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Igneous Rocks
By Cindy Grigg
The picture shows
pumice containing air
pockets.
Rocks can be put into
three main groups. They
are grouped by how the
rocks formed. Igneous
(IG-knee-us) rocks form
from magma that began
deep inside the mantle of
the Earth where it is very
hot. "Igneous" comes
from the Latin word "ignis," which means fire. Igneous rocks can
form above ground or below it.
Igneous rocks start out as magma deep inside the Earth. Magma is
a hot mixture of melted rock and gases. When magma erupts onto
Earth's surface, it is called lava. The rocks that form when lava cools
are called extrusive igneous rocks. (The prefix "ex-" means "out.")
Extrusive rocks cool quicker since they form on Earth's surface,
away from the heat deep inside. They cool so quickly that crystals or
grains often don't form at all. Obsidian (OB-sid-EE-ahn) is a
glassy-looking rock formed in this way.
If some crystals do form, they are usually small. Basalt
(BAH-salt) is another extrusive rock. It often has small crystals or
grains. Pumice (PUM-ihss) is a type of extrusive igneous rock that
forms from magma that contains a lot of gases. It cools very quickly,
and the gases escape. As a result, pumice has large air holes in it
where the gases were. Pumice is so lightweight that it can float in
water.
Magma cools as it rises. Sometimes magma cools and forms rock
still inside the Earth. These are called intrusive igneous rocks. (The
prefix "in-" means "in or inside.") Intrusive and extrusive rock looks
different. Intrusive rocks cool slowly. The hot mixture of minerals
inside the liquid rock cools at different rates. The minerals form
crystals as they cool. Intrusive rock has large grains.
Granite is a type of an intrusive igneous rock. Many people have
granite counter tops in their kitchens. Granite is prized for its
patterns and colors. These patterns and colors are due to the minerals
that crystallized as the melted rock cooled. Intrusive rocks are
exposed by the weathering away of the Earth's surface.
Igneous rocks can be changed by heat and pressure inside the
Earth. Then they are called metamorphic rocks. Igneous rocks on the
surface of the Earth are gradually weathered. They may become
parts of sedimentary rocks. Earth's rocks are always changing. They
are always on the move. The rock cycle is what we call this cycle of
change.
The rocks born of fire are the beginning of almost all rocks on
Earth. Before metamorphic rock changed, it first cooled from magma.
Before surface rocks weathered into sediment, many of them were
formed by the fiery furnace inside Earth. Rocks melt and cool. They
are squeezed and pushed upward. They are weathered and broken.
Learning about the rock cycle tells us many things about the Earth.
Igneous Rocks
Questions
1. How does the author's explanation of the meaning of word
prefixes help the reader?
A. Knowing the meaning of a prefix helps the reader
understand the meaning of a new, unfamiliar word.
B. Knowing the meaning of a prefix helps the reader
understand where igneous rocks formed.
C. both a and b
D. none of the above
2. What causes pumice to float in water?
A. Pumice cools quickly and has large air holes in it.
B. Pumice forms from volcanoes.
C. Pumice is made of sand.
D. none of the above
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3. Extrusive igneous rocks form:
A. either inside or outside the Earth
B. outside the Earth
C. inside the Earth
9. Igneous rocks form from:
A. magma
B. basalt
C. pumice
D. sediments
10. Name one specific type of igneous rock.
4. Intrusive rock has large grains because ______.
A. It cools slowly.
B. It cools quickly.
C. It has fewer minerals in it.
D. none of the above
5. What can the reader infer after reading Paragraph 8?
A. that most rocks are igneous
B. that most rocks are intrusive
C. the names of the three groups of rocks
D. that most rocks are extrusive
6. The phrase "formed by the fiery furnace" is an example of
______.
A. a synonym
B. a simile
C. onomatopoeia
D. alliteration
7. Which of these words does NOT describe obsidian?
A. igneous
B. glass-like
C. extrusive
D. large grained
8. What are the three types of rocks?
Write a short summary of this text. Tell what igneous rock is and give
some examples. Explain how the different types form.
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Write three sentences that are facts about igneous rocks. Then write
two sentences that are opinions about them.