Chapter 11: Igneous Rocks
An igneous rock is any rock that has crystallized from a molten state. As we‟ll see, such rocks
form in a variety of environments.
Like almost every other aspect of geology, the formation of most igneous rocks can be linked to
plate tectonic processes. A couple of important terms you should learn right away are magma
(molten rock material in Earth‟s interior) and lava (molten rock material at Earth‟s surface).
For now, you can just skim the discussion of the rock cycle found at the beginning of this
chapter. The rock cycle is of vital importance, but we‟ll delay a detailed discussion of it until
later in the course.
Igneous rocks form in two basic places—at or near Earth‟s surface and in Earth‟s interior.
Igneous rocks that cool and crystallize at Earth‟s surface are termed, extrusive (or volcanic),
whereas those that cool and crystallize in Earth‟s interior are termed intrusive (or plutonic).
If igneous rocks are those that cool and crystallize from molten rock material (magma and lava),
where does such molten material come from?
Ultimately, from the melting of pre-existing rocks! So let‟s consider that conditions under which
rock in Earth‟s interior can be melted. Instead of reading this chapter in order, I recommend that
you skip ahead at this point to the section labeled HOW MAGMA FORMS. Once you‟re covered
this, I‟ll refer you back to the earlier part of this chapter.
At first thought, you might think that it‟s rather obvious how magma forms—by extreme heating
and melting of pre-existing rock. Surprisingly, adding heat energy to a rock mass is not the only
way, or even the most common way, that rock melts. It is one way, however. Figure 11.17
should convince you that Earth‟s interior is very hot--under some conditions, hot enough to melt
solid rock. When hot rock or magma works its way upward, due to convection (buoyant rise), for
example, the heat from the rising rock body can sometimes be sufficient to melt the surrounding
cooler rock that the rising, hot rock material comes into contact with. Although your book
doesn‟t use this term, think of melting due to the addition of heat energy as thermal melting.
A quick digression…
Heat and temperature are not the same thing. Heat is a form of energy, whereas temperature is a
measure of average molecular and atomic vibration.
Heat can be envisioned as the energy exchanged when a higher temperature and a lower temperature
object are brought into close contact. As the ice in your drink melts, heat energy flows from the higher
temperature liquid into the ice. The result? As the liquid loses heat energy to the ice, and its temperature
drops (i.e., it cools off…and you’ve got yourself a chilly, refreshing diet coke). As the ice absorbs heat
energy from the surrounding liquid, that energy is used to melt the ice.
Interestingly, when ice first melts, the temperature doesn’t drop; rather, ice at 0º centigrade is converted
into water at 0º centigrade; then, with additional input of thermal energy (heat), the temperature of the
water increases.
Temperature, on the other hand, is best thought of as the average vibrational motion (kinetic energy for
you physics buffs) that the atoms and molecules in a substance possess. Take your drink, for example.
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Some of the atoms in your diet coke are vibrating very fast, and others very slow, while most of the
atoms are vibrating at some intermediate speed. If, however, we were to generalize, we’d say that the
atoms in your diet coke are moving at some average speed. That average vibrational speed is
essentially what a thermometer measures—temperature.
Okay, back to rocks…
Another way to melt rock is to lower the pressure. This is called decompression melting. As
hot, buoyant rock rises, it experiences lower confining pressure, which, in turn, allows the rock
to expand and lowers its melting temperature, causing the rock to melt.
At divergent plate boundaries, as the plates move apart, the lithosphere is stretched and
thinned. This lowers the pressure (and therefore the melting temperature) of the underlying
mantle rock, allowing it to melt.
In the early days of plate tectonic theory, some researchers thought that the magma oozing out
of the mid-ocean ridge system forced the plates apart as it rose upward. Some websites on
plate tectonics still repeat this misconception.
Now we know, however, that the process of plate divergence is what generates such magma in
the first place, via decompression melting. Pull two plates apart, and the underlying rock
material in Earth‟s mantle will melt.
On the other hand, a rising blob of hot, buoyant rock beneath a plate will cause the plate to be
domed upward and thinned, as shown in Figure 11.25. Such doming and thinning eventually
fractures the plate, leading to lowered confining pressure underneath the plate, and finally, to
decompression melting.
A third important way to melt rock is to add pressurized water. We‟ll call this hydration
melting. As your book explains, pressurized water (specifically water vapor) breaks the siliconoxygen bonds in silicate minerals, causing the rock to liquefy. Surprisingly, only a small
percentage of water is required for this process to work effectively.
Can you think of a plate tectonic environment where hydration melting might be important?
Let‟s see, if I ask the question in a more specific way, the answer may become more obvious…
Can you think of a plate tectonic environment where large amounts of water could be dragged
down into Earth‟s interior, causing mantle rocks to melt via hydration melting? If a subduction
zone came to mind, congratulations, you‟re right! Please see me for an honorary “A” for this
chapter .
Let‟s illustrate these three melting mechanisms (thermal, decompression, and hydration melting)
with a graph (below):
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Increasing Temperature (T)
Increasing
Pressure (P)
1
Melting Curve
(wet granite)
2
4
(dr
y
Melting
Curve
to
(dry granite)
we
t)
Melting Curve
(dry basalt)
3
(
d
r
Melting Pathways:
y
t
Dry Basalt: 1 to 2 (increasing T @ constant P) = Thermal Melting
o
Dry Basalt: 3 to 2 (decreasing P @ constant T) = Decompression Melting
Dry Granite to Wet Granite: 4-dry to 4-wet (add water @ constant T & P) = Hydration
Melting
w
e
Melting mechanisms.
t)

