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44 Earth

A volcano is a vent or weak spot in the Earth’s crust

through which magma (hot, melted rock) escapes as

. LAVA. In some places, lava oozes slowly out of

the ground. In others, there’s a violent eruption.

HOW DOES A VOLCANO ERUPT?

A volcano erupts when magma wells up from deep

inside the Earth. In violent eruptions, the magma fills

a hollow chamber below a vent blocked by cooled

and hardened rock. Gas and water mingle with the

magma, forming an explosive mixture. The pressure

builds up in the chamber until the magma, gas, and

steam are forced upwards and blast through the vent.

ARE VOLCANOES ALL THE SAME SHAPE?

Volcanoes are different shapes depending on the type

of lava and shape of their vents. Shield volcanoes

have broad, shallow cones and are made of runny,

flowing lava. Fissure volcanoes are long cracks in

the crust. Violent eruptions usually produce steep-

sided conical mountains. Composite cones are built

up from alternate layers of lava and ash.

ARE ALL VOLCANOES DANGEROUS?

There are around 25 major volcanic eruptions on land

every year and thousands of minor ones, many of

which take place under the sea. Active volcanoes are

those that may erupt at any time. Dormant (sleeping)

volcanoes have not erupted for centuries but may still

do so. Extinct volcanoes are no longer likely to erupt.

Lava is the name given to magma once

it has reached the Earth’s surface. Lava

may be thick and sticky or thin and

runny, depending on the minerals the

lava contains and the temperature and

pressure when it was formed.

Volcanoes

4 A VIOLENT ERUPTION

Violent eruptions fling out

red-hot lumps of rock and

scorching ash, which cool

to form a distinctive cone-

shaped mountain.

LAVA

1 AFTERMATH

A huge flow of lava can destroy

everything in its path, like this

village near Mount Kilauea

Volcano in Hawaii, USA.

1 CLOUD OF ASH

The eruption of Mount St Helens

in Washington State, USA, sent

a cloud of fine ash 20 km

(13 miles) into the sky.

1 FOUNTAIN OF FIRE

Some volcanoes with narrow

vents shoot jets of magma up

to 200 m (660 ft) into the sky,

splattering the surrounding land.

Magma chamber

forms deep underground

beneath the Earth’s crust

A cloud of gas,

steam, and rock fragments

bursts out of the volcano

Magma is forced up the main

vent and through narrower side

vents called branch pipes

Layers of ash and lava

build up to form a

volcanic mountain

Red-hot lava

flows down the side

of the volcano

SHIELD VOLCANO FISSURE VOLCANO COMPOSITE CONE VOLCANO

Branch

pipe

Magma Magma

Lava

Fissure

Ash

Vent

Magma

Vent

FIND OUT MORE. Earthquakes 43 • Islands 42 • Rocks 46–47

volcanoes