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Name:
Lab Period:
Lab 6: Igneous Rocks and Processes
Geologic material is constantly being recycled into different compositions and textures
through a set of processes known as the rock cycle, illustrated below. The three major types of
rock are igneous, sedimentary, and metamorphic. These rock types differ in both (1) source
material and (2) the processes leading to their formation, both of which are constrained by the
paths shown on the cycle. Therefore, identifying a rock’s type will enable us to infer the general
history of a specimen. Note that rocks do not necessarily follow the path around the outside of
the circle; they can follow any of the short-circuits from one stage to another. As will be shown
in the next several labs, a more precise knowledge of a rock’s history may be obtained from a
more detailed classification scheme.
Igneous rocks are those that form directly from the cooling and hardening (i.e. crystallization
and/or glassification, which we will imprecisely lump together as "solidification") of molten rock
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(magma). The composition and location of this magma at the time of solidification will
determine the features of the resulting igneous rock. Therefore, description of the texture and
composition of an igneous rock provides information about the processes that formed that rock.
This lab introduces the common textures and compositions of igneous rocks and their
relationship to igneous processes.
Igneous Environments
One comprehensive model for the occurrence of the various types of igneous rocks is
based upon plate tectonics [A detailed account is included in your textbook.]. The convection of
the mantle and the corresponding movement of lithospheric plates produce upwelling of molten
material at areas such as mid-ocean ridges, and result in burial of material at subduction zones to
depths where it undergoes partial melting. Specific tectonic processes are confined to certain
portions of the convection cycle, so the characteristic igneous rocks associated with each process
are typically found in similar environments wherever they occur.
In particular, the composition of igneous rocks is sensitive to the environment in which
they form. The primary constituent of most magmas is silica (SiO2), and the precise amount of
silica in a magma exerts so much control on its behavior that the classification of magma into its
common types is based on this criterion. Mafic magmas contain about 50% silica, tend to flow
easily, and have high density. Mafic magmas are most often associated with mantle sources, such
as mid-ocean ridges or oceanic hot spots. An example of a mafic rock is basalt. Felsic magmas
contain about 70% silica, flow very poorly, and have low densities. These felsic compositions
are associated with continental crustal sources, such as melted crustal rocks above continental
hot spots or in subduction zones. An example of a felsic rock is rhyolite. Intermediate magmas
contain about 60% silica and have intermediate densities and viscosities. These compositions are
often associated with subduction zones where oceanic crust is partially melted and assimilates
continental crustal material as it rises to the surface. The classic example of an intermediate rock
is andesite.
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Bowen’s Reaction Series
Common igneous minerals can be arranged according to their order of crystallization, as
summarized in Bowen’s reaction series. Minerals with high crystallization/melting temperatures
(around 1200°C) such as olivine and Ca-rich plagioclase, are at the top. These are the mafic
minerals, rich in iron, magnesium, and calcium, and tend to be dark (green to black). Minerals
with low crystallization/melting temperatures (around 600°C) such as quartz are at the bottom.
These are felsic minerals, rich in potassium, aluminum, and silica, and tend to be lighter in color
(pink to white to colorless). Igneous rocks are classified as mafic, intermediate, or felsic based on
the abundances of these different types of minerals.
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Visual guide to estimating percentages of mafic minerals.
Igneous Structures
The texture of an igneous rock – size and shape of mineral grains in the rock – is a result
of how and where it crystallizes. Magmas that solidify underground are called intrusive and
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produce plutonic rocks. Magmas that reach the surface before solidifying (lavas) are called
extrusive and form volcanic rocks.
Intrusive Rock Structures and Textures
Intrusive magmas cool slowly, insulated by the surrounding rock, which provides time
for their minerals to grow large enough for individual crystals to be seen with the unaided eye.
This coarse-grained igneous rock texture is known as phaneritic. When and intrusive magma
completely solidifies into a large body of plutonic rock, the rock structure is known as a pluton.
Plutons are classified by their geometry, orientation, and size. A planar body that cuts through
the layers of surrounding rock (country rock) is called a dike. Dikes usually mark the channels
along which magma flowed upward. Cylindrical channels that feed volcanic vents are termed
volcanic pipes or necks. Planar bodies that have intruded parallel to the surrounding rock layers
are sills. If a sill flexes the overlying material upward to form a dome, then it becomes a
laccolith. Very large plutons, which often serve as source reservoirs for all of the above features,
are called batholiths.