In the above graph, the melting temperature of dry basalt and dry granite increases with increasing
pressure. That is, these rocks melt at higher and higher temperatures as pressure increases (i.e., as
one goes deeper into Earth‟s interior).

For each melting curve above, if you‟re on the left side of the line, the rock is solid (i.e., hasn‟t yet
melted), and if you‟re on the right side of the line, the rock is liquid (i.e., it‟s melted).

For example, at point 1, temperature and pressure conditions are such that dry basalt would be solid;
however, if we don‟t change the pressure and increase the temperature, the dry basalt eventually
melts as we cross from the left to the right side of the dry basalt melting curve, on our way to point 2.
We‟ve only increased the temperature moving from point 1 to point 2, thus, when our dry basalt
melts, it‟s via thermal melting.

Moving from point 3 for dry basalt (high temperature, high pressure) to point 2 (high temperature, low
pressure), our rock also melts, but not because we‟ve pumped thermal energy into it. Rather, the
rock melts because the melting temperature of the rock is reduced, NOT the actual temperature of
the rock itself. Thus, from point 3 to 2 the rock has melted not by heating, but by lowering the
pressure (decompression melting).

Finally, let‟s focus on point 4 in the graph above. At first, we have a hot piece of dry granite. Since
point 4 is to the left of the melting curve for dry granite, things are still solid. However, when we add
a small amount of water to the dry granite, neither the temperature nor the pressure of the granite
changes. Instead, the melting temperature of the granite is reduced.
So, our applicable melting curve for point 4 switches from the dry granite to the wet granite. As you
can see, point 4 is to the right of the melting curve for wet granite, and so our granite melts (hydration
melting).
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Your book also mentions that when mixed minerals are present, the melting temperature of the
various minerals can be lowered. Luckily for us, most igneous rocks consist of a mixture of
various silicate minerals, so the melting temperatures of most silicate minerals inside rocks are
lower together than they‟d be by themselves.
Before we introduce a classification scheme for common igneous rocks we need to develop
some background in terms of igneous rock and magma chemistry. You‟ll notice that in Figure
11.6, some igneous rocks (and also various magmas) are enriched in silicon and oxygen.
Silicon and oxygen are referred to collectively as silica. Igneous rocks and magmas enriched in
silica (silicon and oxygen) are termed felsic (or silicic). This is really just a made-up term we
get by combining the “fel” from feldspar with the “sic” from silica. So, fel + sic = silica.
Felsic rocks and magmas also tend to be enriched in sodium and potassium as well and also
tend to lack abundant calcium, magnesium, and iron. Igneous rocks and magmas that contain
medium amounts of various chemical constituents (silica, sodium, calcium, potassium, iron,
magnesium) are termed intermediate in composition. Igneous rocks and magmas that contain
abundant iron and magnesium but not very much silica, sodium, and potassium are termed
mafic. Here again, this is just a made-up term we get by combining the “ma” in magnesium with
the “fic” in iron…Er, wait a minute, there‟s no “fic” in iron, so what‟s going on? Well, one form of
iron is known as ferric iron (Fe 3+), so we‟re cheating a bit, but you get the idea. If very high
amounts of iron and magnesium and very low amounts of silica are present, such rocks and
magmas are termed ultramafic. Think of ultramafic rocks as mafic rocks on steroids.
Knowing the chemical/temperature trends associated with felsic, intermediate, and mafic
igneous rocks is critical (hint, hint)! This is something that would be good to memorize.
Here‟s a summary chart to help you in this effort:
Magma/Rock Composition
Mafic
Intermediate
Explanation
Felsic
SiO2 = silica
K = potassium
Increasing SiO2, K, Na
Na = sodium
Ca = calcium
Increasing Ca, Fe, Mg
Fe = iron
Increasing crystallization temperature
Mg = magnesium
Composition and temperature variation in magma and igneous rock.
Let‟s summarize:

Felsic rocks: enriched (high) in silica, potassium, and sodium; depleted (low) in calcium, iron,
magnesium.
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
Intermediate rocks: moderate amounts of silica, potassium, sodium, calcium, iron, and magnesium.

Mafic rocks: enriched in iron, magnesium, calcium; depleted in silica, potassium, sodium.

