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Identifying Rock Types
ss
AC T I V I T Y OV E RV I E W
LAB O R AT
Students observe rock samples and use a table of identifying characteristics to help
identify each rock as either igneous, metamorphic, or sedimentary. Students then use
this knowledge to identify the type of rock brought back by the forest hikers and decide
whether or not the hikers were likely to have seen diamonds.
KEY CONCEPT S AND PROCESS SKILLS
(with correlation to NSE 5-8 Content Standards)
1.
One way to gather data is through observations. Accurate and complete observations are important for making conclusions about the natural world. (Inquiry:1)
2.
Rocks are grouped into three categories—sedimentary, igneous, and metamorphic—based on how they are formed. Sedimentary rocks are formed by the
ongoing deposition of rocks and other sediments that are cemented together.
Igneous rocks are formed by the cooling of magma and metamorphic rocks are
formed from earth's heat and pressure on rocks. (EarthSci: 1)
KEYVO C ABUL ARY
igneous
metamorphic
rock
sedimentary
B-71
Activity 20 • Identifying Rock Types
M AT E RI AL S AN D AD VAN C E P R E P AR AT I O
N
For the teacher
1
Scoring Guide: GROUP INTERACTION
For the class
Number
of samples
8
Rock
coal
Rock type
sedimentary
Color of paint dot
yellow
8
limestone
sedimentary
blue
8
sandstone
sedimentary
black
8
gabbro
igneous
purple
8
granite
igneous
light blue
8
obsidian
igneous
white
8
gneiss
metamorphic
red
8
phyllite
metamorphic
brown
For each group of four students
1
rock sample collected by the hikers (garnet schist)
8
rock samples
For each pair of students
1
magnifying lens
For each student
1
Student Sheet 20.1, "Rock Data"
1
copy of Scoring Guide: GROUP INTERACTION (optional)
Each type of rock should have a small spot of paint on it. Because some of the colors
may appear similar and/or blend in with the rock color, be sure that you can identify each
rock sample prior to starting the activity.
This activity provides an opportunity to assess a student's ability to work in groups. You
can use your copy of the GROUP INTERACTION Scoring Guide to assess your students' ability
to work as a team and to respectfully consider each other's ideas.
Masters for all Scoring Guides can be found in Teacher Resources III: Assessment.
T E AC H I NG S UMMARY
Getting Started
1.
Discuss the ways in which rocks can be identified.
Doing the Activity
2.
(GI ASSESSMENT) Students examine and identify rock samples by rock type.
Follow-Up
3.
B-72
Students decide whether the hikers were likely to have seen diamonds.
Identifying Rock Types • Activity 20
B AC K G RO U NDI NF O R M AT I O N
Identifying Rock Types
Rocks are generally grouped based on how they are formed and not on how they look.
As a result, every identifying physical characteristic based on rock formation has
exceptions. For example, both granite and obsidian are igneous rocks, even though one
has large crystals and the crystals aren't even visible in the other. This variation is one
reason it can be very difficult for students to sort a random collection of rocks based on
how they are formed. (It is much easier to use a key to identify a particular rock, such as
limestone, and then deduce how it was formed afterwards.) The rocks used in this
activity have been selected to facilitate identification by rock formation by students of all
ages. Note that Table 1, "Rock Characteristics," will not always work sucessfully with
other rock samples.
Geologists use several kinds of information to identify rock types. These include the size
of crystal grains, the type of layering that may be present, relative hardness, the types of
minerals present in the rock (which can be indicated by color), and texture. However, not
all rocks are easy to categorize, and even experienced field geologists may occasionally
have difficulties classifying a particular rock.
All rocks are made up of one or more minerals. It is these minerals that give different
rocks their distinct appearance. For example, granite is mostly made up of the miner- als
quartz, orthoclase, and plagioclase. When referring to the size of grains in a rock, it is
the size of these different mineral crystals that is being examined.
Granite, an igneous rock, has large crystals of its different component minerals that can
easily be seen by the human eye. Granite is a plutonic igneous rock, or one that cooled
below the surface of the earth. Plutonic rocks—named for Pluto, the Roman god of the
underworld—tend to have large crystals that can be easily seen. Igneous rocks that cool
on the earth's surface are known as volcanic because they usually have erupted from
volcanoes. Volcanic rocks may have small crystals, or may lack crystals entirely and
have a smooth, glassy appearance. Obsidian is one example of a volcanic rock with a
glassy surface.
