VANDERBILT STUDENT VOLUNTEERS FOR SCIENCE
http://studentorgs.vanderbilt.edu/vsvs
Igneous Rocks
Fall – VINSE/TnScore
2013
Goal: To introduce students to the types of igneous rocks, how they form, and what minerals combine
to form them.
Fits Tennessee Standards GLE 0707.7.1 Describe the physical properties of minerals
GLE 0707.7.2: Summarize the basic events that occur during the rock cycle
I. Introduction – What are Igneous Rocks?
A. Definitions – How are igneous rocks formed?
B. Volcano Diagram – Where does rock material come from?
C. Lava vs. Magma, Intrusive vs. Extrusive
II. Examining Igneous Rocks
A. Analyzing Intrusive vs. Extrusive Igneous Rocks
B. Minerals of Igneous Rocks
III. Examining Pegmatite
IV. Special Types of Volcanic Rock
Materials:
8 Plastic Cases with two sets of Igneous Rocks – box #1
8 Plastic Cases with two sets of Minerals – box #2
2 Plastic Cases containing 33 samples of Pegmatite total – box #3
8 Plastic Cases with two sets of Volcanic Rocks – box #4
1 VSVS Set of Rocks & Minerals
32 Magnify Glasses
32 Igneous Rock Observation sheets
16 Laminated Mats for Igneous Rocks
16 Laminated Venn Diagram mats for minerals in igneous rocks
16 Volcano diagrams in sheet protectors
Divide class into 8 groups of 4 students. Groups will work in pairs but share boxes of rocks and
minerals. Hand out an Igneous Rock observation sheet to each student.
I. Introduction – What Are Igneous Rocks?
Write the following vocabulary on the board: magma, lava, intrusive igneous rock, extrusive igneous
rock
A. Definitions – How are igneous rocks formed?
There are 3 types of rocks - sedimentary, metamorphic and igneous. This lesson focuses on igneous
rocks.
Ask students if they know how igneous rocks are formed and what are they formed from? (Question
1 on the observation sheet)
Igneous rocks form when the melted rock material from the Earth cools.
Cooling and hardening of melted rock material can occur on or underneath Earth’s surface.
B. Volcano Diagram – Where does rock material come from?
Pass out the laminated mats for igneous rocks (1 per pair). Refer to the images as you talk about key
terms in C.
C. Lava vs. Magma, Intrusive vs. Extrusive
Ask students if they know what intrusive and extrusive rocks are? (Questions 2&3 on the observation
sheet)
Tell them that melted rock material is called magma when it is underneath the Earth’s surface.
Igneous rocks made from magma form underneath the Earth’s surface. These rocks are called
intrusive igneous rocks. (Reference the igneous rocks mat, noting the cooling below the earth’s
surface.)
When the melted material is on or above the Earth’s surface, it is called lava. Igneous rocks formed
from lava form on or above the Earth’s surface. These rocks are called extrusive igneous rocks.
(Reference to igneous rocks mat again).
Tell the students that they will be looking at different samples of igneous rocks and they:
o will be looking for visible differences between intrusive and extrusive igneous rocks
o will also be learning about some of the different minerals that make up different types of
igneous rocks
o will examine some different types of volcanic rocks.
II. Examining Igneous Rocks
In this section, students will gain hands-on experience looking at and classifying rocks.
Materials:
16 Igneous Rock Mats (contains columns for 3 different types of igneous rocks and diagram showing
where these rocks form).
8 boxes of igneous rocks (Box #1, rocks A-F) (2 sets of rocks per box)
A. Intrusive Versus Extrusive Igneous Rocks
Hand out igneous rock samples (Box #1 with rocks A-F)
Each pair of students will share a chart and each group of four shares a box of rocks (there are two
sets of rocks in each box).
Tell students to:
• Match the rocks to the letters in the table. Walk around and help them to do this as needed.
Your Notes:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Explain that:
• These rocks are all igneous rocks.
• The top row of rocks are Extrusive Igneous Rocks that formed from lava on the Earth’s surface
• The bottom row of rocks are Intrusive Igneous Rocks that formed from magma below the
Earth’s surface
Ask students:
1. What differences do you see between the Intrusive and Extrusive rocks? (Question 4)
Answers may include: The extrusive rocks (top row) are solid colors while the intrusive rocks
(bottom row) are ‘speckled’. Students may or may not know that the "specks" are crystals of
minerals. Other terms could be fine-grained or course-grained.
Have students record their answer on their observation sheet, question 4.
Explanation:
 Scientists can classify rocks as fine-grained or coarse-grained. Coarse-grained rocks
have large crystals of different minerals, whereas fine-grained rocks have very small
crystals that are difficult to see.
 Extrusive igneous rocks cool and harden much more quickly since they form at the
Earth’s surface where the temperature is cooler. Since they cool quickly there is not as
much time for large, visible crystals to form.