A. Pluton B. Laccolith C. Sill D. Dike E. Volcanic neck or feeder F. Lava flows or pyroclastic deposits
Extrusive Rock Structures and Textures
Magmas cool quickly upon exposure to air or water (most present-day volcanic activity is
submarine) so that only small crystals form, leading to a fine-grained, aphanitic texture. If the
magma cools so rapidly that no crystals form, then the texture is said to be glassy. If the magma
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produces gas bubbles that fail to escape during cooling, then the resulting frothy texture is called
vesicular and the bubbles are known as vesicles. Basaltic lavas have low viscosity, require only
low pressures to erupt, and tend to flow gently from their vents and spread widely into low,
broad structures known as shield volcanoes, such as Kilauea in Hawaii. Andesitic and rhyolitic
lavas, with their higher viscosity, require higher pressures to force an eruption. This usually
culminates in a sudden catastrophic explosion, often ejecting material into the atmosphere where
it cools and descends as pyroclastic debris. This material forms a steep pyroclastic cone around
the vent. If eruptions continue to occur from the same vent, repeated layers of flows and
pyroclastic debris will accumulate into a stratovolcano such as Mount Rainier.
Complex Cooling Histories and Texture
Magmas can have complex cooling histories, during which different minerals will
nucleate (begin to form) and grow at different pressures and temperatures. This can lead to rocks
with crystals of more than one size. This multi-modal texture is porphyritic. For example, a
basaltic magma may remain underground cooling slowly, forming large olivine crystals (olivine
is stable at high temperatures so it crystallizes early in the cooling process). If the magma, with
its large olivine crystals, then erupts so that the remaining melt cools rapidly, then the other
minerals will be small. The large crystals are called phenocrysts and the smaller grains constitute
the groundmass. The order and rates of crystallization for the minerals in an igneous rock can
often be determined by textural relationships such as this.
Part I. Igneous environments and compositions
1. List the minerals you recognize in each of the following specimens. Determine the
percentage of mafic minerals. Most people will tend to emphasize the dark-colored
minerals, leading one to overestimate their abundance. Use the abundance diagram to
help compensate for this. Identify each specimen as mafic, intermediate, or felsic.
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Specimen
Minerals Present
Percent Mafic
Minerals
Composition
2
4
6
2. Mt. St. Helens is now intermittently spewing ash with an intermediate composition and
has in the past produced lavas ranging from rhyolitic to basaltic in composition. What
does this indicate about the regional tectonic environment of the Cascades and why?
3. What type of lava is most common globally: basaltic, andesitic, or rhyolitic? What are
two distinct reasons for this? (Hint: One is related to tectonic environment and source, the
other to a physical property of the magma.)
4. The Palisades Sill, located in New Jersey, is a classic natural example of fractional
crystallization. Use Bowen’s reaction series and the figure below for the following
questions:
A. What is the overall composition of this sill (mafic, intermediate, or felsic)?
B. Using the letters on the figure, fill in the order of crystallization of the different units:
Crystallized first ______ ______ ______ ______ ______ Crystallized last
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C. After the olivine layer crystallized, did the remaining melt have a similar, more mafic,
or more felsic composition than the original melt? Briefly explain.
D. After the basalt layers crystallized, did the remaining melt have a similar, more mafic,
or more felsic composition than the original melt? Briefly explain.
E. At approximately what temperature did the sill finish crystallizing?
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Part II. Igneous structures and textures
5. Identify textures (aphanitic, phaneritic, porphyritic, vesicular, glassy), cooling rates
(slow, moderate, fast, two-stage), and origin (intrusive, extrusive, intrusive-extrusive) for
the following samples.
Specimen
1
Texture
Cooling Rate
Origin
3
5
6
7
8
9
5. Sample 9 has a felsic composition. Why is it so dark?
6. Use the figure on the top of the next page to answer the following questions:
a. Which unit will have the largest crystals? Why?
b. Which unit will have the smallest crystals? Why?
c. Superficially, a sill (C on the figure) exposed on the surface and a lava flow (F on the figure)
can look very similar: a horizontal or nearly horizontal “sheet” of igneous rock. Describe two
ways to tell the difference between them.
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F
Part III.
Review of Igneous Composition
Igneous rock composition can be described with varying levels of detail. Generally,
composition is based on the relative abundance of felsic and mafic minerals. Mafic minerals
include olivine, pyroxene, and Ca-rich plagioclase, which are typically dark in color. Thus,
igneous rocks which contain mainly of dark-colored minerals will probably be mafic in
composition. Light-colored minerals, including quartz, potassium feldspar (orthoclase), and Narich plagioclase, are felsic and the igneous rocks made mainly of these light-colored minerals are
called felsic rocks. Igneous rocks that contain a mixture of light- and dark-colored minerals are
called intermediate. Greater detail comes from estimating or measuring the percentage of
different minerals found in a rock specimen. For example, one type of granite (a felsic igneous
rock) would be more fully described by saying it is composed of 40% quartz, 35% potassium
feldspar, and 25% plagioclase.
Review of Igneous Texture
Igneous rock texture describes the size and arrangement of the crystals that comprise the
rock, and is largely a function of the cooling rate. The texture of igneous rocks with large, visible
crystals is called phaneritic. If the crystals are small and difficult to see with the unaided eye, the
texture is termed aphanitic. Some igneous rocks contain large crystals within a groundmass of
smaller crystals; this texture is called porphyritic. If an igneous rock cools so quickly that
crystals do not have time to form at all, a glassy texture is produced. Finally, if an igneous rock
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cools relatively quickly, thereby trapping gas bubbles (vesicles) in its structure, it is called
vesicular.
