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112 Essentials of Geology
CONCEPT CHECKS 4. 6
1. Define Bowens reaction series.
2. How does the crystallization and settling of the
earliest formed minerals affect the composition
of the remaining magma?
3. Compare the processes of assimilation and
magma mixing.
> Figure 4. 23 Magma mixing This is one of the ways
> the composition of a magma body can change.
A. During the ascent of two chemically distinct magma bodies,
the more buoyant mass may overtake the slower rising body.
B. Once joined, convective flow mixes the two magmas,
generating a mass that is a blend of the two magma bodies.
Magma body A
Mixing
Magma body B
Recall from Bowens reaction series that rocks with a
granitic composition are composed of minerals with the
lowest melting (crystallization) temperaturesnamely,
quartz and potassium feldspar (see Figure 4. 20). Also
note that as we move up Bowens reaction series, the
minerals have progressively higher melting tempera-
tures, and that olivine, which is found at the top, has the
highest melting point. When a rock undergoes partial
melting, it forms a melt that is enriched in ions from
Recall that igneous rocks are composed of a mixture of
minerals and, therefore, tend to melt over a temperature
range of at least 200C. As rock begins to melt, the min-
erals with the lowest melting temperatures are the first
to melt. If melting continues, minerals with higher melt-
ing points begin to melt, and the composition of the melt
steadily approaches the overall composition of the rock
from which it was derived. Most often, however, melting
is not complete, a process known as partial melting .
# 4. 7 Partial Melting & Magma Composition
Describe how partial melting of the mantle rock peridotite can generate a basaltic
(mafic) magma.
> Figure 4. 22 Assimilation
> of the host rock by a
> magma body
As magma rises through Earths brittle upper crust, it may
dislodge and incorporate the surrounding host rocks. Melting
of these blocks, a process called assimilation , changes the
overall composition of the rising magma body.
Rising
magma
Host
rock
Fractures
are brittle, the magma pushing upward can cause the
overlying rock to fracture into numerous pieces. The
force of the injected magma is often sufficient to dislodge
and incorporate blocks of the surrounding host rock
(Figure 4.22 ). Melting of these blocks, a process called
assimilation , changes the overall chemical composition
of the magma body.
Another means by which the composition of magma
can be altered is called magma mixing . Magma mix -
ing may occur during the ascent of two chemically
distinct magma bodies as the more buoyant mass over -
takes the more slowly rising body ( Figure 4.23 ). Once
they are joined, convective flow stirs the two magmas,
generating a single mass that has an intermediate
composition.
> M04_TARB6622_13_SE_C04.indd 112 11/11/16 12:58 PM
CHAPT ER 4 Igneous Rocks & Intrusive Activity 113
minerals with the lowest melting temperatures, while the
unmelted portion is composed of minerals with higher
melting temperatures (Figure 4.24 ). Separation of these
two fractions yields a melt with a chemical composition
that is richer in silica and nearer the felsic (granitic) end
of the spectrum than the rock from which it formed. In
general, partial melting of ultramafic rocks tends to yield
mafic (basaltic ) magmas , partial melting of mafic rocks
generally yields intermediate (andesitic ) magmas , and
partial melting of intermediate rocks can generate felsic
(granitic ) magmas .
# Formation of Basaltic Magma
Most magma that erupts at Earths surface is basaltic
in composition and has a temperature range of 1000
to 1250C. Experiments show that under the high-
pressure conditions calculated for the upper mantle,
partial melting of the ultramafic rock peridotite can gen-
erate a magma of basaltic composition. Further evidence
that many basaltic magmas have a mantle source are the
inclusions of peridotite, a rock that basaltic magmas often
carry up to Earths surface from the mantle.
Basaltic (mafic) magmas that originate from partial
melting of mantle rocks are called primary or primitive
magmas because they have not yet evolved. Recall that
partial melting that produces mantle-derived magmas
may be triggered by a reduction in confining pressure
during the process of decompression melting. This can
occur, for example, where hot mantle rock ascends as
part of slow-moving convective flow at mid-ocean ridges
(see Figure 4. 18). Basaltic magmas are also generated at
subduction zones, where water driven from the descend-
ing slab of oceanic crust promotes partial melting of the
mantle rocks that lie above (see Figure 4. 19).
# Formation of Andesitic & Granitic Magmas
If partial melting of mantle rocks generates most basaltic
magmas, what is the source of the magma that crystal-
lizes to form andesitic (intermediate) and granitic (felsic)
rocks? Recall that silica-rich magmas erupt mainly along
the continental margins. This is strong evidence that
continental crust, which is thicker and has a lower density
than oceanic crust, must play a role in generating these
more highly evolved magmas.