Ultramafic rocks: same trends as for mafic rocks, but even more so.
You might be wondering how to tell at a glance whether a rock is felsic, intermediate, mafic, or
ultramafic. Simply put, by the minerals!
As we‟ll learn below, some minerals are enriched in silica (quartz, K-spar, and some types of
plagioclase, for example), whereas some lack abundant silica (for example, olivine and
pyroxene).
It also turns out that silica-rich minerals are generally light colored, non-ferromagnesian
silicates, whereas silica-poor minerals are dark colored, ferromagnesian silicates.
So…in general, light-colored igneous rocks tend to be felsic, medium-colored rocks tend to be
intermediate, and dark-colored rocks tend to be mafic or ultramafic.
Moving on to igneous rock classification, think of things this way: the most common rock-forming
minerals are silicates, which are enriched in silicon and oxygen. Therefore, it makes sense, in a
way, that we‟d use the silica content of an igneous rock or magma as part of the basis for
classifying it based on composition.
If you know something about the composition of an igneous rock (i.e., whether it‟s a felsic,
intermediate, or mafic rock), then the only other thing you need to know is whether the rock
formed intrusively (in Earth‟s interior) or extrusively (at Earth‟s surface).
Most of the time it‟s quite easy to distinguish between intrusive and extrusive rocks. Intrusive
rocks cool very slowly, deep underground. This allows a long time for minerals to form, which
means that they can grow large enough to be seen with the naked eye.
Remember, a mineral can only be assembled one atom at a time, so the longer the cooling time
available, the larger the mineral can become. In contrast, extrusive rocks cool quickly, which
doesn‟t allow much time for the minerals in the rock to grow very large.
So, extrusive igneous rocks are characterized by small, fine-grained minerals usually no larger
than 1 millimeter and often invisible to the naked eye.
Intrusive igneous rocks are characterized by larger, visible crystals, typically larger than 1
millimeter and easily visible to the naked eye.
The top of Figure 11.6 puts this all together to create a classification scheme for the seven most
common types of igneous rocks. In general, you can think of “course grained” as equivalent to
“intrusive” and fine-grained as equivalent to “extrusive”, although there are exceptions to this
rule that we won‟t worry about in this course.
The top two rows of Figure 11.6 are VERY IMPORTANT, with the exception of komatiite, which is
very rare. All I have to say about these top two rows is this: know „em--learn „em--live „em!
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If given information about the composition of an igneous rock (felsic, intermediate, mafic, or
ultramafic) and also the size of the minerals in the rock (fine-grained or coarse-grained), make
sure that you can name (classify) the rock.
Maybe now you can now appreciate why it‟s important not to take all igneous rocks “for granite”
…
Here‟s a summary chart:
Origin
Intrusive
Extrusive
Felsic
Granite
Rhyolite
Intermediate
Diorite
Andesite
Mafic
Gabbro
Basalt
Ultramafic
Peridotite
n/a
Rock Photos
Here are some photos of the igneous rocks listed in the table above:
K-spar (pink)
Quartz (light gray)
Biotite (black)
Granite.
(photo courtesy of USGS; http://geomaps.wr.usgs.gov/socal/geology/transverse_ranges/index.html)
Notice the overall light color of granite (above) and the abundance of K-spar and quartz.
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Rhyolite (courtesy Wikipedia: http://en.wikipedia.org/wiki/Rhyolite).
