homework encyclopedia

46 Earth

The Earth’s crust is made of rocks, and rocks are made

of natural substances called. MINERALS. There are

three main types of rock – sedimentary, igneous, and

metamorphic rock. Each type is formed in different

ways. The oldest rocks on Earth were formed over

4.25 billion years ago.

HOW ARE METAMORPHIC ROCKS CREATED?

Metamorphic rocks are formed when existing

rocks are changed underground by great heat or

pressure, or both. When volcanoes erupt and when

mountains are formed by the movement of the Earth’s

tectonic plates, rocks are heated and squeezed. The

minerals in the rocks are then changed, forming

metamorphic rocks.

WHAT ARE SEDIMENTARY ROCKS MADE OF?

Sedimentary rocks are made of fine rock particles

that have been worn away and then carried by rivers,

glaciers, or the wind and collect in lakes and oceans.

The tiny fragments are then compressed (squashed)

and cemented together to form sedimentary rock in

a process called lithification.

WHAT ARE IGNEOUS ROCKS?

Igneous rocks form when magma (molten rock) rises

from deep underground and cools and solidifies at

or near the Earth’s surface. Igneous rock that forms

under the ground may later reach the surface because

of geological upheaval. It may also be exposed as the

rocks above are worn away.

1 THE ROCK CYCLE

The rocks that form Earth’s crust are continually destroyed and remade

in an endless process called the rock cycle. Rock is formed by melting;

by cooling and solidifying; by changing through heat and pressure; by

weathering and erosion; and by compression and cementation.

Rocks

Heat

and pressure

can alter sedimentary

and intrusive igneous rocks

Hexagonal columns formed

when the molten basalt lava

flow cooled, contracted,

and split

Basalt, an igneous rock, is the

most common rock on

the Earth’s surface

SEDIMENTARY ROCK

This rock includes sandstone

(above), limestone, and chalk.

Igneous rocks

lava forms extrusive

igneous rocks

METAMORPHIC ROCK

Metamorphic rock includes

marble (above), slate, and schist.

IGNEOUS ROCK

Igneous or volcanic rock includes

granite (above) and basalt.

Metamorphic

rock melts and

may rise to the

Earth’s surface

as magma

Rivers wear away rock and

carry rock fragments and

sediment to the sea

Glaciers erode rock and

carry fragments downhill

Sedimentary rocks form

from layers of sediment

on lake and ocean beds