One way andesitic magma can form is when a ris-
ing mantle-derived basaltic magma undergoes magmatic
differentiation as it slowly makes its way through the
continental crust. Recall from our discussion of Bowens
reaction series that as basaltic magma solidifies, the
silica-poor ferromagnesian minerals crystallize first. If
these iron-rich components are separated from the liquid
by crystal settling, the remaining melt has an andesitic
composition (see Figure 4. 20).
Andesitic magmas can also form when rising basaltic
magmas assimilate crustal rocks that tend to be rich in
silica. Partial melting of basaltic rocks is yet another way
in which at least some andesitic magmas are thought to
be produced.
Although granitic magmas can be formed through
magmatic differentiation of andesitic magmas, most gra -
nitic magmas probably form when hot basaltic magma
ponds (becomes trapped because of its greater density)
below continental crust ( Figure 4.25 ). When the heat
Vent
Conduit
Atmosphere
Continental
crust
Lithosphere
Asthenosphere
Magma
chamber
Partial melting of continental
crust generates magma with
a felsic composition.
Basaltic magma ponds
beneath less dense crustal
rocks.
Basaltic magma buoyantly
rises through lithospheric
mantle.
Partial melting of peridotite
generates basaltic magma.
> SmartFigure 4. 25
> Formation of granitic
> magma Granitic
> magmas are generated
> by the partial melting
> of continental crust.
ANIMATION
> https://goo.gl/Wd57Cw
Key
Partial melting of a
hypothetical rock
composed of the
minerals on Bowens
reaction series yields
two products.
A melt having
an intermediate
to felsic
composition.
An unmelted residue having
a mafic composition.
Olivine
Quartz
Plagioclase feldspar
Potassium feldspar
Pyroxene
Amphibole
> SmartFigure 4. 24
> Partial melting
> Partial melting generates
> amagma that is nearer
> the felsic (granitic) end
> of the compositional
> spectrum than the parent
> rock from which it was
> derived.
TUTORIAL
> https://goo.gl/xGLCN6
> M04_TARB6622_13_SE_C04.indd 113 11/11/16 12:58 PM
114 Essentials of Geology
emplacement of magma into preexisting rocks are
called intrusions or plutons . Because all intrusions
form far below Earths surface, they are studied pri -
marily after uplifting and erosion (covered in later
chapters ) have exposed them. The challenge lies in
reconstructing the events that generated these struc -
tures in vastly different conditions deep underground,
millions of years ago.
Intrusions are known to occur in a great variety of
sizes and shapes. Some of the most common types are
illustrated in Figure 4.26 . Notice that some plutons have
a tabular (tabula = table) shape, whereas others are
Although volcanic eruptions are occasionally violent and
spectacular events, most magma crystallizes at depth,
without fanfare. Therefore, understanding the igneous
processes that occur deep underground is as important
to geologists as studying volcanic events, which are the
focus of Chapter 5 .
# Nature of Intrusive Bodies
When magma rises through the crust, it forcefully
displaces preexisting crustal rocks, termed host rock
or country rock . The structures that result from the
# 4. 8 Intrusive Igneous Activity
Compare and contrast these intrusive igneous structures: dikes, sills, batholiths,
stocks, and laccoliths.
CONCEPT CHECKS 4. 7
1. Briefly describe why partial melting results in a
magma whose composition is different from that of
the rock from which it was derived.
2. How are most basaltic magmas thought to have
formed?
3. What is the process that is thought to generate most
granitic magmas?
from the hot basaltic magma partially melts the overly -
ing crustal rocks, which are silica rich and have a much
lower melting temperature, the result can be the produc -
tion of large quantities of granitic magmas. This process
is thought to have been responsible for the volcanic
activity in and around Yellowstone National Park in the
distant past.
A. Relationship between volcanism and intrusive igneous activity.
C. Extensive uplift and erosion exposed a batholith composed of several
smaller intrusive bodies (plutons).
B. Basic intrusive structures, some of which have been exposed by erosion.
Cinder
cones
Composite
cones
Fissure
eruption
Exposed portion of the
Sierra Nevada Batholith
Magma
chamber Solidified magma bodies
(plutons)
Solidified magma bodies
(plutons)
Magma
chamber
Laccolith
> Belinda Images/SuperStock
Volcanic
necks
Dike
Batholith Batholith
Sills Sills
Dikes
Conduit
Laccolith
Dikes Sill
SmartFigure 4. 26
Intrusive igneous
structures (Photo: Belinda
> Images/SuperStock)
ANIMATION