Like granite, rhyolite is typically light colored due to an abundance of non-ferromagnesian
(iron/magnesium-poor) minerals like quartz and feldspar. Rhyolite is fine-grained due to its
extrusive origin.
Plagioclase feldspar
(white)
Dark-colored
ferromag minerals
(e.g., hornblende, augite)
Diorite (photo courtesy of USGS; http://libraryphoto.cr.usgs.gov/).
Diorite (photo above) contains approximately equal amounts of dark-colored ferromagnesian
minerals and light-colored plagioclase feldspar.
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Andesite (courtesy of USGS; http://volcanoes.usgs.gov/images/pglossary/andesite.php).
Andesite (shown above) is the extrusive equivalent of diorite. Notice that this rock is finer
grained than diorite, although it has about the same overall composition. Some of the minerals
in the above photo are visible with the naked eye, but most are not.
Plagioclase feldspar
(white)
Dark-colored
ferromag minerals
(e.g., hornblende, augite)
Gabbro (courtesy of Wikipedia; http://en.wikipedia.org/wiki/Gabbro).
In this photo of gabbro (above), the “grainy” appearance is due to an abundance of minerals
large enough to see with the naked eye. Although this rock contains a mixture of
ferromagnesian and non-ferromagnesian minerals, the ferromags are more abundant, giving
gabbro a dark color overall.
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Basalt (courtesy of Wikipedia; http://en.wikipedia.org/wiki/Basalt).
In the above photo of basalt, the small holes are vesicles—solidified holes created by escaping
gases as the rock was crystallizing. Although this rock is too fine grained to distinguish
individual minerals, the abundance of ferromagnesian minerals like olivine and augite give the
rock a dark overall color.
Pyroxene
(black)
Olivine
(light brown)
Peridotite.
Peridotite (above photo) is a relatively rare rock within Earth‟s crust; however, it‟s the most
common rock in the upper mantle. Peridotite is an ultramafic intrusive igneous rock composed
almost exclusively of olivine and pyroxene.
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There are several other common igneous rocks, all extrusive, that don‟t fit neatly into the above
table, including obsidian (volcanic glass), pumice (highly vesicular obsidian), tuff (lithified
volcanic ash), and volcanic breccia (composed of volcanic rock fragments). These rocks are
discussed in Chapter 10 of your text.
There is one short paragraph in the section of your text entitled ABUNDANCE AND
DISTRIBUTION OF PLUTONIC ROCKS that nicely summarizes the “big picture”:
Granite, then, is the predominant igneous rock of the continents…basalt and gabbro are the
predominant rocks underlying the oceans. Andesite (usually along continental margins) is
the building material of most young volcanic mountains. Underneath the crust, ultramafic
rocks make up the upper mantle.
I couldn‟t have said it better myself!
Bowen’s Reaction Series (Figure 11.20 in your text)
This is a really important diagram! In summary, Bowen developed this diagram after
much experimental work—he actually melted and crystallized rock in his laboratory!
Unfortunately, he used liquid mercury to cool his experimental magmas and later in life
suffered the effects of mercury poisoning from breathing those nasty vapors…bummer!
Bowen‟s Reaction Series is really just a summary diagram that tells us the
crystallization order of various silicate minerals and mineral groups in a cooling magma.
Here‟s a sketch:
Intermediate
Olivine
Pyroxene
Amphibole
Continuous Branch
Mafic
Discontinuous Branch
Low silica/high Temp.
(≥1200ºC)
Biotite
Felsic
Ca Plagioclase
Ca/Na Plag
Na Plagioclase
Potassium
Feldspar
Muscovite
Quartz
High silica/low Temp.
(~600ºC)