 Intrusive rocks form deep within the Earth where they cool much more slowly because
of a higher temperature. Crystals have more time to grow larger.
Have students answer question 5 on their observation sheet.
2. What differences do you notice among the rocks in the 3 columns? (Question 6)
The color from left to right gradually gets lighter. The rocks in the blue column are
lighter in color than those in the red column. Students may or may not know that the color of a
rock depends on its chemical composition.
Your Notes:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Have students answer question 6 on their observation sheet.
Explain the columns (Question 7)
• The color of a rock depends on the elements in the minerals in the rock.
• Felsic and Mafic are words created by scientists to describe the chemical composition of rocks.
• Felsic rocks are light-colored because they contain minerals that have more silicon, sodium, aluminum
and potassium (don’t emphasize elements, focus on the color). These rocks are in the blue column.
• Granite is the most common felsic rock and felsic rocks sometimes are called granitic rocks (as in
your text book).
• Mafic rocks (in the red column) are dark-colored and contain minerals that have more calcium, iron
and magnesium. Basalt is the most common mafic rock, and mafic rocks are sometimes called
basaltic rocks.
Have students answer question 7 on their observation sheet.
Tell students that the reason for the difference in colors will be more obvious after they have examined
the minerals that make up the rocks.
Have the students place the rocks on their labels on the diagram where the rocks form.
Tell students to notice where these rocks are forming.
Darker mafic rocks form from the ocean ridges
Lighter felsic rocks form on the continents.
Tell students you will address the different rock colors in the next section, when thy look at the
mineral content of the rocks.
B. Minerals of Igneous Rocks
Materials (in addition to the igneous rocks mat):
16 Laminated Venn Diagrams – Minerals of Igneous Rocks
8 Plastic Cases containing two sets of minerals – box #2
32 Magnify Glasses
Briefly explain the Venn Diagram to the students:
Note: Keep the igneous rocks in position where they form on the igneous rock mat.
• Remind students that MINERALS are the building blocks of rocks. The rocks are different
combinations of these minerals.
• Explain that the next case (box #2) contains minerals found in the igneous rocks on the mat.
• The minerals (labeled with numbers) will all be placed on the Venn Diagram.
Your Notes:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
• The colored circles/ovals in the Venn Diagram correspond to the three columns in the table:
 Any mineral in the blue circle can be found in a felsic/granitic rock.
 Any mineral in the black circle can be found in an intermediate rock.
 Any mineral in the red circle can be found in a mafic/basaltic rock.
 Minerals in overlapping ovals are in both corresponding rock types
Hand out cases containing mineral samples (box #2 with minerals labeled 12-19), 1 to each group
of four (there are two sets of minerals per case). Note that minerals 13 and 15 are not present
Tell Students to:
 Place Venn Diagram at the bottom of the igneous rock mat and place the numbered mineral on its
numbered position on the Venn diagram
Ask students
 What difference do they see in the colors of the minerals?
The color gradually gets lighter from left to right.
 What is the relationship between the color in the minerals and rocks?
The color of the rock depends on the minerals that make up the rock. The minerals that make up
the mafic rocks tend to be darker than those that make up the felsic rocks. Intermediate rocks are
made from some felsic minerals and some mafic minerals.
 Which type of igneous rock would be easiest to identify the different minerals in, intrusive or
extrusive? Intrusive because you can see different crystals.
Tell students to look at the large-grained rocks with the magnifying glasses to observe the minerals.
Note: Do NOT spend any more time than 1 minute on this – it is more
important to move on to the pegmatite sample and the last section on
Volcanic rocks.
III. Examining Pegmatite
Materials:
2 Plastic Cases (Box #3) containing 32 samples of pegmatite
32 Magnify Glasses
Tell students that you are now going to give them a different rock that has even bigger crystals than the
ones they currently have.
Your Notes:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Tell them that the rock you are giving them is called pegmatite.
Note: the name pegmatite refers to an igneous rock with especially large mineral crystals. It
does not have a specific mineral composition – it can be felsic, intermediate, or mafic in
composition.
Pass out one sample of pegmatite to EACH student.
Walk through the minerals of the pegmatite with the students.
Using the minerals on the Venn Diagram as a reference, ask
students if they can see: Orthoclase feldspar (it might be pink in the
pegmatite), quartz, muscovite, and biotite.
Record the mineral names on their observation sheet. (Question 8)
Note: If the samples have a salmon/pink colored mineral, point out
to students that it is a type of orthoclase feldspar (12) that has
impurities that makes it pink instead of the white mineral they have in front of them.
Tell students that other minerals are present but that we have listed only the largest/easiest to see.
Based on the minerals listed, ask them which igneous rock group they think it belongs to (i.e. intrusive
or extrusive AND granitic, intermediate, or basaltic) and record their answer on their observation sheet.
