4 8 rocks and minerals (earth science)

How do fossils help scientists fi gure out the age of a layer of rock2. different forms during the rock cycle?. Vocabulary igneous rock luster metamorphic rock mineral sediment sediment

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Scott Foresman Science 4.8

Nonfi ction Summarize • Charts

• Captions

• Diagrams

• Glossary

Rocks and Minerals

ISBN 0-328-13881-9

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Scott Foresman Science 4.8

Nonfi ction Summarize • Charts

• Captions

• Diagrams

• Glossary

Rocks and Minerals

ISBN 0-328-13881-9

ì<(sk$m)=bdiibe< +^-Ä-U-Ä-U

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1 What are some of the words used to

describe a mineral’s luster?

2 How do fossils help scientists fi gure out

the age of a layer of rock?

3 What is magma?

different forms during the rock cycle

Write to explain how rocks change from one form to another Include details from the book to support your answer

5 Summarize Explain the main

differences between sedimentary, igneous, and metamorphic rocks

What did you learn?

Vocabulary

igneous rock

luster

metamorphic rock

mineral

sediment

sedimentary rock

weathering

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ISBN: 0-328-13881-9

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by Mary Miller

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Earth’s crust is made of a

layer of rocks All rocks are

made of minerals

calcite

Mineral Crystals

Minerals are part of our everyday lives The milk

you drink contains a mineral called calcium that

builds strong bones and teeth The dirt in your school

playground has minerals in it Minerals are nonliving

solid crystals that make up rocks Minerals are not made

by humans but are found in nature

garnet

opal yellow sulfur

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Scientists have identifi ed more than three thousand minerals Each mineral has crystals that form a unique shape For example, a quartz crystal is six-sided, similar

to the shape of your pencil Topaz crystals can be very large Some weigh over two hundred pounds

A mineral has the same chemical makeup no matter where it forms Sulfur found in Florida has the same chemicals as sulfur found in Hawaii

About thirty minerals are found in Earth’s crust

These minerals are called the rock-forming minerals

Each kind of rock is made of a different mix of minerals For example, hornblende granite is always made of feldspar, quartz, and mica Some types of rock have only one or two minerals A type of coal called anthracite is nearly all carbon

quartz

feldspar

hornblende granite mica

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Scientists use different tests to tell one mineral from

another They test its physical properties, such as color,

luster, hardness, streak, and cleavage

Quartz can be many different colors.

Identifying Minerals

Color

Color is the most obvious property of a mineral

because it is easy to see However, a color test has

limitations Some minerals come in many different

colors For example, quartz can be purple, yellow, pink,

or brown Another problem is that many minerals are

the same color, or even colorless

Luster

Scientists also study a mineral’s luster Luster is the

way a mineral’s surface refl ects light The luster can be

dull, metallic, pearly, glassy, greasy, or silky A quartz

rock crystal has a glassy luster In contrast, pyrite has a

metallic luster Most minerals have a glassy luster

smoky quartz

citrine rock crystal

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Hardness

Streak

The streak test is another way to tell minerals apart

A mineral is scratched across a special kind of plate The color of the powder left behind is the mineral’s streak

Even though minerals can come in many colors,

a mineral’s streak will always be the same color For example, fl uorite can look yellow, green, blue, purple,

or even black But its streak is always white The streak test is one way to tell gold from “fool’s gold,” or pyrite

Real gold leaves a yellow streak Pyrite leaves a greenish black streak

Mohs Scale for Hardness

corundum

calcite gypsum

talc

A mineral’s hardness is how easily it can be scratched Scientists measure hardness using a chart called the Mohs Scale for Hardness The scale ranks minerals from 1 to 10 A mineral with a high number can scratch all minerals lower on the scale For example,

a diamond is a 10 Feldspar is a 6 If a diamond is rubbed against a piece of feldspar, the diamond will scratch the feldspar

10 9

8 7 6 5

4 3

2 1

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Sedimentary Rocks

Rock is always changing Mountains and rocks are

battered by natural forces Rain, wind, and ice break

down rocks into mineral particles These particles are

tossed together with bits of shells and soil Dead plant

and animal remains are also swept into this mix

Wind, running water, and gravity move these

particles from one place to another This is erosion

The eroded material that settles at the bottoms of lakes,

rivers, and oceans is called sediment.

