Geology 2 – Physical Geology Lab
Lab #5 – Igneous Rock Identification
Student Outcomes:
 Be able to identify and name the nine common igneous rock types examined in lab
 Recognize and name the textures that differentiate volcanic and plutonic rocks
 Understand that igneous rocks are classified by both texture and by mineral type, as well as the
percentages of minerals present.
 Be able to identify and name the minerals common in the different types of igneous rocks.
 Be able to use new terminology for rock identification and for describing their source.
Materials:
 Part I - Nine stations of igneous rock samples with accompanying photographs and figures.
 Part II – List of terminology used is the study of igneous rocks
 Part III – 11 mystery igneous rock samples
 Hand lens or magnifying glass
 Student textbook with the descriptions of the igneous rocks
Logistics: The lab is in two parts.
Part I – Rotate to various stations at which you can observe various samples of igneous
rocks. Become familiar with the differences in texture and mineral composition of the
different igneous rocks. Some stations also have accompanying photographs and figures
with important information about the rocks. Answer the questions on the lab worksheet.
Part II – Add your notes and definitions to the list of new terminology for your own use.
Part II - Determine the names of eleven mystery igneous rock samples by identifying the
texture and mineral composition.
Part I. Classification, Characteristics, and Identification of Igneous Rocks
We classify igneous rocks by texture and mineral composition.
 Texture: The first step in classifying igneous rocks is to observe the texture. Rock texture is
determined by its cooling history, and the amount of dissolved gases in the magma.
 Coarse-grained texture rocks are plutonic (intrusive). Large grain size indicates that
the rocks cooled slowly within the lithosphere, providing the opportunity for the
growth of larger crystals.
 Fine-grained texture rocks are volcanic (extrusive). Volcanic rocks cool quickly
above or near the surface of the earth and have smaller interlocking crystals that are
not visible to the naked eye, or have no crystals at all.
 Glassy texture describes rock that has an absence of any internal crystalline structure.
Glassy rocks are born from rapidly quenched lava that cools so quickly that crystals
do not form.
 Composition: The second step in classifying igneous rocks is to examine the minerals that make
up the rock. The chemical composition of an igneous rock determines the minerals that form.
The following chart on the following page shows how rock names depend on mineralogical
variations.
Geology 2 – Physical Geology
Igneous Rock Identification
This graph shows that as the rocks grade from granite/rhyolite to gabbro/basalt, the proportion of
felsic minerals decreases and the proportion of mafic minerals increases.
 Felsic minerals include quartz, K-feldspar, sodium-rich plagioclase, and the micas. These
minerals are light colored. Felsic is a term derived from Feldspar and Silica.
 Mafic minerals include olivine, pyroxene, and amphibole. Mafic minerals are dark colored.
Mafic is a term derived from Magnesium and Fe (iron).
(Remember these familiar minerals from the previous lab?)
Identification of Igneous Rock Hand Samples
For the igneous rock quiz in lab, you are responsible for identifying and naming the following nine
igneous rocks:
Granite
Diorite
Gabbro
Rhyolite
Andesite
Basalt
Obsidian
Pumice
Tuff
You will examine and observe several other types of igneous rocks in the lab so you can gain
experience in recognizing key minerals in rock samples and become familiar with various igneous
rock textures. However, you will be tested on only the nine rock types listed above and described
next.
Plutonic Rock Identification
Recall that plutonic rocks are coarse-grained and have visible mineral crystals. Plutonic rock hand
sample identification depends on the proportion of dark (mafic) to light (felsic) minerals.
1) Granite contains mostly light minerals of quartz, K-spar and plagioclase. Granite contains Kspar and less than 50% dark (mafic) minerals.
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Geology 2 – Physical Geology
Igneous Rock Identification
2) Diorite appears to have roughly equal amounts of light and dark minerals. In fact, Diorite
contains a bit less than 50% dark (mafic) minerals, and no K-spar.
3) Gabbro contains more dark minerals than light minerals. It has more than 50% dark (mafic)
minerals and no quartz.
Volcanic Rock Identification
Recall that volcanic rocks are fine-grained with usually no visible crystals. In the laboratory,
volcanic rocks are identified by their chemical composition, especially their silica (SiO2) content.
High silica (more than 65%) volcanic rocks are rhyolite, medium silica rocks are andesite, and low
silica (less than 50%) rocks are basalt. However, we need a classification for identifying volcanic
rocks in the field where it is not possible to analyze the chemical composition of the rock. The best
criterion for identifying volcanic rocks in the field is phenocryst mineralogy. A phenocryst is a
conspicuously large crystal embedded in a matrix, or groundmass, of finer-grained crystals. As with
plutonic rocks, we can use visible crystals to help us identify the rock type. If we cannot see
phenocrysts, we use the color of the rock to identify the rock.