(Question 9)
These rocks are mostly intermediate (some may be granitic). Granite is the best answer. It must
be an intrusive rock because of large crystals.
Tell students to make sure all of the minerals and rocks are all on the correct places on the mat (including
the pegmatite) and to move the mat to the side so that a VSVS member can collect them during the next
activity.
IV. Special Types of Volcanic Rocks
Materials:
16 Handouts with detailed information on these specimens
16 Volcano diagrams
8 Plastic Cases with two sets of Volcanic Rocks (Box #4 - 4
students will share the case)
Samples provided include Vesicular Basalt, Scoria/Cinders,
Pumice, Obsidian with visible lava flow (red), Ash Tuff, and
Vitric (Glass) Tuff.
Your Notes:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Tell students that you will be giving them a case of special volcanic rocks that are different from the
igneous rocks they have already looked at.
When they get the rocks, ask them if they are intrusive/extrusive (extrusive). Why? There are no
crystals in them.
They are to work with their partner to make observations about each rock. As they make
observations, they should fill in the chart for question ten and be ready to discuss what makes
each rock special.
If time is short, have the class discuss the differences between the rocks and note the differences on
the board.
Pass out the cases of volcanic rocks (M-T) labeled “Use 4th”. Each group of four should get one case that
contains two sets of rocks.
While the students are working, walk around and engage them in conversation about what they are
observing and make sure that they are recording their observations on their observation sheet.
After students have worked for a couple of minutes, pass out the handouts that contain the information
about the volcanic rocks and collect the igneous rocks, pegmatite and minerals.
Have the students stop working, and ask them about their observations. Work through each rock, asking
what they observed, adding in anything special about the rock that they left out, explaining the processes
that formed each rock, and pointing out to them where on the volcano diagram this rock is
depicted/formed.
Clean up: Collect all volcanic rocks and put into cases in their labeled positions.
Lesson written by:
Edited by:
Reference:
Pat Tellinghuisen, Coordinator of VSVS, Vanderbilt University
Courtney Luckabaugh, Lab Manager of VSVS, Vanderbilt University
Kyle H. Broach, VSVS Training Committee, Vanderbilt University
Lucas Loffredo, VSVS Training Committee, Vanderbilt University
Chernicoff, S., & Whitney, D. (2007). Geology: An Introduction to
Physical Geology. Upper Saddle River, New Jersey: Pearson.
Your Notes:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Your Notes:
____________________________________________________________________________________________________________
____________________________________________________________________________________________________________
____________________________________________________________________________________________________________
____________________________________________________________________________________________________________
____________________________________
Special Types of Volcanic Rock
M. Vesicular Basalt
Vesicular Basalt has the same mineral composition as the basalt you saw in the first part of
the lesson. The word vesicular means that the rock has vesicles or small cavities (air
pockets). These cavities or holes are caused by the expansion of gas bubbles or steam when
the lava was cooling to form the rock. Since the gas could not escape before the rock cooled
fully, it left air pockets throughout the rock.
N. Scoria (Cinders)
Scoria can be felsic, intermediate, or mafic. What makes a rock scoria is not what it is made
up of but how it is made and what its texture is like. It is made from lava that has a lot of
gases trapped inside. These gases form large bubbles in the lava which remain as holes or
cavities in the solid rock. Even with these holes, however, the rock generally will still sink
in water. Scoria can either form from flowing lava or from lava that explodes out the top of
the volcano. It is not a very useful rock. Its name actually comes from a word for waste.
O. Pumice
Like scoria, pumice does not have a specific mineral composition, though it most commonly
forms from felsic lava. When lava is extremely rich in gases, it can begin frothing or
foaming. When this foam solidifies, pumice is formed. There are so many small holes
throughout pumice that is can be more air than rock. Some pumice is so porous that it can
float on water. Pumice is a very useful rock because it is light weight, has sharp edges, and
is fairly hard. It is commonly used as scouring stones or in exfoliating creams.
P. Obsidian
Obsidian is also known as volcanic glass. It is formed when felsic/granitic lava from a
volcano flows into water (a lake or ocean). The water causes the lava to cool so quickly that
no mineral crystals can form, not even the smallest crystals that most extrusive igneous
rocks have. Without this crystalline structure, obsidian has a smooth, glassy appearance.
The black color of your sample of obsidian comes from the presence of magnesium and iron
impurities in the lava. Your sample also has red streaks through it. The lava in this sample
had not mixed fully before it cooled so it is both red and black. The red streaks tell us how
the lava was flowing when it cooled. In the past, obsidian was used to make arrowheads and
other tools.
