Rocks that contain substantial dark silicate minerals and calcium-rich plagioclase
feldspar (but no quartz) are said to have a
basaltic composition (see Figure 3.3).
Basaltic rocks contain a high percentage of
ferromagnesian minerals, so geologists also
refer to them as mafic (from magnesium
and ferrum, the Latin name for iron).
Because of their iron content, mafic rocks
are typically darker and denser than
granitic rocks. Basaltic rocks make up the
ocean floor as well as many of the volcanic
islands located within the ocean basins.
Basalt also forms extensive lava flows on
the continents.
Other Compositional
Groups
As you can see in Figure 3.3, rocks with a
composition between granitic and basaltic
rocks are said to have an intermediate
composition, or andesitic composition after
the common volcanic rock andesite. Intermediate rocks contain at least 25 percent
dark silicate minerals, mainly amphibole,
pyroxene, and biotite mica with the other
dominant mineral being plagioclase
feldspar. This important category of
igneous rocks is associated with volcanic
activity that is typically confined to the
margins of the continents.
Another important igneous rock,
peridotite, contains mostly olivine and
pyroxene and thus falls on the opposite
side of the compositional spectrum from
granitic rocks (see Figure 3.3). Because
peridotite is composed almost entirely of
ferromagnesian minerals, its chemical
composition is referred to as ultramafic.
Although ultramafic rocks are rare at Earth’ s
surface, peridotite is the main constituent
of the upper mantle.
Silica Content
as an Indicator
of Composition
An important aspect of the chemical composition of igneous rocks is silica (SiO 2 )
content. Typically, the silica content of
crustal rocks ranges from a low of about
40 percent in ultramafic rocks to a high of
more than 70 percent in granitic rocks (see
Figure 3.3). The percentage of silica in
igneous rocks actually varies in a systematic
manner that parallels the abundance of
other elements. For example, rocks that are
relatively low in silica contain large
amounts of iron, magnesium, and calcium.
By contrast, rocks high in silica contain
very little iron, magnesium, or calcium but
are enriched with sodium and potassium.
Consequently, the chemical makeup of an
igneous rock can be inferred directly from
its silica content.
Further, the amount of silica present in
magma strongly influences its behavior.
Granitic magma, which has a high silica
content, is quite viscous (“thick”) and may
erupt at temperatures as low as 700 °C.
On the other hand, basaltic magmas are
low in silica and are generally more fluid.
Basaltic magmas also erupt at higher temperatures than granitic magmas—usually at
temperatures between 1100 and 1250 °C
and are completely solid when cooled to
1000 °C.
In summary, igneous rocks can be
divided into broad groups according to the
proportions of light and dark minerals they
contain. Granitic (felsic) rocks, which are
composed almost entirely of the lightcolored minerals quartz and feldspar, are at
one end of the compositional spectrum
(see Figure 3.3). Basaltic (mafic) rocks,
which contain abundant dark silicate
minerals in addition to plagioclase feldspar,
make up the other major igneous rock
group of Earth’ s crust. Between these
groups are rocks with an intermediate
(andesitic) composition. Uultramafic rocks,
which lack light-colored minerals, lie at
the far end of the compositional spectrum
from granitic rocks.
C O N C E P T C H E C K 3 . 3
Igneous rocks are composed mainly of
which group of minerals?
Differentiate between felsic and mafic
igneous rocks.
In what way are the dark (ferromagnesian)
silicate minerals different from the light
(nonferromagnesian) silicate minerals?
3
2
1
Igneous Textures:
What Can They
Tell Us?
Igneous Rocks
Igneous Textures
The term texture is used to describe the
overall appearance of a rock based on the
size, shape, and arrangement of its mineral
grains (FIGURE 3.4). Texture is an important
property because it reveals a great deal
about the environment in which the rock
formed. This fact allows geologists to make
inferences about a rock’ s origin based on
careful observations of grain size and other
characteristics of the rock.
Factors Affecting
Crystal Size
Three factors influence the textures of
igneous rocks: (1) the rate at which molten
rock cools; (2) the amount of silica present;
and (3) the amount of dissolved gases in the
magma. Among these, the rate of cooling
tends to be the dominant factor.
A very large magma body located many
kilometers beneath Earth’ s surface will cool
over a period of perhaps tens to hundreds of
thousands of years. Initially, relatively few
crystal nuclei form. Slow cooling permits
ions to migrate freely until they eventually
join one of the existing crystalline structures.
Consequently, slow cooling promotes the
growth of fewer but larger crystals.
On the other hand, when cooling
occurs rapidly—for example, in a thin lava
flow—the ions quickly lose their mobility
GEODe
ESSENTIALS
OF GEOLOGY
D I D Y O U K N O W ?
People have been making glass in
roughly the same way for at least 2000
years. The process involves melting
certain Earth materials and cooling the
liquid quickly before the atoms have
time to form an orderly crystalline
structure. This is the same way that
natural glass, called obsidian, is
generated from lava. It is possible to
produce glass from a variety of
materials, but most commercial glass is
produced from quartz sand and lesser
amounts of carbonate minerals.