4) Rhyolite is a light colored, sometimes reddish, fine-grained volcanic rock. It contains quartz, kspar, or mica phenocrysts; and may also contain plagioclase.
5) Andesite is a medium gray colored, fine-grained volcanic rock. It commonly may have dark
colored phenocrysts of amphibole or pyroxene and light colored phenocrysts of plagioclase.
It does not contain olivine, quartz, k-spar, or mica phenocrysts.
6) Basalt is a very dark colored, fine-grained volcanic rock that may contain green olivine
phenocrysts. It sometimes may also contain plagioclase and pyroxene phenocrysts.
There are other types of volcanic rocks that are classified almost entirely by their texture, such as:
7) Obsidian is a non-crystalline, glassy volcanic rock. It is typically glassy black and can be dark
brick red or brown. Obsidian cools so quickly that mineral crystallization does not occur.
8) Pumice is a light colored volcanic rock with very abundant adjacent holes, or vesicles. Pumice
feels surprisingly light because it has a very low density, which allows it to float on water.
9) Tuff is a pyroclastic rock composed of tiny ash particles welded together. Pyroclastic material is
volcanic rock ejected during an eruption. You will see more samples of pyroclastic rocks in
the lab.
Igneous Rocks Stations
The igneous rock stations in this lab give you an opportunity to closely examine a variety of igneous
rocks. You will gain experience in recognizing key minerals found in igneous rocks that are
essential for rock identification. You will also become familiar with different textures used in
describing igneous rocks. Many of the stations have photographs or figures that accompany the
rock samples. Look at the photographs and figures closely and study how they relate to the rock
samples. Some of the lab worksheet questions relate to the diagrams and figures as well as the
rocks samples.
Igneous Rock Minerals
It is critical to be able to identify individual grains of quartz, plagioclase, and potassium feldspar (Kspar) when describing igneous rocks. Here are a few hints:
Quartz is usually appears as translucent gray. Quartz crystal grains are usually small and equant.
Sometimes you can see conchoidal fracture.
Plagioclase is usually blocky with two good cleavages. It is most often milky white.
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Geology 2 – Physical Geology
Igneous Rock Identification
Potassium Feldspar (K-spar) is positively identified by its typical salmon-pink color or presence
of exsolution lamellae. K-Spar color can range from dark pink to milky-white and look
similar to plagioclase. Luckily, at MPC, all our samples with K-spar have pink colored
feldspars.
Plutonic Rock Stations
1. Gabbro. Gabbro is a coarse-grained plutonic rock that appears to have more dark than light
minerals. The light mineral is plagioclase. The dark minerals include pyroxene, hornblende, and
biotite.
 Looking at the figure at this station, where in the crust does gabbro most commonly form?
In oceanic crust or continental crust?
2. Diorite. Diorite is a coarse-grained plutonic rock that appears to have roughly equal amounts of
light and dark minerals. The light mineral is plagioclase; there is not much quartz. You can tell the
light minerals are mostly plagioclase because they are not as translucent as quartz. The dark
minerals are primarily hornblende with small amounts of biotite.
 Sample 816 has light minerals of plagioclase and quartz. Use the magnifying glass or hand
lens to look at the minerals closely. The abundant milky white plagioclase have well-formed
crystal faces. The small rounded grains of quartz appear translucent gray. The dark minerals
are mostly hornblende with some biotite.
 Look at the diagrams and photographs at this diorite station. The photographs show silicic
plutonic rocks as they occur in the field. In the lower diagrams, what is the evidence that
suggests the plutonic rocks intruded into the darker rocks?
3. Granite. Granite is a coarse-grained plutonic rock that has more light minerals than dark
minerals. Notice that each granite sample contains pink potassium feldspar (K-spar) crystals. The
presence of potassium feldspar (K-spar) is the most important feature of granite. All the granite
samples in this lab have salmon-colored K-spar. Remember that outside the MPC lab that K-spar is
sometimes pink, but not always. Be sure to notice that each sample has all three of the light
minerals: K-spar (pink crystals, sometimes with exsolution lamellae), plagioclase (milky white,
blocky crystals), and quartz (translucent or glassy gray crystals). These minerals are present in
different proportions, but each mineral is always present. See if you can identify individual grains
of K-spar, plagioclase and quartz in each of the granite samples at Station 3A. Look at all of the
granite samples and become familiar with how to recognize each of them as granite.