The particles in sediment have different sizes and

shapes Some are smooth, while others have sharp edges

Rivers carry bits of rock into the ocean.

Ice wears away rock.

Rock settles at the bottom of the ocean.

Wind wears

away rock.

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Rock Layers

Over time, new sediment fl ows over the old rock particles The new layers press down on the older layers

The weight of the layers helps to cement the particles

together They harden and form sedimentary rock The

kind of sedimentary rock that is made depends on what makes up the sediment

Most sedimentary rocks are formed by erosion

Rocks are broken down and moved to new places

These bits are cemented together to form new rocks

Sandstone can start with tiny quartz grains that are round in shape and uniform in size The quartz mixes with grains of feldspar and sand These tiny grains are glued together with bits of silica, calcite, or iron

Kinds of Sedimentary Rocks

limestone shale

kinds of sedimentary rocks

sandstone

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Some sedimentary

rocks are formed in

part from the remains

of plants and animals

Most fossils are found in

sedimentary rocks When

fi sh and other sea animals

die, their bodies fall to

the ocean fl oor Their

hard skeletons and shells

are saved in the layers

of sediment Calcite

and other dissolved

minerals cement these

layers together to create

limestone

Sometimes clay hardens into sedimentary rock

Tiny particles of clay settle at the bottoms of oceans

and lakes Over time these minerals harden to form

mudstone or shale Mudstone contains quartz, feldspar,

and mica It is used to make bricks and cement

Chemicals in shale can be used to make fuel and other products

Mudstone

When an animal dies,

it may sink

to the ocean

fl oor.

It may become buried

in the sediment.

Soft parts

of the animal decay, but hard parts are left.

The fossil becomes part of the rock around it.

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How Rocks Become Soil

The soil at your local park was once a large rock—or even part of a mountain Over time, rocks are broken apart by heat and cold Rainwater drips through cracks

in the rock and freezes When the ice thaws, it leaves behind even larger cracks Deep-rooted plants can also dig into rocks and create grooves and cracks

Eventually, pieces of rock break off These pieces are blown or washed away This battering of rock by natural

forces is called weathering Over millions of years, even

mountains can be worn down to a pile of rocks

These bits of weathered rock mix with decayed plants and animals to form soil Soil contains living things, such as insects, bacteria, and fungi These life forms make nutrients to help plants grow

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What Rocks Can Tell Us

Some sedimentary rocks contain fossilized remains

of life forms that existed millions of years ago Many

animals that once roamed Earth no longer exist Fossils

are our only record that they ever existed Fossils give

scientists clues about Earth’s history They show how

prehistoric plants and animals lived

Scientists might fi nd fossilized footprints made by

a dinosaur 100 million years ago The distance between

the footprints can tell scientists how fast the dinosaur

could move Fossils also give clues about prehistoric

animal behavior The discovery of fossilized nest eggs

revealed that some dinosaurs laid eggs and took care

of their young Fossils of teeth can tell scientists if the

animal was a meat or plant eater

dinosaur eggs

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How Fossils Develop

Fossils are found in layers of rock The age of a fossil can help scientists fi gure out the age of the rock layer

The Archaeopteryx had feathers like a bird but teeth

and a tail like those of a reptile It lived about 150 million years ago An Archaeopteryx fossil found in a layer of rock

means the layer formed in that time period Scientists study fossils in rock layers to learn how animals and plants have changed over time

Precambrian Era (more than 544 million years ago)—

Life appeared Rocks from this era have fossils of early jellyfi sh.