R & T. Ash Tuff & Vitric Tuff
Volcanic tuff is rock formed when debris from an explosive volcano piles up and is later
compressed into a solid rock from the pressure of other debris, ground, etc that settles
above it. Tuff can form from many different materials including volcanic ash, and broken up
pieces of volcanic glass and rock. Sample R is called ash tuff because it is mainly composed
of volcanic ash pressed together to form a solid rock. Sample T is called vitric tuff. Vitric
means glassy, so vitric tuff is mainly composed of bits and pieces of volcanic glass.
Name _______________________________________
1.
2.
3.
4.
5.
6.
7.
8.
9.
Igneous Rock Observation Sheet
What are igneous rocks? How are they formed and what are they formed from?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
What are extrusive rocks made from and where are they formed?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
What are intrusive rocks made from and where are they formed?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
What differences do you notice between the extrusive and the intrusive igneous rocks
(between the top row and the bottom row of rocks)?
________________________________________________________________________
________________________________________________________________________
What causes these differences?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
What differences do you notice between granitic/felsic igneous rocks, intermediate
igneous rocks, and basaltic/mafic igneous rocks (between the three columns of rocks)?
________________________________________________________________________
________________________________________________________________________
What causes these differences?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
What minerals do you observe in the pegmatite sample?
________________________________________________________________________
________________________________________________________________________
What type of igneous rock (of the six listed in your chart) do you think that the pegmatite
is based on the minerals you have identified?
________________________________________________________________________
10. To fill out this chart, record your observations of each volcanic rock sample and then
write down some key facts you learned about each rock from the information sheet.
Weight
(very
Solid? Has
Texture
heavy,
holes?
(smooth,
heavy,
Volcanic
Size of
rough,
light, very
Rock
Color
holes?
gritty, etc)
light)
Why is this rock special?
Vesicular
Basalt (M)
Scoria (N)
Pumice (O)
Obsidian (P)
Ash Tuff (R)
Vitric Tuff (T)
ANSWER KEY
Igneous Rock Observation Sheet
1. What are igneous rocks? How are they formed and what are they formed from?
Igneous rocks are rocks formed by the cooling of lava or magma (molten or melted rock
and minerals).
2. What are extrusive rocks made from and where are they formed?
Extrusive rocks are made from lava (melted/molten rock on the Earth’s surface) and are
formed on the Earth’s surface.
3. What are intrusive rocks made from and where are they formed?
Intrusive rocks are made from magma (melted/molten rock below the Earth’s surface)
and are formed below the Earth’s surface.
4. What differences do you notice between the extrusive and the intrusive igneous rocks
(between the top row and the bottom row of rocks)?
The top row (extrusive igneous rocks) are a solid color. The bottom row (intrusive
igneous rocks) are speckled – they have many different colors.
5. What causes these differences?
Extrusive igneous rocks cool more quickly since it is on the Earth’s surface where it is
cooler, so the mineral crystals do not have as much time to form and are smaller
because of this. The mineral crystals are so small, you cannot differentiate one mineral
crystal from another, so the rock looks uniform.
6. What differences do you notice between granitic/felsic igneous rocks, intermediate
igneous rocks, and basaltic/mafic igneous rocks (between the three columns of rocks)?
The rocks on the granitic/felsic rocks are lighter in color while the basaltic/mafic rocks
are much darker. The intermediate rocks are in between the granitic and basaltic rocks
in color.
7. What causes these differences?
The color depends on the minerals that make up the rocks. Those that make up the
mafic rocks tend to be darker than those that make up the felsic rocks. Intermediate
rocks are made from some felsic minerals and some mafic minerals.
8. What minerals do you observe in the pegmatite sample?
Answers could include: Orthoclase feldspar (might be pink in the pegmatite), quartz,
muscovite, and biotite.
9. What type of igneous rock (of the six listed in your chart) do you think that the
pegmatite is based on the minerals you have identified?
These rocks are mostly intermediate (some may be granitic). Granite is the best answer.
It must be an intrusive rock because of large crystals.
10. To fill out this chart, record your observations of each volcanic rock sample and then
write down some key facts you learned about each rock from the information sheet.
Texture
(smooth,
rough,
gritty, etc)
Weight
(very heavy,
heavy, light,
very light)
Volcanic Rock
Color
Solid? Has
holes? Size
of holes?
Vesicular
Basalt (M)
Black
Large Holes
Rough
Very heavy
Has holes
Still Heavy
Scoria (N)
Red/Brown
Medium
Holes
Rough
Heavy-Light
Has holes
Very Rough
Sinks
Pumice (O)
White
Small Holes
Rough
Very Light
Has Holes
Rough
Floats
Obsidian (P)
Black &
Red
Solid
Smooth
Heavy
Glassy
No crystals
Shows lava flow
Ash Tuff (R)
White/Tan
Solid
Ashy/Gritty
Light
Ashy/Gritty
Mix of volcanic material
Vitric Tuff (T)
White/Tan
Solid
Gritty
Light
Gritty
Mix of volcanic material
Why is this rock special?