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Miami University Module 4 Igneous Rock Reading Summary
VideosVideo 2: Igneous Rock Textures
Video 3: Naming Igneous Rocks
.1 INTRODUCTION
All rocks found on the Earth are classified into one of three groups: igneous, sedimentary, or metamorphic. This rock classification is based on the origin of each of
these rock types, or if you prefer, based on the rock-forming process that formed the
rock. The focus of this chapter will be on igneous rocks, which are the only rocks that
form from what was once a molten or liquid state. Therefore, based on their mode
of origin, igneous rocks are defined as those rock types that form by the cooling of
magma or lava. You would be right in thinking that there is more to the classification
of igneous rocks than stated in the previous sentence, as there are dozens of different
igneous rocks that are considered commonplace, and dozens of more types that are
less common, and also quite a few igneous rock types that are quite scarce, yet each
igneous rock has a name that distinguishes it from all the rest of the igneous group of
rocks. So, if they all start out as molten material (magma or lava), which must harden
to form a rock, then it is logical to assume that these igneous rocks differ from one another primarily due to: 1) the original composition of the molten material from which
the rock is derived, and 2) the cooling process of the molten material that ended up
forming the rock. These two parameters define the classification of igneous rocks,
which are simplified into the two terms: composition and texture. Igneous rock composition refers to what is in the rock (the chemical composition or the minerals that
are present), and the word texture refers to the features that we see in the rock such
as the mineral sizes or the presence of glass, fragmented material, or vesicles (holes)
in the igneous rock.
8.1.1 Learning Outcomes
After completing this chapter, you should be able to:
• Classify igneous rock types based on color, texture, and mafic color index
• Identify, when possible, the minerals present in an igneous rock
• Determine the cooling history of the igneous rock
8 Igneous Rocks
Karen Tefend
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8.1.2 Key Terms
8.2 IGNEOUS ROCK ORIGIN
8.2.1 Magma Composition
It seems like a bad joke, but before any igneous rock can form, there must be
molten material known as magma produced, which means that you must first have
a rock to melt to make magma in order for it to cool and become an igneous rock.
Which brings more questions: what rock melted to form the magma? Was there
more than one rock type that melted to form that magma? Did the rocks completely melt, or did only certain minerals inside of those rocks melt (a process known as
partial melting)? Once that melted material formed, what happened to it next? Did
some other process occur to change the composition of that magma, before ending
up as the igneous rock that we are studying? These are just a few of the questions
that a person should consider when studying the origin of igneous rocks.
Most rocks (there are very few exceptions!) contain minerals that are crystalline solids composed of the chemical elements. In your chapter on minerals you
learned that the most common minerals belong to a group known as the silicate
minerals, so it makes sense that magmas form from the melting of rocks that most
likely contain abundant silicate minerals. However all minerals (not just the silicates) have a certain set of conditions, such as temperature, at which they can
melt. Since rocks contain a mixture of minerals it is easy to see how only some of
the minerals in a rock may melt, and why others stay as a solid. Furthermore, the
temperature conditions are important, as only minerals that can melt at “lower”
temperatures (such as 600°C) may experience melting, whereas the temperature
would have to increase (for example, to 1200°C) in order for other minerals to also
melt (remember the lower temperature minerals are still melting) and thus add
their chemical components to the magma that is being generated. This brings up
an important point: even if the same types of rocks are melting, we can generate
different magma compositions purely by melting at different temperatures!
Once magma is generated, it will eventually start to rise upward through the
Earth’s lithosphere, as magma is more buoyant than the source rock that generat-
• Aphanitic
• Extrusive
• Felsic (Silicic)
• Ferromagnesian
• Glassy
• Intermediate
• Intrusive
• Mafic
• Nonferromagnesian
• Phaneritic
• Phenocryst
• Plutonic
• Porphyritic
• Ultramafic
• Vesicular
• Volcanic
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ed it. This separation of the magma from the source region will result in new thermal conditions as the magma moves away from the heated portion of the lithosphere and encounters cooler rocks, which results in the magma also cooling. As
with melting, minerals also have a certain set of conditions at which they form, or
crystallize, from within a cooling magma body. You would be right in thinking that
the sequence of mineral crystallization is the opposite sequence of crystal melting.
