4
LESSON
GOALS
You will learn
• how minerals are
identified.
• how igneous rocks form.
• how sedimentary rocks
form.
• how metamorphic rocks
form.
mineral (min/or ol),
nonliving solid matter from
the earth.
Salt crystal
Calcite crystals
258
How Are Rocks Made?
Imagine you are on a road going up the side of a
mountain. The road is cut into the mountain. You
might see different kinds of rocks as you ride along.
Minerals That Make Up Rocks
Rocks have different colors and shapes, but all
rocks are made of one or more minerals. A mineral is
nonliving, solid matter from the earth. Gold and
diamond are minerals.
The atoms in a mineral join together in a certain
pattern to make crystals. A crystal has a certain
shape. Minerals do not all have the same shape of
crystals. Notice in the picture that the shape of the
salt crystal is different from the calcite crystals.
Minerals have different physical properties that
help people identify them. Sometimes color helps
people identify a mineral. The color of gold helps
people identify it. However, most minerals are not
always the same color. A mineral can have different
colors depending on how it formed.
Another property that helps people identify
minerals is how light reflects from the mineral.
Minerals, such as the pyrite (pi/rit) in the picture,
look like shiny metal. Other minerals, like the talc in
the picture, do not look like metal.
A mineral can also be identified by how hard it is.
A mineral is given a number from I to 10 for its
hardness. Talc is the softest mineral and has a
hardness of 1. Diamonds, like the ones in the picture,
are the hardest mineral and have a hardness of 10.
Hardness of a mineral is found by trying to scratch
it with different objects or with other minerals. A
mineral can only be scratched by an object or other
mineral that is harder. Testing the hardness of an
unknown mineral can help identify it.
Talc
Pyrite
Igneous Rocks
igneous (ig/ne os) rock,
rock that forms from
magma.
Granite and a granite building
260
Rocks form in different ways. Some rocks are
made from magma deep inside the earth. These rocks
are called igneous rocks. The magma slowly moves
up through cracks in the earth's crust. As the magma
rises, it cools and hardens. This hardened magma is
igneous rock. Most of the earth's crust is made of
igneous rock.
Some igneous rocks form under the ground. The
granite (gran lit) in the pictures on the left is an
igneous rock that forms under the ground. Other
igneous rocks form above ground when magma pours
out from a volcano or through cracks in the ground.
When this magma-. now called lava-cools,
it
hardens into igneous rock. Obsidian (ob sidle on)
in the pictures on the right is an igneous rock that
forms above the ground.
Obsidian and obsidian tools
Sedimentary Rocks
Some rocks are made from tiny pieces of rocks,
sand, shells, and other materials. These tiny bits are
sediments. Rivers and streams carry sediments into
lakes and oceans. The sediments sink to the bottom of
the water. Over the years, many layers of sediment
collect on the lake or ocean floor. The top layers press
down on the bottom layers. The bottom layers are
pressed into rock. Sedimentary rocks are made
from sediments that are pressed together. The
sedimentary rock in the picture was once under water.
The sedimentary rock in the picture is shale. This
sedimentary rock is made from layers of mud and
clay. Traces of plants and animals-or fossils-are
often found in sedimentary rocks. The organisms
were trapped in layers of sediment. As the sediments
hardened, the organisms' bodies decayed and left
their traces in the rock. Find the fossils of the frog
and leaf in the pictures.
SCIENCE IN YOUR LIFE
When you write with
chalk, you might be. using
a sedimentary rock. A rock
called chalk is made from
tiny seashells that have
been pressed together.
sedimentary
(sed/c men! tor e) rock, a
rock that is made from
sediments that have been
pressed together.
Shale and fossils
Metamorphic Rocks
Sometimes, one kind of rock changes into another
kind. Rock that is buried under layers of other rock is
under pressure. This pressure causes heat. Over many
years, great heat and pressure can change igneous or
sedimentary rock into metamorphic rock. The
limestone in the picture is a sedimentary rock that
becomes the metamorphic rock called marble. The
head in the picture is made from marble.
metamorphic
(rnet/o mor/fik) rock, a
rock that forms when
igneous or sedimentary
rock is changed by heat
and pressure.
Lesson Review
1. How is the hardness of a mineral found?
2. How do igneous rocks form?
3. How do sedimentary rocks form?
4. How do metamorphic rocks fo~?
5. Challenge! Scientists have found fossils of water
plants and animals in rock high in mountains.
What kind of rock are the fossils in and where did
the rock form?
Limestone
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Studyon
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FIND OUT
ON YOUR OWN
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EN'
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.
your own, pages 394-395.
Copy the chart below on a sheet of paper. Use
an encyclopedia to find out the missing
information. Then, complete the chart.
Kind of
rock
Granite
262
Uses for
rock
from magma
that cools
and hardens
Marble
statues,
buildings,
and furniture
Limestone
sedimentary
Slate
metamorphic
Pumice
How rock
forms
scrubbing
and polishing
LESSON 4
pages 258-262
1. Gold and diamonds are
2. A mineral's color is one of its physical EN .
3. What is one of the ways you can identify the mineral
(.
pynte?
4. What is the hardness of the hardest mineral?
5. Most of the earth's crust is made of WN .
6. Name an igneous rock that forms above ground.
(,
7. What is shale made from?
8. Where are fossils often found?
9. Great heat and pressure inside the earth can change __
into marble.
10. What kind of rock is marble?
11. What kind of rocks form in each of the areas marked in
the pictures below?