C H A P T E R
T H R E E
Igneous Rocks and Intrusive Activity
in Review
Igneous rocks form when magma cools and solidifies. Extrusive,
or volcanic, igneous rocks result when lava cools at the surface.
Magma that solidifies at depth produces intrusive, or plutonic,
igneous rocks.
As magma cools, the ions that compose it arrange themselves
into orderly patterns—a process called crystallization. Slow
cooling results in the formation of relatively large crystals.
Conversely, when cooling occurs rapidly, the outcome is a solid
mass consisting of tiny intergrown crystals. When molten
material is quenched instantly, a mass of unordered atoms,
referred to as glass, forms.
The mineral composition of an igneous rock is the consequence of the chemical make-up of the parent magma and the
environment of crystallization. Igneous rocks are divided into
broad compositional groups based on the percentage of dark and
light silicate minerals they contain. Felsic rocks (e.g., granite and
rhyolite) are composed mostly of the light-colored silicate minerals potassium feldspar and quartz. Rocks of intermediate composition, (e.g., andesite and diorite) are rich in plagioclase feldspar
and amphibole. Mafic rocks (e.g., basalt and gabbro) contain
abundant olivine, pyroxene, and calcium-rich plagioclase
feldspar. They are high in iron, magnesium, and calcium, low
in silica, and dark gray to black in color.
The texture of an igneous rock refers to the overall appearance
of the rock based on the size, shape, and arrangement of its mineral grains. The most important factor influencing the texture
of igneous rocks is the rate at which magma cools. Common
igneous rock textures include aphanitic (fine-grained), with grains
too small to be distinguished without the aid of a microscope;
phaneritic (coarse-grained), with intergrown crystals that are
roughly equal in size and large enough to be identified without
the aid of a microscope; porphyritic, which has larger crystals
(phenocrysts) embedded in a matrix of smaller crystals
(groundmass); and glassy.
The mineral make-up of an igneous rock is ultimately determined by the chemical composition of the magma from which it
crystallizes. N. L. Bowen discovered that as magma cools in the
laboratory, those minerals with higher melting points crystallize
before minerals with lower melting points. Bowen’s reaction series
illustrates the sequence of mineral formation within magma.
During the crystallization of magma, if the earlier-formed minerals are more dense than the liquid portion, they will settle to
the bottom of the magma chamber during a process called crystal
settling. Owing to the fact that crystal settling removes the earlierformed minerals, the remaining melt will form a rock with a
chemical composition that is different from the parent magma.
The process of developing more than one magma type from a
common magma is called magmatic differentiation.
Once a magma body forms, its composition can change
through the incorporation of foreign material, a process termed
assimilation, or by magma mixing.
Magma originates from essentially solid rock of the crust and
mantle. In addition to a rock’ s composition, its temperature,
depth (confining pressure), and water content determine whether
it exists as a solid or liquid. Thus, magma can be generated by
raising a rock’s temperature, as occurs when a hot mantle plume
“ponds” beneath crustal rocks. A decrease in pressure can cause
decompression melting. Further, the introduction of volatiles (water)
can lower a rock’ s melting point sufficiently to generate magma.
A process called partial melting produces a melt made of the
low-melting-temperature minerals, which are higher in silica
than the original rock. Thus, magmas generated by partial
melting are nearer to the felsic end of the compositional
spectrum than are the rocks from which they formed.
Intrusive igneous bodies are classified according to their shape
and by their orientation with respect to the country or host rock,
generally sedimentary or metamorphic rock. The two general
shapes are tabular (sheet-like) and massive. Intrusive igneous
bodies that cut across existing sedimentary beds are said to be
discordant; those that form parallel to existing sedimentary beds
are concordant.
Dikes are tabular, discordant igneous bodies produced when
magma is injected into fractures that cut across rock layers.
Nearly horizontal, tabular, concordant bodies, called sills, form
when magma is injected along the bedding surfaces of sedimentary rocks. In many respects, sills closely resemble buried lava
flows. Batholiths, the largest intrusive igneous bodies, sometimes
make up large linear mountains, as exemplified by the Sierra
Nevada. Laccoliths are similar to sills but form from less fluid
magma that collects as a lens-shaped mass that arches overlying
strata upward.
Some of the most important accumulations of metals, such as
gold, silver, lead, and copper, are produced by igneous processes.
The best-known and most important ore deposits are generated
from hydrothermal (hot-water) solutions. Hydrothermal deposits
are thought to originate from hot, metal-rich fluids that are remnants of late-stage magmatic processes. These ion-rich solutions
move along fractures or bedding planes, cool, and precipitate the
metallic ions to produce vein deposits. In a disseminated deposit
(e.g., much of the world’ s copper deposits), the ores from
hydrothermal solutions are distributed as minute masses
throughout the entire rock mass.
