaped coral atoll

with a central lagoon is

all that remains after a

seamount is submerged.

FORMATION OF A

CORAL ATOLL

islands