Chapter 3
Igneous Rocks, Intrusive Activity,
and the Origin of Igneous Rocks
Index
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Picture on pg. 53
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Igneous Rocks

Igneous Rocks – A rock formed or apparently formed from
solidification of magma.

Igneous rocks may be either extrusive if they form at the earth’s
surface (e.g., basalt) or intrusive if magma solidifies
underground.
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How do we know?

Unlike the volcanic rock in Hawaii, nobody has ever seen
magma solidify into intrusive rock. So what evidence suggests
that bodies of granite (and other intrusive rocks) solidified
underground from magma?
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Proof I
1.
2.
3.
4.
Mineralogically and chemically, intrusive rocks are essentially
identical to volcanic rocks.
Volcanic rocks are fine-grained.
Experiments have confirmed that most of the minerals in these
rocks can form only at high temperatures. More evidence comes
from examining intrusive contacts, such as shown in Fig. 3.1 and
Fig. 3.2. (A contact is a surface separating different rock types.
Preexisting solid rock, country rock, appears to have been
forcibly broken by an intruding liquid, with the magma flowing
into the fractures that developed. Country rock is an accepted
term for any older rock into which an igneous body intruded.
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Proof II
5.
6.
7.
Close examination of the country rock immediately adjacent to
the intrusive rock usually indicates that it appears “baked” close
to the contact with the intrusive rock.
Rock types of the country rock often match xenoliths, fragments
of rock that are distinct from the body of igneous rocks in which
they are enclosed.
In the intrusive rock adjacent to contacts with country rock are
chill zones, finer-grained rocks that indicate magma solidified
more quickly here because of the rapid loss of heat to cooler heat.
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Different Types of Igneous Rocks
Fine-grained rocks – A rock in which most of the
mineral grains are less than one millimeter
across (igneous) or less than 1/16 mm
(sedimentary).
 Plutonic rocks – Igneous rock formed at great depth.
 Coarse-grained rocks –Rock in which most of the
grains are larger than 1 millimeter (igneous) or
2 millimeters (sedimentary). (Fig. 3.3)

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Identification of Igneous Rocks

Igneous rock names are based on texture (notably grain
size) and mineralogical composition (which reflects
chemical composition). Mineralogically (and chemically)
equivalent rocks are granite-rhyolite, diorite-andesite, and
gabbro-basalt. The relationships between igneous rocks
are shown in Fig. 3.4.
Table 3.1
Fig. 3.5
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Intrusive Bodies

Intrusions, or intrusive structures, are bodies of intrusive rock
whose names are based on their size and shape, as well as their
relationship to surrounding rocks. They are important aspects of
the architecture, or structure, of the earth’s crust. The various
intrusions are named and classified on the basis of the following
considerations: (1) Is the body large or small? (2) Does it have a
particular geometric shape? (3) Did the rock form at a
considerable depth or was it a shallow intrusion? (4) Does it
follow layering in the country rock or not?
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
Volcanic neck
 A volcanic
neck is an intrusive structure
apparently formed from magma that solidified
within the throat of a volcano. One of the best
examples is Ship Rock in New Mexico. (Fig. 3.6)
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Dikes and Sills




Dike – A tabular, discordant intrusive structure.
Fig. 3.7
Fig. 3.8
Discordant – Not parallel to any layering or parallel planes.
Sill – A tabular intrusive concordant with the country rock.
Fig. 3.9
Concordant – Parallel to layering or earlier developed planar
structures.
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Intrusives That Crystallize at
Depth
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


Pluton – An igneous body that crystallize deep underground.
Stock – A small discordant pluton with an outcropping area of
less than 100 square kilometers.
Batholith – A large discordant pluton with an outcropping area
greater than 100 square kilometers. (Fig. 3.10)
Diapir – Bodies of rock (e.g., rock salt) or magma that ascend
within the earth’s interior because they are less dense than the
surrounding rock. (Fig. 3.11)
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Geothermal Gradient
Geothermal Gradient – The rate at which
temperature increases with increasing
depth beneath the surface It is, on the
average, to be about 3oC for each 100
meters (30oC/km) of depth in the upper
part of the crust.
 Fig. 3.13

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Factors That Control Melting
Temperatures
Pressure
The melting point of a mineral generally increases with increasing pressure. Pressure
increases with depth in the earth’s crust, just as temperature does.
Water Under Pressure
If enough gas, especially water vapor, is present and under high pressure, a dramatic
change occurs in the melting process. Water vapor sealed in under high pressure by
overlying rocks helps break down crystal structures. High water pressure can
significantly lower the melting points of minerals. (Fig. 3.14)
Effect of Mixed Minerals
Two metals – as in solder – can be mixed in a ratio that lowers their melting
temperature far below that of the melting points of the pure metals. (Fig. 3.15)
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Differentiation and Bowen’s
Reaction Theory
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
Differentiation is the process by which different ingredients
separate from an originally homogenous mixture. In the early part
of the twentieth century, N. L. Bowen conducted a series of
laboratory experiments demonstrating that differentiation is a
plausible way for silicic and mafic rocks to form from a single
parent magma.
Bowen’s reaction series is the sequence in which minerals
crystallize from a cooling magma, as demonstrated by Bowen’s
laboratory experiments. In simplest terms, Bowen’s reaction series
shows that those minerals with the highest melting temperatures
crystallize from the cooling magma before those with lower melting
points. However, the concept is a bit more complicated than that.
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Crystallization

Crystallization begins along two branches, the discontinuous
branch and the continuous branch. In the discontinuous branch,
one mineral changes to another at discrete temperatures during
cooling and solidification of the magma. Changes in the
continuous branch occur gradationally through a range in
temperatures and affect only the one mineral, plagioclase.
Crystallization takes lace simultaneously along both branches.
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Assimilation

A very hot magma may melt some of the country rock
and assimilate the newly molten material into the magma
(Fig. 3.18) This is like putting a few ice cubes into a cup
of hot coffee. The ice melts and the coffee cools as it
becomes diluted.
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Mixing of Magmas

If two magmas meet and merge within the crust, the
combined magma will be compositionally intermediate.
(Fig. 3.19)
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Fig. 3.22
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Pg. 71
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Fig. 3.23
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Pg. 72
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Back to the Beginning
Fig. 3.1
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Pg. 54
Back
Fig. 3.2
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Pg. 54
Back
Fig. 3.3
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Pg. 55
Back
Fig. 3.4
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Pg. 56
Back
Table 3.1
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Pg. 56
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Fig. 3.5
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Pg. 57
Back
Fig. 3.6
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Pg. 60
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Fig. 3.7
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Pg. 61
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Fig. 3.8
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Pg. 61
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Fig. 3.9
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Pg. 62
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Fig. 3.10
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Pg. 62
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Fig. 3.11
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Pg. 63
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Fig. 3.13
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Pg. 64
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Fig. 3.14
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Pg. 65
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Fig. 3.15
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Pg. 66
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Bowen’s Reaction Series
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Pg. 3.16
Back
Fig. 3.18
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Pg. 69
Back
Fig. 3.19
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Pg. 69
Back