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Igneous Rock
2
Where do igneous rocks come from? Here’s a hint: The word
igneous comes from a Latin word that means “fire.”
What You Will Learn
Describe three ways that igneous
rock forms.
Explain how the cooling rate of
magma affects the texture of
igneous rock.
Distinguish between igneous rock
that cools within Earth’s crust and
igneous rock that cools at Earth’s
surface.
Vocabulary
intrusive igneous rock
extrusive igneous rock
READING STRATEGY
Reading Organizer As you read
this section, make a table comparing
intrusive rock and extrusive rock.
S6CS3.b Use metric input units (such as
seconds, meters, or grams per milliliter) of
scientific calculations to determine the proper unit
for expressing the answer.
S6E5 Students will investigate the scientific
view of how the earth’s surface is formed.
S6E5.b Investigate the contribution of
minerals to rock composition.
S6E5.c Classify rocks by their
process of formation.
Igneous rock forms when hot, liquid rock, or magma, cools
and solidifies. The type of igneous rock that forms depends
on the composition of the magma and the amount of time it
takes the magma to cool.
Origins of Igneous Rock
Igneous rock begins as magma. As shown in Figure 1, there
are three ways magma can form: when rock is heated, when
pressure is released, or when rock changes composition.
When magma cools enough, it solidifies to form igneous
rock. Magma solidifies in much the same way that water freezes.
But there are also differences between the way magma freezes
and the way water freezes. One main difference is that water
freezes at 0°C. Magma freezes between 700°C and 1,250°C.
Also, liquid magma is a complex mixture containing many
melted minerals. Because these minerals have different melting points, some minerals in the magma will freeze or become
solid before other minerals do.
Figure 1
Pressure The high pressure deep inside the
Earth forces minerals to
remain solid. When hot
rock rises to shallow
depths, the pressure in
the rock is released, and
the minerals can melt.
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Chapter 4
The Formation of Magma
Composition When
fluids such as water
combine with rock, the
composition of the rock
changes, which lowers
the melting point of the
rock enough to melt it.
Temperature A rise in
temperature can cause
the minerals in a rock to
melt. Different melting
points cause some minerals to melt while other
minerals remain solid.
Rocks: Mineral Mixtures
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