Scientists use a geologic time scale to divide Earth’s history into time periods called eras The earliest era is

at the bottom of the chart This order shows the ages of the layers of sedimentary rock The oldest fossils are in the bottom layers

Paleozoic Era (544 million to 248 million years ago)—Fossils of early reptiles, insects, and fi sh are found in these rocks.

Cenozoic Era (65 million years ago to present)—Fossils show that large mammals and earliest humans appeared.

Mesozoic Era (248 million to 65 million years ago)—Fossil evidence of dinosaurs is found in the rocks of this era This is when dinosaurs became extinct

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Igneous And

Metamorphic Rocks

Sedimentary rocks are not the only kinds of rock

found on Earth The two other kinds are igneous rocks

and metamorphic rocks

Igneous Rocks

Many rocks are hard and can be broken into pieces

Sometimes rock melts This is what happens to the

layer of rock deep below Earth’s crust, where it is very

hot This molten rock is magma Igneous rock forms

from molten rock Most of the ocean fl oor is a type of

igneous rock named basalt

Igneous rocks can form above or below Earth’s

surface Sometimes hot magma explodes out of Earth’s

crust when a volcano erupts Magma that reaches Earth’s

surface is called lava The lava may stream down the

sides of a volcano in a red-hot river It can also be fl ung

out of the volcano in hot chunks Lava cools quickly on

the surface It can harden into solid rock in a few days

Most igneous rock, however, does not form this way

The magma slowly rises It fi lls cracks in Earth’s crust

As the magma cools and hardens, mineral crystals form

in the rock The hardening of magma into igneous rock

within Earth can take a very long time

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The Giant’s Causeway

A causeway is a road built over water Stone pillars may hold up these roads The Giant’s Causeway is a natural formation of basalt columns on the coast of Northern Ireland Nearly forty thousand pillars lead from the cliffs to the sea Some reach a height of more than forty feet Many of the columns are six-sided

The columns are 50 to 60 million years old As fl owing lava reached the sea, it cooled rapidly As it

cooled, pressure on the lava caused

it to squeeze together Cracks split the rock from top to bottom and formed

the pillars

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Over millions of years, rocks in Earth’s crust are

squeezed by the weight of other rocks This can cause

rocks to change Some rocks begin to melt Rock that

is changed by heat and pressure is called metamorphic

rock It has undergone a metamorphosis, or a change

in form

Sedimentary and igneous rock can change into

metamorphic rock Limestone is a sedimentary rock

Pressure deep below Earth’s surface can turn it into

marble The igneous rock shale can become slate

Metamorphic rocks change in many ways as they are

formed The rocks’ mineral crystals can change They

may become new crystals that have a different size or

shape Sometimes the minerals settle in layers as a result

of heat and pressure This can cause some metamorphic

rocks to break into fl at slabs

zeolite crystals

Metamorphic Rocks

Marble forms from limestone.

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The rock cycle is the process in which old rocks recycle into new ones Heat, pressure, weathering, and erosion help keep the rock cycle going

Sedimentary, igneous, and metamorphic rocks can all change into a different kind of rock at any time

Metamorphic rock can melt into magma and later become igneous rock when the lava cools Igneous rock can wear away, and its pieces can become sedimentary rock

All three kinds of rocks are made of minerals These minerals have many properties that can be used to

describe them Different combinations of minerals form different kinds of rocks The next time you see a rock, think about how many changes it will go through as it moves through the rock cycle

The Rock Cycle

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igneous rock rock that is formed when

magma or lava cools and hardens

luster the way a mineral’s surface

refl ects light

metamorphic rock rock changed by great heat and

pressure inside Earth

mineral a naturally occurring,

nonliving crystal that makes

up rocks

sediment the eroded material that settles

on the bottoms of lakes, rivers, and oceans

sedimentary rock rock created as layers of

sediment are weighed down and hardened together

weathering the battering of rocks by wind,

rain, heat, and ice

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