The New Childrens Encyclopedia

38

EARTH

Most sedimentary rocks are made

of small particles of other rocks,

transported by water, wind, or

ice. The particles may all be the

same size or a mixture of sizes.

These build up in layers, and

pressure from new layers above

squashes them into hard rock.

Metamorphic rocks These began

as other types of rock but have

been altered by great heat,

pressure, or both, deep inside

the Earth’s crust. Most of the

minerals in the original rock

have changed. These rocks often

show folded or squashed bands.

Rocks

Our planet is a big ball of rock. Rock is what

gives the Earth its features—mountains,

canyons, and plains. Rocks can be massive,

or as small as a grain of sand. All of them

started life deep inside the mantle.

WHAT IS A ROCK?

Rocks are usually made up of several

different minerals. Looking closely at a

rock can tell you a lot about its history.

The shapes of crystals or grains in the rock

and how they fit together reveal whether

the rock is one of three types: igneous,

metamorphic, or sedimentary.

Igneous rocks began as molten rock

deep inside Earth. They are the most

common rocks in the Earth’s crust.

Some were erupted onto the surface

as lava; others solidified underground.

The speed at which they cooled is

shown by the crystals they contain—

big crystals indicate slow cooling.

 STONE CYCLE

Earth’s rocks are

endlessly recycled,

but it takes

millions of years

for rocks to form

and change.

Igneous rocks

Sedimentary rocks Metamorphic rocks