The sequence of mineral formation from magma has been experimentally determined by Norman L. Bowen in the early 1900’s, and the now famous Bowen’s reaction series appears in countless textbooks and lab manuals (Figure 8.1).
This “reaction series” refers to the chemical reactions that are the formation
of minerals, through chemical bonding of elements within the magma, in a sequence that is based on falling magma temperatures. Close examination of Figure
8.1 shows that the first mineral to crystallize in a cooling magma of ultramafic
composition is olivine; the length of the arrow indicates the range of temperatures
at which olivine can form. Once temperatures fall below this range, olivine crystals
will no longer form; instead, other minerals such as pyroxene will start to crystallize
(a small interval of temperatures exists where both olivine and pyroxene can crystallize). Minerals that form in cooling magma are called crystals, or phenocrysts; as
Figure 8.1 | Bowen’s Reaction Series, showing the progression of mineral crystallization as magma temperatures
drop from ~1400 °C to ~500 °C. Note the corresponding names for igneous rock composition (underlined and
bolded) and some example rock types within each compositional group.
Author: Karen Tefend
Source: Original Work
License: CC BY-SA 3.0
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INTRODUCTORY GEOLOGY IGNEOUS ROCKS
these phenocrysts are forming, they are removing chemical elements from the magma. For example, olivine phenocrysts take magnesium (Mg) and iron (Fe) from the
magma and incorporate them into their crystal structure. This behavior of mineral
phenocrysts to take certain chemical elements into their structure, while excluding
other elements, means that the composition of the magma must be changing as
phenocrysts are forming!
There can be more than one mineral type crystallizing within the cooling magma, as the arrows in Figure 8.1 demonstrate. The minerals on the left side of Bowen’s reaction series are referred to as a discontinuous series, as these minerals
(olivine, pyroxene, amphibole, and biotite) all remove the iron (Fe), magnesium
(Mg), and manganese (Mn) from the magma during crystallization, but do so at
certain temperature ranges. These iron- and magnesium-rich minerals are referred
to as ferromagnesian minerals (ferro = iron) and are usually green, dark gray, or
black in color due to the absorption of visible light by iron and magnesium atoms.
On the right side of Bowen’s reaction series is a long arrow labelled plagioclase
feldspar. Plagioclase crystallizes over a large temperature interval and represents
a continuous series of crystallization even though its composition changes from
calcium (Ca) rich to sodium (Na) rich. As the magma temperature drops and plagioclase first starts to crystallize (form), it will take in the calcium atoms into the
crystal structure, but as magma temperatures continue to drop, plagioclase takes
in sodium atoms preferentially. As a result, the higher temperature calcium-rich
plagioclase is dark gray in color due to the high calcium content, but the lower temperature sodium-rich plagioclase is white due to the high sodium content. Finally,
at the bottom of the graph in Figure 8.1, we see that three more minerals can form
as temperatures continue to drop. These minerals (potassium feldspar, muscovite,
and quartz) are considered to be the “low temperature minerals”, as they are the last
to form during cooling, and therefore first to melt as a rock is heated. The previous
removal of iron and magnesium from the magma results in the formation of the latest-forming minerals that are deficient in these chemical elements; these minerals
are referred to as nonferromagnesian minerals, which are much lighter in color.
For example, the potassium-rich feldspar (also known as orthoclase) can be a pale
pink or white in color. The references to mineral color are necessary, as the color of
any mineral is primarily due to the chemical elements that are in the minerals, and
therefore the color of an igneous rock will be dependent on the mineral content (or
chemical composition) of the rock.