67
Igneous Textures: What Can They Tell Us?
feldspar (but no quartz) are said to have a
basaltic composition (see Figure 3.3).
Basaltic rocks contain a high percentage of
ferromagnesian minerals, so geologists also
refer to them as mafic (from magnesium
and ferrum, the Latin name for iron).
Because of their iron content, mafic rocks
are typically darker and denser than
granitic rocks. Basaltic rocks make up the
ocean floor as well as many of the volcanic
islands located within the ocean basins.
Basalt also forms extensive lava flows on
the continents.
Other Compositional
Groups
As you can see in Figure 3.3, rocks with a
composition between granitic and basaltic
rocks are said to have an intermediate
composition, or andesitic composition after
the common volcanic rock andesite. Intermediate rocks contain at least 25 percent
dark silicate minerals, mainly amphibole,
pyroxene, and biotite mica with the other
dominant mineral being plagioclase
feldspar. This important category of
igneous rocks is associated with volcanic
activity that is typically confined to the
margins of the continents.
Another important igneous rock,
peridotite, contains mostly olivine and
pyroxene and thus falls on the opposite
side of the compositional spectrum from
granitic rocks (see Figure 3.3). Because
peridotite is composed almost entirely of
ferromagnesian minerals, its chemical
composition is referred to as ultramafic.
Although ultramafic rocks are rare at Earth’ s
surface, peridotite is the main constituent
of the upper mantle.
Silica Content
as an Indicator
of Composition
An important aspect of the chemical composition of igneous rocks is silica (SiO 2 )
content. Typically, the silica content of
crustal rocks ranges from a low of about
40 percent in ultramafic rocks to a high of
more than 70 percent in granitic rocks (see
Figure 3.3). The percentage of silica in
igneous rocks actually varies in a systematic
manner that parallels the abundance of
other elements. For example, rocks that are
relatively low in silica contain large
amounts of iron, magnesium, and calcium.
By contrast, rocks high in silica contain
very little iron, magnesium, or calcium but
are enriched with sodium and potassium.
Consequently, the chemical makeup of an
igneous rock can be inferred directly from
its silica content.
Further, the amount of silica present in
magma strongly influences its behavior.
Granitic magma, which has a high silica
content, is quite viscous (“thick”) and may
erupt at temperatures as low as 700 °C.
On the other hand, basaltic magmas are
low in silica and are generally more fluid.
Basaltic magmas also erupt at higher temperatures than granitic magmas—usually at
temperatures between 1100 and 1250 °C
and are completely solid when cooled to
1000 °C.
In summary, igneous rocks can be
divided into broad groups according to the
proportions of light and dark minerals they
contain. Granitic (felsic) rocks, which are
composed almost entirely of the lightcolored minerals quartz and feldspar, are at
one end of the compositional spectrum
(see Figure 3.3). Basaltic (mafic) rocks,
which contain abundant dark silicate
minerals in addition to plagioclase feldspar,
make up the other major igneous rock
group of Earth’ s crust. Between these
groups are rocks with an intermediate
(andesitic) composition. Uultramafic rocks,
which lack light-colored minerals, lie at
the far end of the compositional spectrum
from granitic rocks.
C O N C E P T C H E C K 3 . 3
Igneous rocks are composed mainly of
which group of minerals?
Differentiate between felsic and mafic
igneous rocks.
In what way are the dark (ferromagnesian)
silicate minerals different from the light
(nonferromagnesian) silicate minerals?
3
2
1
Igneous Textures:
What Can They
Tell Us?
Igneous Rocks
Igneous Textures
The term texture is used to describe the
overall appearance of a rock based on the
size, shape, and arrangement of its mineral
grains (FIGURE 3.4). Texture is an important
property because it reveals a great deal
about the environment in which the rock
formed. This fact allows geologists to make
inferences about a rock’ s origin based on
careful observations of grain size and other
characteristics of the rock.
Factors Affecting
Crystal Size
Three factors influence the textures of
igneous rocks: (1) the rate at which molten
rock cools; (2) the amount of silica present;
and (3) the amount of dissolved gases in the
magma. Among these, the rate of cooling
tends to be the dominant factor.
A very large magma body located many
kilometers beneath Earth’ s surface will cool
over a period of perhaps tens to hundreds of
thousands of years. Initially, relatively few
crystal nuclei form. Slow cooling permits
ions to migrate freely until they eventually
join one of the existing crystalline structures.
Consequently, slow cooling promotes the
growth of fewer but larger crystals.
On the other hand, when cooling
occurs rapidly—for example, in a thin lava
flow—the ions quickly lose their mobility
GEODe
ESSENTIALS
OF GEOLOGY
D I D Y O U K N O W ?
People have been making glass in
roughly the same way for at least 2000
years. The process involves melting
certain Earth materials and cooling the
liquid quickly before the atoms have
time to form an orderly crystalline
structure. This is the same way that
natural glass, called obsidian, is
generated from lava. It is possible to
produce glass from a variety of
materials, but most commercial glass is
produced from quartz sand and lesser
amounts of carbonate minerals.
67
Igneous Textures: What Can They Tell Us?