Igneous Rock Classification
Igneous rocks are classified based on their composition and texture. One basic
classification is a binary classification which treats each property as a single variable when
assigning a rock name. One modification arises in the case of rocks with a porphyritic texture,
wherein the rock name is based on the texture and composition of the groundmass with the
modifier “porphyry.” For example, a mafic rock with large olivine crystals in an aphanitic
groundmass would be classified as a basalt porphyry (alternatively, a porphyritic basalt).
Likewise, a felsic rock with extremely large potassium feldspar crystals in a phaneritic
groundmass would be classified as a granite porphyry. Rocks with vesicular texture are similarly
classified based on the texture and composition of their groundmass. More detailed igneous rock
classifications focus on the abundance (by percentage) of particular minerals in the rock, using
ternary analysis. These classifications are based on the relative proportion of three mineral
components. These classifications require more detailed knowledge of mineral composition, but
provide greater insight into its history. They are most often used when chemical analyses of the
rocks are available, so that compositional data are more quantitative.
Classification and Identification of Igneous Rocks
Igneous rocks are classified by their texture and mineral content. The classification scheme lists
common textures in the left-hand column and typical mineral composition in the top row. Although
rock names occupy distinct spaces in the table, it should be understood that these divisions are
artificial and that rock types are gradational with one another. To identify a rock, determine the
texture and mineral content, and find the name in the “pigeonhole” that satisfies both. For example,
a rock that is coarse grained, phorphyritic, and contains quartz and potassium feldspar is a
porphyritic granite.
It makes no difference whether texture or mineral content is determined first. Texture is more
easily recognized though, so start with that, as follows:
1. If crystals are not visible or are barely visible with a hand lens, and:
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a. The rock looks like glass, the texture is glassy.
b. It is not glassy, but has a texture similar to fine or medium sandpaper, the texture
probably is fine-grained.
c. If the texture is fine-grained and vesicles are present, the texture is fine-grained and
vesicular.
d. The rock consists of fragments, the texture is most likely pyroclastic.
2. If crystals are visible without a hand lens, and:
a. Most crystals are larger than 3 centimeters, the texture is pegmatitic.
b. Most crystals are smaller than 3 centimeters, the texture is coarse-grained.
c. Crystal are of two sizes, the texture is porphyritic.
1. If the groundmass is fine-grained, the texture is fine-grained and porphyritic.
2. If the groundmass is coarse-grained, the texture is coarse-grained and porphyritic.
d. There are features that look like phenocrysts but have rounded shapes (like the shapes
of large vesicles) and the groundmass is fine-grained, the texture is amygdaloidal.
Next identify the major minerals. The important mineralogical criteria are:
1. Presence or absence of quartz (only the granites and rhyolites in column 2 have quartz);
2. The type of feldspar (whether orthoclase or plagioclase, and if plagioclase, whether sodiumrich or calcium-rich
3. The proportions and kinds of dark-colored minerals (biotite, hornblende, augite, and olivine).
Minerals in the groundmass of fine-grained rocks commonly cannot be identified; however,
phenocrysts in such rocks can be helpful. According to the table, if quartz phenocrysts are
present, the rock is rhyolite.; if only hornblende phenocrysts are present, it is andesite; if only
augite and olivine are present, it is basalt; and if feldspar phenocrysts are present, the feldspar
must be identified on the basis of presence or absence of striations.
Procedures for classifying igneous rocks:
1. Identify the rock’s texture.
2. Identify the rock’s color index (i.e., estimate the percentage of light-colored –v- dark-colored
minerals) and, if possible, the identity and abundance of specific minerals.
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a. If the rock is coarse-grained, estimate the color index and percentage abundance of
quartz, feldspars, and mafic minerals. With this information you can characterize
the rock as felsic, intermediate, mafic or ultramafic.
b. If the rock is fine-grained (i.e., mineral grains are too small to be seen with the naked
eye) you can approximate the composition based on the rock’s color index. Felsic
fine-grained rocks tend to be pink, white, or pale brown. Intermediate fine-grained
rocks tend to be gray to greenish-gray. Mafic and ultramafic rocks tend to be dark
gray to black.
3. Classify the rock using the igneous rock classification chart:
Guide to estimating percentages of ferro-magnesian (dark) minerals
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Sample
Number
1
2
3
4
5
6
7
8
9
Texture
Composition
Other Properties
Notes
Rock Name
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Part IV. Synthesis
8. Based on the compositions from Question 1 and the textures from Question 5, choose a likely
setting for each sample from the diagram below (pick a letter). A sample may fit in more than
one setting!
Specimen
Setting
A
B
C
D