Britannica Student Encyclopedia - 2010

skin is exposed to sunlight. Bacteria in

the intestines produce vitamin K.

Certain vitamins (the B complex vitamins

and vitamin C) dissolve in water.

The body stores a small amount of these

vitamins but gets rid of most of those it

does not use. For this reason, people

need to take in plenty of these vitamins

every day.

Other vitamins (A, D, E, and K) do not

dissolve in water. The vitamins that the

body does not use right away are stored

in the body’s fat and liver. Getting too

much of these vitamins can be dangerous

over time.

#More to explore

Food and Nutrition

Volcano

A volcano is an opening in Earth’s crust.

When a volcano erupts, hot gases and

melted rock from deep within Earth

find their way up to the surface. This

material may flow slowly out of a fissure,

or crack, in the ground, or it may

explode suddenly into the air. Volcanic

eruptions may be very destructive. But

they also create new landforms.

A Volcanic Eruption

During a volcanic eruption, hot melted

rock called magma escapes from a vent,

or opening, in Earth’s surface, or crust.

Magma released from a volcano is

known as lava. Fresh lava ranges from

1,300° to 2,200° F (700° to 1,200° C)

in temperature. It glows red as it flows

out of the volcano’s opening. As it cools,

it hardens into rock.

Strong volcanic eruptions throw bits of

magma into the air. These bits cool into

tiny pieces of rock, called volcanic dust

or volcanic ash.Wind can carry volcanic

Apples and other fresh fruits contain many

of the vitamins that growing children as well

as adults need.

An erupting volcano threatens a village in

Indonesia. Glowing liquid rock called lava

flows down the side. Clouds of smoke and

ash fill the air.

BRITANNICA STUDENT ENCYCLOPEDIA Volcano 99

 

dust thousands of miles away. Volcanic

ash can coat the land for miles around

the volcano.

Steam and poisonous gases also escape

from volcanoes. Sometimes these gases

mix with ash to create destructive fiery

clouds.

Where Volcanoes Form

Earth’s crust is made up of huge, rocky

pieces called plates. The plates move

slowly over the crust. Most volcanoes lie

along the boundaries between these

plates.

Some of the most violent eruptions take

place where the edge of one plate is

forced beneath the edge of another. This

forces magma to rise to the surface. Hot

gases in the magma make these volcanoes

very explosive. Most volcanoes of

this type are found around the edges of

the Pacific Ocean. This huge circle of

volcanoes is known as the ring of fire.

Volcanoes also form in places where two

plates slowly pull apart. Molten rock

rises through the gap between the plates.

It causes fissure eruptions, in which lava

flows out over the ground. This type of

volcano is common along the Mid-

Atlantic Ridge, a mountain chain under

the Atlantic Ocean. Volcanoes in the

northern part of this ridge formed the

island country of Iceland.

A small number of volcanoes are not

located along the edges of plates. They

form at “hot spots” in Earth’s crust. At a

hot spot, molten rock rises from deep

below the crust. The volcanoes of

During a volcanic eruption, lava flows from

vents. Ash and gas shoot into the air.

100 Volcano BRITANNICA STUDENT ENCYCLOPEDIA

 

Hawaii are the best examples of hot-spot

volcanoes.

Volcanic Landforms

Volcanic eruptions create new landforms

that are also called volcanoes. The two

most common types are stratovolcanoes

and shield volcanoes.

Stratovolcanoes are mountains shaped

like cones. They have a narrow top with

steep sides and a wide bottom. A crater,

or bowl-shaped pit, usually lies at the

top. Stratovolcanoes are made up of

layers of hardened lava and ash. Thousands

of eruptions left these layers over

millions of years. Mount Fuji in Japan is

a stratovolcano.

Shield volcanoes are dome-shaped

mountains built by lava flows. They are

not as steep as stratovolcanoes. The volcanoes

of Hawaii are shield volcanoes.

Sometimes the top of a volcano collapses

and forms a pit called a caldera. A

caldera is larger than a crater. Some

calderas fill up with water to form lakes.

A somma volcano forms when a new

volcanic cone partially fills a caldera.

A complex volcano has more than one

vent. A volcano can have more than one

vent when two cones overlap one

another. Or a volcano can form new

vents during an explosion.

Hot Springs, Geysers, and

Fumaroles

Hot springs, geysers, and fumaroles are

other types of volcanic activity. They

happen in places where magma heats

underground water. A hot spring is a

place where warm water comes up

through the ground. A geyser is a kind

of hot spring that shoots water and

steam into the air. Fumaroles are vents

that release gas and steam.

Studying Volcanoes

Volcanology is the branch of geology

that focuses on volcanoes. Scientists who

study volcanoes keep track of earthquakes

and other signs of volcanic activ-

Eruptions of volcanoes can create several

different types of landforms.

BRITANNICA STUDENT ENCYCLOPEDIA Volcano 101

 

ity. They try to predict when an

eruption may take place.

History

Volcanoes have a long history of destruction.

In AD 79 the eruption of Mount

Vesuvius destroyed the Roman cities of

Pompeii and Herculaneum. Two of the