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Key Concept
Igneous rocks are
classified according to
their origin, texture, and
mineral composition.
Igneous rock is rock that
forms from magma or lava.
Igneous rock may form on
or below Earth’s surface.
Where igneous rock forms
is its origin.
Extrusive rock is igneous
rock that forms on Earth’s
surface, when lava cools.
Lava cools quickly, forming
small crystals. As a result,
extrusive rock has a smooth
texture. The most common
extrusive rock is basalt.
Spectacular organlike basalt
columns at Svartifoss, Skaftafell
National Park, Iceland
Intrusive rock is igneous
rock that forms below
Earth’s surface, when
magma cools.
Magma cools slowly,
forming big crystals. As a
result, intrusive rock has a
rough texture. The most
common intrusive rock is
granite.
At an elevation of 14,505 ft., Mt.
Whitney reaches the highest peak in
the conterminous United States, and
like much of the Sierra Nevada, it
consists of Cretaceous granodiorite,
a rock intermediate in composition
between granite and diorite.
Igneous rocks differ in how
much silica they contain.
Silica is a material formed
from oxygen and silicon.
From left to right, these rocks
are arranged in order of
decreasing silica content:
granite, diorite, and gabbro.
Low-silica rocks, such as
basalt, are dark-colored rocks.
High-silica
rocks, such as
granite, are
light-colored
rocks.
Why do intrusive rocks have a
rough texture?
a. Intrusive rocks have big
crystals.
b. Intrusive rocks form quickly.
c. Intrusive rocks form from
lava.
1. Label each circle in the Venn diagram with the kind of
igneous rock it describes.
a.
extrusive
Rock
Forms on
the surface
b.
intrusive
Rock
Forms when
magma or lava
cools
Forms
under the
surface
If an igneous rock is lightcolored it probably contains
a lot of ?
silica
Key Concept:
People throughout history
have used igneous rock
for tools and building
materials.
Igneous rocks have several
uses. Examples of useful
igneous rocks are granite,
pumice, and obsidian.
Granite is very hard and
dense. Granite has long
been used for buildings and
bridges. Today, granite is
also used for curbstones
and kitchen counters.
Pumice is very rough.
Pumice is used for cleaning
and polishing.
Obsidian is sharp and
smooth like glass. Obsidian
was used by ancient native
Americans to make knives
and other sharp tools.
a. A rock used for curbstones
and kitchen counters is granite
b. A rock used for cleaning and
polishing is pumice
c. A rock used for making knives
and other sharp tools
is obsidian