Sedimentary rocks have crystals of varying size. Clay and mudstone are examples of
sedimentary rocks with such small particles that it is often difficult to distinguish individual pieces. Layering, relative softness, crumbliness, and the presence of fossils are
often more useful in identifying sedimentary rocks.
Metamorphic rocks tend to have small crystals and may sometimes appear to have
crystals that are more densely packed than the other rock types. In some cases
(foliated metamorphic rock), foliation, or banding, may be seen as a series of compact
layers. Foliation is often more visible when the layers are composed of different-colored
minerals. These layers may sometimes appear wavy, particularly in samples of gneiss.
B-73
Activity 20 • Identifying Rock Types
T E AC H I N G S U G G E S T I O N
S
students will confuse the layering seen in sedimentary rocks, such as coal, with the foliation seen in
metamorphic rocks, such as phyllite.
G E T T I N G S TA R T E D
1.
Discuss the ways in which rocks can be
identified.
Review Table 1 "Rock Characteristics" in the Student
Book to begin discussing how to identify rocks by
rock type. Use the background information to provide
students with any necessary information about how
to do this.
Review what is meant by the word "crystals." Unlike
the mineral samples examined so far, the size of
most mineral crystals is small. The large crystals
referred to in the table would be no larger than 0.5
centimenters, while small crystals are generally not
even visible without magnification. Refer students to
the figure in the Student Procedure comparing large
and small crystal size.
DOING THE ACTIVIT Y
2.
(GI ASSESSMENT) Students examine and
identify rock samples by rock type.
Students may realize that they are being asked to
identify the rock type of three rocks that they have
already examined (granite, limestone, and coal) and
read about (coal). Point out that in this activ- ity, they
will still need to record those observations that would
be most useful in identifying these rocks, regardless
of whether or not they can already iden- tify its rock
type.
Pairs of students can work together to examine a
sample and then trade samples with other pairs of
students until they have examined all eight rock
samples. You may wish to use the GROUP INTERACTION
(GI) Scoring Guide to assess how well students are
working together to examine the samples and to
discuss their ideas.
Students may have particular difficulty in identify- ing
sedimentary vs. metamorphic samples. This is
because sedimentary rocks are formed from the layering of sediments, and, when exposed to earth's
heat and/or pressure, they form consecutive thin
layers that look like bands within the rock. Many
B-74
If students have identified a rock or rocks incorrectly, inform them of the number of identifications
they have correct and ask them to carefully reexamine the rocks to see if they might want to
change any of their IDs. You may want to clarify the
type of banding (striation) students should see or
show them how to determine crumbliness.
For Part B, there are eight samples of the hikers'
rocks. Students should work together in groups of
four to examine and discuss the sample. Explain that
the reddish-brown crystals are garnets. When
examining crystal size, students should focus on the
size of the crystals in the rest of the rock (they are
very small); making them more likely to correctly
identify the rock as metamorphic. If students
observe the large crystals and think the rock might be
igneous, ask them if the crystals meet the other
characteristics of large crystals in igneous rock,
such as interlocking patterns (they do not). If students observe the banding and think that it might
represent the layering found in sedimentary rocks,
ask them to determine if the sample has other common characteristics of sedimentary rock, such as
crumbliness.
A sample response to Student Sheet 20.1, "Rock
Data" is shown on the next page.
FOLLOW-UP
3.
Students decide whether the hikers were
likely to have seen diamonds.
Use the student responses to Questions 1 and 2 to
help summarize the activity.
Discuss Question 3, which helps explain how students can deduce whether diamonds are likely to
have been seen in Roughpoint National Forest. Students' identification of the hikers' rock may vary; it is
more important that students support their identification with relevant and accurate evidence (in
Question 3b) from Table 2. The primary difference
between the data on kimberlite and garnet schist is
Identifying Rock Types • Activity 20
Sample Response to Student Sheet 20.1, "Rock Data"
Paint Color
Name of Rock
Observations of Identifying Characteristics
Igneous,
Metamorphic, or
Sedimentary?