For the purposes of this lab, all the samples at this station we will call granite. In lab we use the
term granite loosely as a general term for all light colored, coarse-grained igneous rocks. The
mineralogical definition we use here is broader than the usage by more specialized geologists. Also
recognize that granite includes a range mineral composition and grain sizes. The next two examples
show how composition and grain size can vary in granite.
Look at the potassium feldspar-rich granite samples (Station 3B). These samples are almost
entirely composed of potassium feldspar, (K-spar). Become familiar with the appearance of the Kspar crystals. Try to find some of the few quartz grains; they will look more translucent or glassy
gray.
 Now read the comparison between granite and ice cream. It was written about granite, and
applies to K-spar-rich granite as well. How is ice cream an analogy for granite?
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Geology 2 – Physical Geology
Igneous Rock Identification
The last sample at this station is pegmatitic granite (Station 3C). A Pegmatite is a very coarsegrained igneous rock, typically granite. They have very large crystals. Pegmatites form in plutonic
magma chambers where crystallization in a water-rich environment is believed to be responsible for
the very large crystals. You should be able to identify easily the quartz, K-spar, and plagioclase
crystals in this rock. Pegmatitic granite is named for its specific coarse-grained texture.
 Look at the photographs of the pegmatitic granite. How big can minerals in a pegmatite
grow?
 Look at the geologic map figure. The plutonic granitic rocks are shown in pink on the map.
How can you identify plutonic rocks by their map pattern, or shape they display, on the
geologic map?
Volcanic Rocks Stations
4. Basalt. Basalt is a dark, fine-grained volcanic rock. It is the fine-grained extrusive equivalent of
the coarse-grained intrusive Gabbro. Basalt is positively identified by the presence of olivine
crystals within a dark fine-grained matrix or groundmass. Some basalts lack any phenocrysts. They
appear as dark colored fine-grained rocks with no visible crystals.
 First look at the sample “Bas-1”. It has the best examples of olivine phenocrysts. You will
need to use a hand lens to see the olivine phenocrysts clearly. Can you identify the grains of
olivine in each?
 Now look at sample 170826. Notice it is completely fine-grained and is identified as basalt
solely on the basis of its dark color.
 Examine the pillow basalt sample. Pillow basalt forms at the mid-ocean ridge.
Basalt is probably the most common volcanic rock type on Earth. Basalt forms at divergent margins
and hot spots, comprises the sea floor, and is common at convergent margins.
 Look at the photographs. Viscosity is a measure of the fluid’s resistance to flow. What is
the viscosity of basalt lava? What kind of volcano does it most commonly form?
5. Andesite. Andesite is generally a medium gray, fine-grained volcanic rock. It is the fine-grained
extrusive equivalent of the coarse grained intrusive Diorite. Andesite can have dark, prismatic
hornblende phenocrysts or milky-white, blocky plagioclase phenocrysts. Sometimes andesite
contains no phenocrysts making identification difficult without microscopic examination. Andesite
can be tricky to identify when it is as dark as basalt, but andesite will never contain olivine
phenocrysts as does basalt. Andesite can also resemble lighter colored rhyolite but it does not have
the quartz, K-spar or micas found in rhyolite.
 Look at the variety of andesite samples and find the plagioclase and hornblende phenocrysts.
Using a hand lens examine the plagioclase phenocrysts that look milky white and blocky,
and the typical dark, prismatic hornblende phenocrysts.
The large, steep-sided stratovolcanoes of the Andes are composed largely of andesite.
Stratovolcanoes, or composite volcanoes, are the most common type of volcano produced by
andesitic lava. Stratovolcanoes have explosive eruptions and steep sides, and are composed of both
lava flows and ash deposits. It is the high viscosity of the magma that allows the volcano to have
such steep sides.
 Looking at the figures, what is the most common tectonic setting of andesite?
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Geology 2 – Physical Geology
Igneous Rock Identification
6. Rhyolite. Rhyolite is a usually buff to pink or occasionally very light gray, fine-grained volcanic
rock. It is the fine-grained extrusive equivalent of granite. Rhyolite frequently contains glass
fragments and voids, resulting from the rapid cooling of the lava above ground. Rhyolite can
contain small phenocrysts of biotite, quartz, or potassium feldspar.
 Look at sample EC6. This rhyolite contains small biotite phenocrysts.
 The small light grayish samples are fine-grained and called rhyolite because of their light
color.
 Rhyolite eruptions form either huge craters, called calderas (see Crater Lake, top photo), or
domes. A dome is essentially a lava flow that was not able to flow anywhere because of its
great viscosity. Can you see the rhyolitic dome on the figure?
