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Igneous Rock
Igneous Rock
 Igneous rocks form
when magma (beneath
the surface) or lava
(reaches the surface)
cools and hardens
 Igneous comes from the
Latin word ignis, which
means fire
 There are two types of
igneous rock, intrusive
and extrusive
Intrusive Igneous Rocks
 Intrusive igneous rocks
form when magma
hardens beneath Earth’s
surface
 As magma rises, it
cools, allowing elements
to combine and form
minerals
 Minerals grow in size,
forming a solid mass of
interlocking crystals
Granite is a common
intrusive igneous rock
Extrusive Igneous Rocks
 Extrusive igneous rocks
form when lava hardens
at the Earth’s surface
 Lava is similar to
magma, except that most
of the gases have
escaped in lava
Rhyolite
Basalt
Obsidian
Classification of Igneous Rocks
 Texture and composition are two
characteristics used to classify igneous
rocks
 Texture describes the appearance of an
igneous rock based on its size, shape,
and arrangement of its interlocking
crystals
 Composition of igneous rocks are based
on the proportions of light and dark
minerals in the rock
Classification of Igneous Rocks
 Texture describes the appearance of an
igneous rock based on its size, shape,
and arrangement of its interlocking
crystals
 There are four types of texture:
Coarse-grained
 Fine-grained
 Glassy
 Porphyritic

Coarse-grained Texture
 Slow cooling of
magma allows ions,
charged atoms, to
move large distances
within the magma
 Slow cooling results in
the formation of large
crystals
 Igneous rocks with
large crystals show
coarse-grained texture
Gabbro
Diorite
Fine-grained Texture
 Rapid cooling of
magma or lava causes
ions to lose their motion
and quickly combine
 Rapid cooling results in
the formation of many
small, interconnected
mineral grains
 Igneous rocks with
small grains show finegrained texture
Rhyolite
Andesite
Glassy Texture
 When lava spews onto
Earth’s surface, there
may not be time for the
ions to arrange
themselves into
crystals
 The ions are randomly
distributed causing a
glassy texture in the
igneous rock
Obsidian (compact glass)
Pumice (frothy glass)
Porphyritic Texture
 Minerals that crystallize
from magma do not
form at the same rate or
the same time
 Some crystals will
become large while
others even start to form
 Igneous rocks with large
crystals and finegrained minerals show a
porphyritic texture
Andesite
Classification of Igneous Rocks
 Composition of igneous rocks are based
on the proportions of light and dark
minerals in the rock
 There are four types of composition
Granitic
 Basaltic
 Other Compositional Groups


Ultramafic and andesitic
Granitic Composition
 Made almost entirely of the light-
colored silicate minerals, quartz
and feldspar
 Most granitic rocks contain 10%
dark silicate minerals, often
biotite and amphibole
Basaltic Composition
 Made almost entirely of the
dark-colored silicate
minerals and plagioclase
feldspar
 Basaltic rocks are rich in the
elements magnesium and
iron
Other Compositional Groups
 Rocks with a composition
 Rocks composed entirely of
between granitic and basaltic
rocks have an andesitic
composition
 Named after the common
volcanic rock andesite
 Contains at least 25% dark
silicate minerals
dark silicate minerals, iron and
magnesium, is referred to as
ultramafic
 Ultramafic rocks are rare at
Earth’s surface because they
make up the mantle.
Peridotite
Igneous Rock Summary
 Igneous rocks form when magma or lava
cools and hardens
 Intrusive rocks form from magma deep
within Earth
 Extrusive rocks form from lava at Earth’s
surface
 Igneous rocks can be classified
according to texture and composition
Table 1 on page 74