yellow
coal
Very light relative to its size and crumbly (flaky)
sedimentary
purple
gabbro
Individual crystals are large and interlock
igneous
red
gneiss
Different bands of color, very small crystals
metamorphic
light blue
granite
Individual crystals are large and interlock
igneous
blue
limestone
Contained a fossil
sedimentary
white
obsidian
Surface smooth like glass
igneous
brown
phyllite
Bands sandwiched together, crystals very small
metamorphic
black
sandstone
Can feel sand, which can be crumbled off the sample
sedimentary
—
hikers' rock
Bands sandwiched together, crystals very small
metamorphic
that kimberlite is igneous and garnet schist is metamorphic. If students identified the rock sample as
metamorphic, the rock should be identified as gar- n e
t s c h i s t . I t s h o u l d m a t c h a l l o f t h e d a ta i n t h e
table. You may want to ask, How could we make the
identification more positive? What else could we do?
Possible responses include finding more data on the
characteristics of kimberlite and garnet schist and
determining if the rock sample strongly matches the
data on the garnet schist.
Complete the story of the hikers' rock by asking,
What do you think Ranger Ormond should do now? If
they have identified the rock as garnet schist, they
are likely to recommend that nothing be done, since
diamonds are not found in association with garnet
schist. Some students may consider the gar- net
schist valuable and recommend going out to the site
to gather or look for samples of large garnet crystals.
SUGGES TED ANSWERS TO QUES TIONS
1.
Do all rocks of the same type, such as all
igneous rocks, look the same? Support
your answer with evidence from your observations.
No, they do not. In this activity, the igneous and
sedimentary rock samples showed a lot of variety. The obsidian does not look like either of the
other igneous rock samples (granite or gabbro).
The coal, limestone, and sandstone all look very
different from each other, even though they are all
sedimentary rocks.
2.
What characteristic was most useful in
identifying each type of rock listed below? Explain.
a. igneous
The large, interlocking crystals were most
helpful in identifying granite and gabbro,
while the shiny surface was most helpful in
identifying obsidian.
B-75
Activity 20 • Identifying Rock Types
b. metamorphic
c.
The bands of color sandwiched together were
No, because diamonds are found in kimber-
t he most helpful in identifying gneiss and
phyllite.
lite. The hikers brought back a sample of
garnet schist. So there are probably not
diamonds there.
sedimentary
The presence of fossils was most helpful in
identifying limestone, the crumbliness was
most helpful in identifying the sandstone, and
lightness relative to its size was most helpful
in identifying coal.
3.
c. Could the hikers have seen diamonds? Explain.
4.
Imagine having to identify another rock.
Which of the following facts would be the most helpful to you? Explain your reasoning.
The rock is light gray in color.
The rock is shiny.
Help Ranger Ormond identify the hikers'
rock by examining the table below [in student book].
The rock was found high on the slope of an
a. Based on your observations of the rock sample,
The rock is small enough to fit a kid's hand.
which rock do you think the hikers found?
Garnet schist.
Some students may conclude that the rock
sample is igneous and identify it as kimberlite. Because it can be difficult to identify rock
types, accept kimberlite as an answer if it is
supported with relevant evidence in Question 4b. Identifying the rock as kimberlite also
means that, in response to 4c, students will
conclude that the hikers could have seen
diamonds.
b. Support your answer with evidence from your
investigation.
The rock sample is metamorphic because it
had bands of color sandwiched together.
Garnet schist is metamorphic and kimberlite is
not (it is igneous).
B-76
extinct volcano.
Knowing that it was found on the slope of an
extinct volcano is extremely useful because this
makes it probable (though not certain) that the
rock is igneous. The color and shininess do not
provide enough information without seeing the
rock itself, because there are metamorphic and s e
d i m e n t a r y ro c k s t h a t c o u l d b e d e s c r i b e d a s
gray and shiny. Size is not helpful in identifying a
rock.
Obsidia
n
Phyllit
e
Sandsto
ne
hikers'
rock
whit
e
brow
n
blac
k
_
_
Gneis
s
re
d
Limesto
ne
Gabbr
o
purpl
e
Granit
e
Coa
l
yello
w
light
blu
e
blu
e
Name of
Rock
Colo
r
©2006 The Regents of the University of
California
Observations of Identifying
Characteristics
Igneou
s,
Metamorphic,
or
Sedimentar
y?
Name Date
Rock Data
Issues and Earth Science • Student Sheet 20.1 B-77