Bowen’s Reaction Series
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A number of important points apply in regard to Bowen‟s Reaction Series:
1. The discontinuous branch of Bowen‟s Reaction Series consists of ferromagnesian silicates,
whereas the continuous branch applies to non-ferromagnesian silicates.
2. In the discontinuous branch (left side), silicate structures change as one mineral or mineral
group stops crystallizing and another starts as the temperature drops. As the magma cools,
we go from isolated tetrahedra (olivine) to single-chain silicates (pyroxenes) to double-chain
silicates (amphiboles) to sheet-silicates (biotite).
3. A subtle feature of the discontinuous branch is that when one mineral or mineral group
stops crystallizing, the previously formed minerals react with the remaining magma to
convert themselves into the lower-temperature silicate structure. For example, olivine is the
first mineral to crystallize in a cooling mafic magma. However, below a certain temperature,
olivine will stop crystallizing. Also (and this is strange…), the pre-formed olivine will begin to
react with the magma to convert itself into pyroxene.
4. In the continuous branch (right side), the silicate structure doesn‟t change as progressively
lower-temperature plagioclase forms. The composition of the plagioclase crystals does
change, however. At relatively high temperature, the first plagioclase to crystallize is rich in
calcium. As the magma cools, plagioclase keeps crystallizing, and more and more of the
calcium gets pulled out of the magma and locked up into plagioclase crystals. Pretty soon,
there‟s not very much calcium left, and the plagioclase that crystallizes at lower
temperatures is instead sodium-rich. Notice, too, that the silicate structure (in this case, a 3D framework structure) doesn‟t change along the continuous branch.
5. Check out this animation of Bowen’s Reaction Series (click on Figure 11.19)
6. High-temperature minerals and mineral groups include olivine, pyroxene, and calcium-rich
plagioclase.
7. Medium-temperature minerals and mineral groups include amphibole and plagioclase
containing roughly equal amounts of sodium and calcium.
8. Low-temperature minerals include muscovite, K-spar, and quartz. These are the very last
minerals to crystallize.
9. As we move from high-temperature to low-temperature minerals, the silica content of the
minerals increases. For example, olivine and pyroxene—although silicate minerals—don‟t
contain very much silica. In contrast, quartz, muscovite, and K-spar are enriched in silica.
10. We never find ALL of the minerals listed in Bowen‟s Reaction Series in a single rock.
Instead, we can relate Bowen‟s Reaction Series in a general way to igneous rock
composition. For example, mafic rocks tend to contain olivine, pyroxene, and calcium-rich
plagioclase. Intermediate rocks tend to contain some pyroxene (sometimes), amphibole,
biotite, and plagioclase enriched in both calcium and sodium. Felsic rocks tend to contain
biotite, muscovite (sometimes), sodium-rich plagioclase, K-spar, and quartz.
11. Bowen‟s Reaction Series is very useful because it allows geologists to predict which
minerals should occur together in igneous rocks of different composition. For example,
we‟d expect to find abundant olivine and pyroxene together in mafic igneous rocks, since
both minerals form at high temperature. However, we‟d almost never find abundant quartz
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and olivine together in the same rock, because these minerals crystallize as such different
temperatures.
You should become very familiar with Bowen‟s Reaction Series. That is, you should