87
Chapter in Review
T H R E E
Igneous Rocks and Intrusive Activity
in Review
Igneous rocks form when magma cools and solidifies. Extrusive,
or volcanic, igneous rocks result when lava cools at the surface.
Magma that solidifies at depth produces intrusive, or plutonic,
igneous rocks.
As magma cools, the ions that compose it arrange themselves
into orderly patterns—a process called crystallization. Slow
cooling results in the formation of relatively large crystals.
Conversely, when cooling occurs rapidly, the outcome is a solid
mass consisting of tiny intergrown crystals. When molten
material is quenched instantly, a mass of unordered atoms,
referred to as glass, forms.
The mineral composition of an igneous rock is the consequence of the chemical make-up of the parent magma and the
environment of crystallization. Igneous rocks are divided into
broad compositional groups based on the percentage of dark and
light silicate minerals they contain. Felsic rocks (e.g., granite and
rhyolite) are composed mostly of the light-colored silicate minerals potassium feldspar and quartz. Rocks of intermediate composition, (e.g., andesite and diorite) are rich in plagioclase feldspar
and amphibole. Mafic rocks (e.g., basalt and gabbro) contain
abundant olivine, pyroxene, and calcium-rich plagioclase
feldspar. They are high in iron, magnesium, and calcium, low
in silica, and dark gray to black in color.
The texture of an igneous rock refers to the overall appearance
of the rock based on the size, shape, and arrangement of its mineral grains. The most important factor influencing the texture
of igneous rocks is the rate at which magma cools. Common
igneous rock textures include aphanitic (fine-grained), with grains
too small to be distinguished without the aid of a microscope;
phaneritic (coarse-grained), with intergrown crystals that are
roughly equal in size and large enough to be identified without
the aid of a microscope; porphyritic, which has larger crystals
(phenocrysts) embedded in a matrix of smaller crystals
(groundmass); and glassy.
The mineral make-up of an igneous rock is ultimately determined by the chemical composition of the magma from which it
crystallizes. N. L. Bowen discovered that as magma cools in the
laboratory, those minerals with higher melting points crystallize
before minerals with lower melting points. Bowen’s reaction series
illustrates the sequence of mineral formation within magma.
During the crystallization of magma, if the earlier-formed minerals are more dense than the liquid portion, they will settle to
the bottom of the magma chamber during a process called crystal
settling. Owing to the fact that crystal settling removes the earlierformed minerals, the remaining melt will form a rock with a
chemical composition that is different from the parent magma.
The process of developing more than one magma type from a
common magma is called magmatic differentiation.
Once a magma body forms, its composition can change
through the incorporation of foreign material, a process termed
assimilation, or by magma mixing.
Magma originates from essentially solid rock of the crust and
mantle. In addition to a rock’ s composition, its temperature,
depth (confining pressure), and water content determine whether
it exists as a solid or liquid. Thus, magma can be generated by
raising a rock’s temperature, as occurs when a hot mantle plume
“ponds” beneath crustal rocks. A decrease in pressure can cause
decompression melting. Further, the introduction of volatiles (water)
can lower a rock’ s melting point sufficiently to generate magma.
A process called partial melting produces a melt made of the
low-melting-temperature minerals, which are higher in silica
than the original rock. Thus, magmas generated by partial
melting are nearer to the felsic end of the compositional
spectrum than are the rocks from which they formed.
Intrusive igneous bodies are classified according to their shape
and by their orientation with respect to the country or host rock,
generally sedimentary or metamorphic rock. The two general
shapes are tabular (sheet-like) and massive. Intrusive igneous
bodies that cut across existing sedimentary beds are said to be
discordant; those that form parallel to existing sedimentary beds
are concordant.
Dikes are tabular, discordant igneous bodies produced when
magma is injected into fractures that cut across rock layers.
Nearly horizontal, tabular, concordant bodies, called sills, form
when magma is injected along the bedding surfaces of sedimentary rocks. In many respects, sills closely resemble buried lava
flows. Batholiths, the largest intrusive igneous bodies, sometimes
make up large linear mountains, as exemplified by the Sierra
Nevada. Laccoliths are similar to sills but form from less fluid
magma that collects as a lens-shaped mass that arches overlying
strata upward.
Some of the most important accumulations of metals, such as
gold, silver, lead, and copper, are produced by igneous processes.
The best-known and most important ore deposits are generated
from hydrothermal (hot-water) solutions. Hydrothermal deposits
are thought to originate from hot, metal-rich fluids that are remnants of late-stage magmatic processes. These ion-rich solutions
move along fractures or bedding planes, cool, and precipitate the
metallic ions to produce vein deposits. In a disseminated deposit
(e.g., much of the world’ s copper deposits), the ores from
hydrothermal solutions are distributed as minute masses
throughout the entire rock mass.
87
Chapter in Review