Other Igneous Rock Stations
7. Obsidian. Sometimes during volcanic eruptions, molten material is ejected into the air, where it
is quenched quickly. Cooling occurs so rapidly that ions are not able to arrange into an orderly
crystalline structure. The result is a non-crystalline texture termed “glassy”. Obsidian is a common
type of natural volcanic glass. It forms from rapidly-cooled rhyolitic lava.
 Look at the photograph. In this photograph, what are the differences in how the terms
“rhyolite,” “obsidian,” and “dome” are used?
8. Vesicular Textures. Fine-grained rocks commonly contain voids, or openings, left by escaping
gas bubbles as the lava cools. The spherical or elongated openings are called vesicles.
 Pumice is a very vesicular volcanic rock that is light in color and in density. It is
surprisingly light in density. One of the samples can float on water because of its low
density, until the vesicles fill up with water. Pumice forms when large amounts of gas
escape through lava to generate a gray, frothy mass. Pumice is another type of volcanic
glass. These samples are rhyolitic in composition because they contain biotite and quartz
phenocrysts.
 Look at the pumice figures. Notice the two diagrams show layers of pumice. Beds
composed of pumice fall into the pyroclastic category of rocks. Pyro means fire and clastic
means made up of bits and pieces of other rock. Thus, pyroclastic rocks have some
characteristics of igneous rocks (formed from cooled magma) and have some characteristics
of sedimentary rocks (show bedding).
 Why is pumice less dense than water?
 Vesicular Basalt is the basaltic version of rhyolitic pumice. Vesicular Basalt is denser and
usually darker than pumice.
 The volcano in the photograph is the top of a cinder cone erupting at night. Basaltic
eruptions typically are not explosive. Gas trapped in the magma causes the explosiveness of
the eruption.
9. Tuff. Tuff forms when ash particles emanating from a hot, explosive eruption weld together as
they reach the surface of the earth. Pumice fragments, crystals, and rock fragments all occur
together in a matrix of welded ash tuff. As the hot ash settles on the ground, the weight from the
overlying ash can compress the underlying material enough to deform, or squish, the pumice
fragments. Essentially sedimentary processes formed these tuff rocks when individual particles
from a volcanic eruption were welded together.
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Geology 2 – Physical Geology
Igneous Rock Identification
 Look at the tuff samples. Find the deformed pumice fragments and layering in the samples
 Why does tuff form a cliff in the photographs?
10. Other Volcanic Rocks. You are not responsible for memorizing these rock types, but they are
interesting to see
 Volcanic Bomb. This volcanic bomb formed when lava erupted from a volcano and spun
around in the air, creating its football-like shape. It is probably basaltic in composition.
o The photograph shows more examples of bombs.
 Basaltic flow texture. This isn’t really a rock name, but this sample is from a basaltic lava
flow. Notice how the lava must have had a low viscosity to make this texture.
o Look at the figure of the volcanic flow. You can imagine the lava flow cooling to
produce a rock with flow structures similar to the sample.
Part II. Igneous Rocks Terminology
There is a lot of new terminology in this lab. This list is to help you focus on the terms you will
need to understand and use when identifying igneous rocks. There are other terms used in the lab
that are important to understanding igneous rocks that are not on this list. You will hear more about
these terms in lecture, but you will not be tested on them in lab. All these terms are found in your
textbook. Part II is for your use to take notes. It will not be graded.
 Coarse-grained texture
 Fine-grained texture
 Plutonic
 Intrusive
 Volcanic
 Extrusive
 Glassy texture
 Felsic
 Mafic
 Granite
 Diorite
 Gabbro
 Rhyolite
 Andesite
 Basalt
 Obsidian
 Pumice
 Tuff
 Phenocryst
 Pyroxene
 Vesicle/vesicular
 Pyroclastic
 Pegmatite/pegmatitic
 Matrix
 Groundmass
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Geology 2 – Physical Geology
Igneous Rock Identification
Part III. Mystery Igneous Rocks
Check out a suite of mystery igneous rocks. Arrange the mystery rocks in numerical order. Identify
each of the samples using the characteristics of igneous rocks that you learned and observed in Part I
of the lab. Work in small groups and discuss your observations with your colleagues as you identify
each igneous rock. Name each rock sample, fill in the list below and turn it in to be graded. If you
get confused, go back to the relevant part I lab station for clues and to remind yourself of the
characteristics and properties of igneous rocks.
Rock identification takes practice. It requires practice recognizing igneous textures, the ability to
recognize a few key minerals that you learned in the previous mineral lab, and understanding the
various compositions of igneous rocks. If you really get stuck, ask me for help.
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Congratulations on completing the Igneous Rock lab! You now know more about the world around
you and how it works!
Geology 2/Labs/Igneous Rocks/Lab #5 - Igneous Rock Identification.doc/LTS 9/18/06
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