learn the overall crystallization order of the minerals by heart (Arrrgh…something else
to memorize!).
Another reason Bowen‟s reaction series is so important is that it gives us clues as to
how magmas and igneous rocks of variable composition can be produced.
Instead of cooling and crystallizing a magma as a single mass, let‟s imagine that we
somehow separate the early-formed, silica-poor crystals from the rest of the magma.
One way for this to happen is when the early olivine and pyroxene crystals settle to the
bottom of a magma chamber (Figure 11.21).
Think of it this way: if we separate out the early, silica-depleted crystals from the
remaining magma, how will the silica content of the magma change? In essence, the
silica content of the remaining magma will increase if we pull out the silica-poor
components. This crystal settling process is an example of what your book refers to as
differentiation (when different ingredients separate out of an originally homogenous
mixture). Here‟s a diagram for clarification:
Cooling MAFIC MAGMA (red)
Early-formed, low-silica olivine
crystals (green) sink to bottom
of magma chamber…
Removal of low-silica component
(olivine) produces a residual magma
(liquid) enriched in silica
Silica-enriched magma cools
intrusively to form INTERMEDIATE
igneous rock (e.g., diorite)
Low-silica olivine “mush” forms at
bottom of magma chamber…
Olivine “mush” cools intrusively to form
ULTRAMAFIC igneous rock (e.g., peridotite).
Magma differentiation by crystal settling.
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To summarize the above diagram, an initially mafic magma differientiates into an
intermediate magma and an ultramafic magma via crystal settling. In other words,
magmas and igneous rocks of variable composition can be derived from a single parent
magma.
Other ways in which magmas of variable composition can be produced include partial
melting (when only some of the minerals in a rock melt), assimilation (when a hot
magma melts and absorbs surrounding rock), and magma mixing (combination of two
magmas).
In partial melting, we just run Bowen‟s Reaction Series in reverse, so that the first
crystals to melt produce a magma higher in silica content than the parent rock.
In the context of igneous rocks, assimilation occurs when hot magma melts and
absorbs the surrounding rock up through which it is intruding. Imagine a very hot, mafic
magma melting felsic rock on its way up toward the surface. If enough felsic rock melts
and gets incorporated into the mafic magma, the silica content of the mafic magma can
increase to the point that it becomes intermediate in composition.
Magma mixing occurs when two magmas combine. Imagine what we‟d get if a mafic
and a felsic magma were to mix in roughly equal proportions…intermediate magma!
In terms of the “big picture”, Table 11.2 nicely summarizes the various magmas
generated in different plate tectonic environments.
Let‟s simplify:

Partial melting of ultramafic, mantle rock will create mafic magmas. Whenever you find a
mafic igneous rock, you can be virtually certain the parent magma from which the rock
crystallized originated in Earth‟s mantle.

On the other hand, felsic magmas originate within Earth‟s crust—the only place where we
find parent rocks high enough in silica to create felsic magmas upon melting.

Basalt and gabbro tend to form at divergent plate boundaries and in plate interiors.

Andesite and diorite typically form near subduction zones. Granite and rhyolite tend to form
at subduction zones and in plate interiors.
If you‟ve made it this far (and have put up with my bad humor), good work! On to
volcanic rocks!
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