8.3 IGNEOUS ROCK COMPOSITION
Often added to the Bowen’s reaction series diagram are the igneous rock classifications as well as example igneous rock names that are entirely dependent on
the minerals that are found in them. For example, you can expect to find abundant
olivine, and maybe a little pyroxene and a little Ca-rich plagioclase, in an ultramafic rock called peridotite or komatiite, or that pyroxene, plagioclase, and pos-
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sibly some olivine or amphibole may be present in a mafic rock such as gabbro or
basalt. You can also expect to see quartz, muscovite, potassium feldspar, and maybe a little biotite and Na-rich plagioclase in a felsic (or silicic) rock such as granite or rhyolite. Figure 8.1 demonstrates nicely that the classification of an igneous
rock depends partly on the minerals that may be present in the rock, and since the
minerals have certain colors due to their chemical makeup, then the rocks must
have certain colors. For example, a rock composed of mostly olivine will be green
in color due to olivine’s green color; such a rock would be called ultramafic. A rock
that has a large amount of ferromagnesian minerals in it will be a dark-colored
rock because the ferromagnesian minerals (other than olivine) tend to be dark colored; an igneous rock that is dark in color is called a mafic rock (“ma-” comes from
magnesium, and “fic” from ferric iron). An igneous rock with a large amount of
nonferromagnesian minerals will be light in color, such as the silicic or felsic rocks
(“fel” from feldspar, and “sic” from silica-rich quartz). So, based on color alone,
we’ve been able to start classifying the igneous rocks.
In Figure 8.2 are examples of igneous rocks that represent the mafic and felsic
rock compositions (Figures 8.2A and 8.2C, respectively), as well as an intermediate rock type (Figure 8.2B). Notice that the felsic rocks can have a small amount of
dark-colored ferromagnesian minerals, but is predominately composed of light-colored minerals, whereas the mafic rock has a higher percentage of dark-colored
ferromagnesian minerals, which results in a darker-colored rock. A rock that is
considered intermediate between the mafic and felsic rocks is truly an intermediate in terms of the color and mineral composition; such a rock would have less
ferromagnesian minerals than the mafic rocks, yet more ferromagnesian minerals
than the felsic rocks.
As previously mentioned, classifying rocks into one of the igneous rock compositions (ultramafic, mafic, intermediate, and felsic) depends on the minerals that
each rock contains. Identification of the minerals can be difficult in rocks such as
Figure 8.2 | Examples of igneous rocks from the mafic (A), intermediate (B), and felsic (C) rock compositions. Photo
scale on bottom is in centimeters.
Author: Karen Tefend
Source: Original Work
License: CC BY-SA 3.0
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INTRODUCTORY GEOLOGY IGNEOUS ROCKS
in Figure 8.2A, as the majority of minerals are dark in color and it can be difficult
to distinguish each mineral. An easy method of determining the igneous rock composition is by determining the percentage of dark-colored minerals in the rock,
without trying to identify the actual minerals present; this method of classification
relies on a mafic color index (MCI), where the term mafic refers to any dark gray,
black or green colored mineral (Figure 8.3). Igneous rocks with 0-15% dark colored minerals (or 0-15% MCI) are the felsic rocks (Figure 8.3A), igneous rocks with
46-85% MCI are the mafic rocks (Figure 8.3C), and igneous rocks with over 85%
MCI are considered ultramafic (Figure 8.3D). This means that any rock with an
intermediate composition or with a 16-45% MCI is an intermediate igneous rock
(Figure 8.3B). Estimating the percentage of dark-colored minerals is only possible
if the minerals are large enough to see; in that case a person can still recognize a
mafic rock by its dark-colored appearance, and a felsic rock by its light-colored
Figure 8.3 | Examples of how igneous rocks can be classified using the Mafic Color Index
(MCI), which is a visual classification based on the amount of ferromagnesian minerals in
the rock: (A) The small amount of tiny black phenocrysts (biotite) gives this rock a 0-15%
MCI value; (B) numerous dark phenocrysts (amphibole) gives this rock a 16-45% MCI value;
(C) this rock lacks visible phenocrysts, but the black color in this rock results in a 46-85%
MCI value; (D) this rock is entirely green in color due to the overwhelming amount of olivine.
Any rock with this much olivine is always classified as having over 85% MCI values.
