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110 Essentials of Geology
> Figure 4. 20 Bowens
> reaction series This
> diagram shows the
> sequence in which
> minerals crystallize from a
> basaltic magma. Compare
> this figure to the mineral
> composition of the rock
> groups in Figure 4. 12.
> Note that each rock group
> consists of minerals that
> crystallize in the same
> temperature range.
Temperature Sequence in which minerals crystallize from magma Composition
(rock types)
High temperatures
(~1200C)
Low temperatures
(~650C)
Olivine
Pyroxene
Amphibole
Biotite mica
> Discontinuous Series
> of Crystallization
Ultramafic
(peridotite/
komatiite) Calcium-
rich
> Plagioclase feldspar
> Continuous Series
> of Crystallization
Potassium feldspar
Muscovite mica
Quartz
Sodium-
rich
+
+
Mafic
(gabbro/basalt)
Intermediate
(diorite/andesite)
Felsic
(granite/rhyolite)
> Cooling magma
# B O W E N ' S R E A C T I O N S E R I E S
Geologists have observed that a single volcano may
extrude lavas that change in composition over time. Such
observations led to the idea that magma might change
over time (evolve) and thus that one magma body could
give rise to igneous rocks with a range of compositions.
To explore this idea, N. L. Bowen carried out a pioneer-
ing investigation early in the twentieth century into the
crystallization of magma.
# Bowens Reaction Series & the
# Composition of Igneous Rocks
Recall that ice freezes at a specific temperature,
whereas basaltic magma crystallizes over a range of at
least 200C of cooling (from about 1200 to 1000C).
In a laboratory setting, Bowen and his coworkers dem -
onstrated that as a basaltic magma cools, minerals tend
to crystallize in a systematic fashion, based on their
melting temperatures. As shown in Figure 4.20 , the first
mineral to crystallize is the ferromagnesian mineral
olivine. Further cooling generates calcium-rich plagio -
clase feldspar as well as pyroxene, and so forth down
the diagram.
During this crystallization process, the composi -
tion of the remaining liquid portion of the magma
# 4. 6 How Magmas Evolve
Describe how magmatic differentiation can generate a magma body that has a
mineralogy (chemical composition) that is different from its parent magma.
also continually changes. For example, at the stage
when about one-third of the magma has solidified, the
remaining molten material is nearly depleted of iron,
magnesium, and calcium because these elements are
major constituents of the minerals that form earliest
in the process. The removal of these elements causes
the melt to become enriched in sodium and potassium.
Further, because the original basaltic magma contained
about 50 percent silica (SiO 2), the crystallization of the
earliest-formed mineral, olivine, which is only about
40 percent silica, leaves the remaining melt richer in
SiO 2. Thus, the magma becomes progressively richer in
silica as it evolves.
Bowen also demonstrated that when the crystals that
form in a magma remain in contact with the remaining
melt, then they (mainly their outer regions) continue to
exchange ions with the melt (react chemically with it).
As a result, the periphery of these mineral grains has a
different, more evolved composition than the interiors.
That is the significance of the arrows in Figure 4. 20.
Stated another way, minerals that remain in contact with
a melt gradually change composition to become the next
mineral in the series Bowen identified. This order of
mineral formation became known as Bowens reaction
series . However, in nature, the earliest-formed minerals
> M04_TARB6622_13_SE_C04.indd 110 11/11/16 12:58 PM
CHAPT ER 4 Igneous Rocks & Intrusive Activity 111
*Recent studies indicate that the Palisades Sill was produced
by multiple injections of magma and does not represent a
simple case of crystal settling. However, it is nonetheless an
instructional example of that process.
> Figure 4. 21 Crystal
> settling results in
> a change in the
> composition of the
> remaining magma A
> magma evolves as the
> earliest-formed minerals
> (those richer in iron,
> magnesium, and calcium)
> crystallize and settle to
> the bottom of the magma
> chamber, leaving the
> remaining melt richer in
> sodium, potassium, and
> silica (SiO 2).
A. Magma having a mafic composition
erupts fluid basaltic lavas.
C. The remaining melt will be enriched
with silica, and should a subsequent
eruption occur, the rocks generated will
be more silica-rich and closer to the
felsic end of the compositional range
than the initial magma.
B. Cooling of the magma
body causes crystals of
olivine, pyroxene, and
calcium-rich plagioclase
to form and settle out, or
crystallize along the
magma bodys cool
margins.
Mafic
magma body
Fluid basaltic
lava flow
> Olivine
> Pyroxene
> Calcium-rich
> plagioclase
Explosive
eruption of
silica-rich magma
Magma Magma
Solid
rock
can separate from the melt, thus halting further chemical
reactions.
The diagram of Bowens reaction series in
Figure 4. 20 depicts the sequence in which minerals
crystallize from a magma of basaltic composition under
laboratory conditions. Evidence that this highly idealized
crystallization model approximates what can happen in
nature comes from analysis of igneous rocks. In particu-
lar, scientists know that minerals that form in the same
general temperature regime depicted in Bowens reaction
series are found together in the same igneous rocks. For
example, notice in Figure 4. 20 that the minerals quartz,
potassium feldspar, and muscovite, which are located in
the same region of Bowens diagram, are typically found
together as major constituents of the intrusive igneous
rock granite.
# Magmatic Differentiation & Crystal
# Settling
Bowen demonstrated that minerals crystallize from
magma in a systematic fashion. But how do Bowens
findings account for the great diversity of igneous rocks?
It has been shown that, at one or more stages during the
crystallization of magma, a separation of various com-
ponents can occur. One mechanism that causes this to
happen is called crystal settling . This process occurs
when the earlier-formed minerals are denser (heavier)
than the melt and sink toward the bottom of the magma
chamber, as shown in Figure 4.21 . When the remaining
melt solidifieseither in place or at another location, if
it migrates into fractures in the surrounding rocksit
will form a rock with a mineral composition that is more
felsic than the parent magma. The formation of a magma
body having a mineralogy or chemical composition that
is different than the parent magma is called magmatic
differentiation .
A classic example of magmatic differentiation is
found in the Palisades Sill, which is a 300-meter-
(1000-foot-) thick slab of dark igneous rock exposed
along the west bank of the lower Hudson River across
from New York City. Because of its great thickness and
consequent slow rate of solidification, crystals of oliv -
ine (the first mineral to form) sank and make up about
25 percent of the lower portion of the Palisades Sill.
By contrast, near the top of this igneous body, where
the last melt crystallized, olivine represents only
1 percent of the rock mass.*
# Assimilation & Magma Mixing
Bowen successfully demonstrated that through mag -
matic differentiation, a single parent magma can gen -
erate several mineralogically different igneous rocks.
However, more recent work indicates that magmatic
differentiation involving crystal settling cannot, by
itself, account for the entire compositional spectrum
of igneous rocks.
Once a magma body forms, the incorporation
of foreign material can also change its composition.
For example, in near-surface environments where rocks
Did You Know?
> The formation of the
> most common chemi-
> cal elements on Earth,
> such as oxygen, silicon,
> and iron, occurred bil-
> lions of years ago inside
> distant massive stars.
> Through various pro-
> cesses of nuclear fusion,
> these stars converted
> the lightest elements,
> mostly hydrogen, into
> these heavier elements.
> In fact, most such ele-
> ments found in the solar
> system, as well as in
> your body, are believed
> to have formed from
> debris scattered by stars
> that formed prior to the
> formation of the solar
> system.
> M04_TARB6622_13_SE_C04.indd 111 11/11/16 12:58 PM
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.