Childrens Illustrated Encyclopedia

441

We live on the surface of a huge ball

of rock, earth. the landscape everywhere is made

up of rocks. Most are covered by soil, trees, or grass.

others, such as uluru (ayers rock) in australia, a

massive lump of sandstone 1,142 ft (348 m) high,

rise from the ground and are visible. the oldest

rocks on earth are about 4 billion years old

other rocks are much more recent, and new rocks

are forming all the time. all rocks contain substances

called minerals. Marble consists mainly of calcite, for

example, and granite contains the minerals mica,

quartz, and feldspar.

rocks form in different ways: from molten

rock within earth, from the fossils of animals

and plants, and by the action of heat and

pressure on ancient rocks inside earth. But

no rocks, however hard, last forever on

earth’s surface. they are slowly eroded,

or worn away, by the action of wind, rain,

and other weather conditions.

hoW rocks forM

all rocks started out as

clouds of dust in space.

the dust particles came

together and formed the

rocks that make up the

planets, moons, and

meteorites. there are

now three main kinds of

rocks on earth’s surface:

igneous, sedimentary,

and metamorphic rocks.

each kind of rock forms

in a different way.

sediMentary rocks

ice, wind, and running water wear

away rocks into pebbles and small

particles called sediment. layers

of sediment containing sand,

clay, and animal skeletons are

buried and squeezed so that they

slowly change into hard rocks

called sedimentary rocks.

Heating and

compressing

limestone turns it

into marble, a hard

metamorphic rock.

MetaMorphic rocks

heat and pressure deep

underground bake and

squeeze sedimentary

and igneous rocks. the

Rocks and minerals 441-

change, often becoming

harder. in this way, they

form new rocks called

metamorphic rocks. after

millions of years, the top

rocks are worn away and

metamorphic rocks

appear on the surface.

Shale forms

from clay at

the riverbed.

Giant’s causeWay

the steps of this unusual

rock formation in northern

ireland are made of

columns of basalt, rock that

developed when lava from a

volcano cooled and set. the

rock cracked into columns

as it cooled.

River carries

sediment

from the land

to the sea.

Clay forms shale,

a sedimentary rock that

crumbles easily.

This rock is slate, the

metamorphic rock that

forms from shale.

Mud and pebbles are buried and

squashed together, producing

a hard sedimentary rock called

conglomerate.

Sedimentary rocks,

such as conglomerate,

form on the beach at the

mouth of a river.

Hot magma solidifies,

forming granite, an

igneous rock.

iGneous rocks

deep underground the heat

is so intense that some rock

is molten (melted). When

it cools, this molten rock,

or magma, sets hard to

produce an igneous rock.

this may happen

underground, or the

magma may rise to the

surface as lava and solidify.

Rocks and mineRals

Limestone contains

the remains of shellfish.

Chalk, another kind

of limestone, is made

of the skeletons of

sea animals.

Red-hot magma heats

surrounding limestone,

turning it into marble.

When magma slowly

cools deep under-

ground, it often forms

granite, a hard rock

that is used as a

building material.

When lava from a

volcano cools on

Earth’s surface, it

forms basalt.

Lava flows from a volcano and

solidifies, forming basalt,

an igneous rock.

Bubbles of gas

trapped in the lava

created holes in

this piece of rock.

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