Author: Karen Tefend
Source: Original Work
License: CC BY-SA 3.0
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INTRODUCTORY GEOLOGY IGNEOUS ROCKS
appearance. An intermediate rock will be somewhat lighter than a mafic rock, yet
darker than a felsic rock. Finally, an ultramafic rock is typically green in color, due to
the large amount of green-colored olivine in the rock. Such rocks that contain minerals that are too small to see are shown in Figure 8.4; note that you can still distinguish
between mafic (Figure 8.4A), intermediate (Figure 8.4B) and felsic (Figure 8.4C) by
the overall color of the rock. The intermediate igneous rock in Figure 8.4B does have
a few visible phenocrysts; this odd texture will be covered later in this chapter.
8.4 LAB EXERCISE
Part A – Igneous Rock Composition
Before attempting to answer the following questions, remove the eight rock
samples from the Igneous Rocks bag in your HOL rock kit (samples I1 – I8) and
place them on a clean sheet of white paper. Your samples should look identical to
the samples in Figure 8.5.
Separate the 8 samples into three groups based on their color (dark, light or intermediate). As a first attempt at classifying rock compositions based on color, most
students end up with three light colored rocks (felsic), two intermediate rocks, and
three dark colored rocks (mafic). There actually are only two mafic, two intermediate, two felsic, and two other rocks that we have yet to discuss, but for now leave your
samples as they are. Take a close look at the dark colored rocks in your mafic pile;
how are they different? When you hold them closer to a light source, notice how they
reflect light; one is dull looking, one has small shiny surfaces, and one is extremely
shiny and smooth. The one that is very smooth and shiny is not a mafic rock, even
though it is dark in color. This particular rock is an example of that rare rock type
that lacks minerals (crystals); instead, it is almost entirely made of glass, and is most
probably felsic in composition. Go ahead and move this rock to its own space on
your piece of paper. Now take a close look at your three light-colored felsic rocks.
One will have several minerals visible that you will see as different colors. The other
two rocks will be fairly uniform in color; one is composed of very tiny minerals that
Figure 8.4 | Examples of igneous rocks from the mafic (A), intermediate (B), and felsic (C) rock
compositions. Notice the difference in appearance between these rocks and those in Figure 8.2
Author: Karen Tefend
Source: Original Work
License: CC BY-SA 3.0
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INTRODUCTORY GEOLOGY IGNEOUS ROCKS
require a microscope to view them, and the other rock seems to be fragile and lightweight. This fragile appearing rock is another example of rock that is again almost
entirely glass with a very few, tiny phenocrysts; set this rock aside with your other
glassy rock. Now you should have four piles, with two rocks in each pile, and you can
now proceed with the following questions. You may want to refer to Figures 8.1
through 8.4 to help with identification.
8.5 IGNEOUS ROCK TEXTURE
The classification of igneous rocks is based not just on composition, but also
on texture. As mentioned earlier, texture refers to the features that we see in the
rock such as the mineral sizes or the presence of glass, fragmented material, or
vesicles (holes) in the igneous rock. We will cover mineral crystal sizes and vesicles in this section.
Since the crystals or phenocrysts form while the magma is cooling, then the
size of the crystals must have something to do with the cooling process. Recall
that each mineral derives its chemical composition directly from the magma, and
that each mineral has a certain temperature interval during which that particular
mineral can form. The chemical elements that become part of the mineral must
migrate from the liquid magma to link or bond with other elements in a certain
way to form the crystal structure that is unique for that mineral. What do you think
will happen if the magma’s temperature drops quickly, or if the magma’s temperature drops slowly? Either way, the time allowed for the migration of the chemical
elements to form a crystal is affected. When magma cools slowly, there is plenty of
time for the migration of the needed chemical elements to form a certain mineral;
that particular mineral can become quite large in size, large enough for a person to
see without the aid of a microscope. As a result, this igneous rock with its visible
minerals is said to have a phaneritic texture (phan = large). The rock samples
shown in Figure 8.2 are all phaneritic rocks. Figure 8.2A is a phaneritic mafic rock
called gabbro, Figures 8.2B and 8.3B are a phaneritic intermediate rock called diorite, and the rock in Figure 8.2C is a phaneritic felsic rock known as granite. If
you refer back to Figure 8.1 (Bowen’s reaction series) you will see that these rock
names are listed on the right side of the diagram.
