IGNEOUS ROCKS
You know that Calvert Cliffs was once part of the ocean floor and that today,
the past is exposed in the rocks of the cliffs. But what types of rocks protect the
story of the cliffs? You remember that Dr. Antomologist gave you
information about three different groups of rocks, igneous, metamorphic,
and sedimentary. You decide to begin your investigation into the cliffs by
investigating igneous rocks.
Objective:
When you have completed this investigation, you should be able to
1.
analyze characteristics of igneous rocks in order to explain how igneous rocks are formed.
Activity 1:
Hey Guys! Your teacher told me that you wanted to check out igneous
rocks and see if they can give you any information about the cliffs! Well,
here are two different igneous rock samples to start you on your way. Each
rock formed differently. One rock formed on the Earth’s surface, while the
other formed deep within Earth’s crust. Can you figure out which is which?
1.
Observe the characteristics of the two rock samples provided by your teacher.
2.
Discuss with your partner the different characteristics that you are able to observe about
each rock (i.e. grain size, texture, pattern, etc.). Write your list of observed characteristics in
the space below.
3.
Predict where each rock sample may have formed. Provide specific characteristics that you
observed in order to help support your prediction.
____________________________________________________________________________
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Pg. 1
Activity 2:
Now that you’ve had a chance to observe some of the characteristics of
igneous rocks, you’ll need more information about their formation in
order to determine if they could have affected the formation of Calvert
Cliffs. Use resources provided by your teacher to gather the following
information about igneous rocks:
 From what materials do igneous rocks form?
 How are igneous rocks classified?
 What’s the difference between intrusive to extrusive
igneous rocks?
 How does cooling rate affect the grain size of igneous
rocks?
5.
Record specific information that you gather on the graphic organizer, “Igneous Rock
Formation.”
6.
Use the information that you gathered and Figure 1 to answer the following questions.
B
Earth’s
surface
Volcanic
layers
A
Figure 1
A. Lava is found at _________ and magma is found at __________.
A, B
A, B
B. Magma cools ______________________ because it is __________________________.
quickly, slowly
under the ground, on Earth’s surface
C. Igneous rocks with large crystals form from _________________________ whereas
lava, magma
igneous rocks with small or no crystals form from ____________________.
lava, magma
Pg. 2
Activity 3:
Think you know enough about igneous rocks to identify them? Use what
you’ve researched about igneous rocks to identify the different rock
samples I’ve sent to your teacher. Good Luck!
Materials (per group):
Various igneous rock samples
Hand lenses
Dichotomous key for igneous rocks
Procedure
1.
Read “Identifying Igneous Rocks.”
IDENTIFYING IGNEOUS ROCKS
A. Obtain the rock samples from your teacher.
B. Use the dichotomous key to identify the igneous rocks based on their observable
characteristics.
C. Place the rocks in the appropriate space on the dichotomous key when you think you have
correctly identified each rock.
D. Ask your teacher to verify your identification of each rock.
E. Determine if each rock was most likely formed from magma or lava. (HINT: use grain
size/crystal size to help you make your decision.
F. Record your decision in Chart 1, “Classifying Igneous Rocks.”
G. Identify the rocks as either intrusive or extrusive by placing an “X” in the appropriate
column.
2.
Reread the procedures and explain how the dichotomous key will be helpful in classifying
the different rock samples.
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3.
Conduct the investigation.
Pg. 3
Chart 1:
Classifying Igneous Rocks
Rock
Formed from
Magma or Lava
Cooled Slowly
(Intrusive)
Cooled Rapidly
(Extrusive)
Andesite
Basalt
Diorite
Gabbro
Granite
Obsidian
Pumice
Rhyolite
Analysis
1.
Make a general statement about how igneous rocks are formed. Use evidence from the
investigation to help you write your statement.
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2.
Explain how the cooling rate of molten material affects the formation and the grain
size of igneous rocks. Provide examples from the investigation to support your
response.
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Fossil Fact:
Determine if it would be possible for the fossils of Calvert Cliffs to
have formed in igneous rocks.
Pg. 4
Forms from
_______________________
(molten material inside the
Earth)
Classified As
_______________________
(molten material on the
Earth’s surface)
Classified As
Intrusive or Extrusive
Intrusive or Extrusive
(circle one)
(circle one)
Cools
Cools
Quickly or Slowly
Quickly or Slowly
(circle one)
(circle one)
Description of Grain Size
Description of Grain Size
Pg. 5
DICHOTOMOUS KEY FOR IGNEOUS ROCKS
Are the rock grains
the size of sand or
smaller (need a hand
lens to see)?
YES
NO
Does the rock
look like black,
shiny glass?
Does the rock contain
pink, white, or black
quartz (looks
speckled)?
YES
NO
Does the rock look
porous (like a
sponge)?
YES
YES
NO
Gabbro
Diorite
Obsidian
Granite
Is the rock mostly dark
grey in color (no
white)?
NO
Pumice
Is the rock dark
gray or black?
NO
NO
Andesite
YES
NO
Does the rock contain
mostly pink or white
quartz (looks mostly
pink or white)?
YES
Basalt
YES
Rhyolite
Pg. 6