Magma that cools relatively quickly will have the opposite result as described
above; there is less time for the migration of the chemical elements to form a min-
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eral, and as a result the minerals will not have time to form large crystals. Therefore, many small crystals of a particular mineral will form in the magma. Igneous
rocks that are composed of crystals too small to see (unless you have a microscope)
are called aphanitic igneous rocks. Figure 8.3C, 8.4A and 8.4C are aphanitic
rocks; because Figure 8.3C (and 8.4A) is dark in color, it is a mafic aphanitic rock
called basalt. The felsic rock in Figure 8.4C is called rhyolite. It is important to note
that basalt and gabbro are both mafic rocks and have the same composition, but
one rock represents a magma that cooled fast (basalt), and the other represents a
mafic magma that cooled slowly (gabbro). The same can be said for the other rock
compositions: the felsic rocks rhyolite and granite have identical compositions but
one cooled fast (rhyolite) and the other cooled slower (granite). The intermediate
rocks diorite (Figure 8.2B) and andesite (Figure 8.4B) also represent magmas that
cooled slow or a bit faster, respectively. Sometimes there are some visible crystals
in an otherwise aphanitic rock, such as the andesite in Figure 8.4B. The texture of
such a rock is referred to as porphyritic, or more accurately porphyritic-aphanitic since it is a porphyritic andesite, and all andesites are aphanitic. Two different
crystal sizes within an igneous rock indicates that the cooling rate of the magma
increased; while the magma was cooling slowly, larger crystals can form, but if the
magma starts to cool faster, then only small crystals can form. A phaneritic rock
can also be referred to as a porphyritic-phaneritic rock if the phaneritic rock contains some very large crystals (ie. the size of your thumb!) in addition to the other
visible crystals. In Figure 8.6 are two porphyritic rocks: a porphyritic-aphanitic
basalt, and a porphyritic-phaneritic granite.
Figure 8.6 | (A) An example of a porphyritic-aphanitic mafic rock with needle-shaped amphibole phenocrysts
(arrow points to one phenocryst that is 1cm in length); No other minerals in (A) are large enough to see. (B)
An example of a porphyritic-phaneritic felsic rock with large feldspars (outlined phenocryst is 3 cm length).
Surrounding these large feldspars are smaller (yet still visible) dark and light colored minerals.
Author: Karen Tefend
Source: Original Work
License: CC BY-SA 3.0
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INTRODUCTORY GEOLOGY IGNEOUS ROCKS
Sometimes the magma cools so quickly that there isn’t time to form any minerals as the chemical elements in the magma do not have time to migrate into any
crystal structure. When this happens, the magma becomes a dense glass called obsidian (Figure 8.7A). By definition, glass is a chaotic arrangement of the chemical
elements, and therefore not considered to be a mineral; igneous rocks composed
primarily of glass are said to have a glassy texture. The identification of a glassy
rock such as obsidian is easy once you recall the properties of glass; any thick glass
pane or a glass bottle that is broken will have this smooth, curve shaped pattern on
the broken edge called conchoidal fracture (this was covered in your mineral chapter). Even though obsidian is naturally occurring, and not man made, it still breaks
in this conchoidal pattern. If you look closely at the obsidian in Figure 8.7A, you will
see the curved (conchoidal) surfaces by noticing the shiny pattern on the rock. Obsidian appears quite dark in color regardless of its composition because it is a dense
glass, and light cannot pass through this thick glass; however, if the edges of the
obsidian sample are thin enough, you may be able to see through the glass.
In Figure 8.7B there is another igneous rock that is also composed primarily of
glass due to a very fast rate of magma cooling. This rock is called pumice, and is commonly referred to as the rock that floats on water due to its low density. The glass in
this rock is stretched out into very fine fibers of glass which formed during the eruptive phase of a volcano. Because these fibers are so thin, they are easy to break (unlike
the dense obsidian) and any conchoidal fractures on these fibers are too small to see
without the aid of a microscope. Pumice can have any composition (felsic to mafic),
but unlike obsidian the color of the pumice can be used to determine the magma
composition, as felsic pumice is always light in color and mafic pumice will be dark in
color. Mafic pumice with a dark grey, red or black color is also known as scoria.
