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Catastrophic Events
Part III
Lesson 22: Exploring Igneous Rock Formation
Focus Question: How are igneous rocks formed?
Goals:
 The rate of cooling of a given solution affects the size of the crystals
formed.
 Intrusive igneous rocks, which form when magma cools slowly beneath
the earth’s surface, have a coarse-grained crystalline texture.
 Extrusive igneous rocks, which form when lava cools quickly on the earth’s
surface, have a fine-grained crystalline texture.
 Some igneous rocks cool in two stages: underground and on the earth’s
surface.
 Some igneous rocks cool in two stages: underground and on the earth’s
surface.
 There are three basic rock types: igneous, sedimentary, and
metamorphic.
 The rock cycle is a continuous process in which each of three basic rock
types transforms into one of the other rock types through episodes of
mountain building, erosion, sedimentation, melting and recrystallization.
Teaching Point:
Student Objectives:
 Examine the texture, or crystal size, of three igneous rock samples under
magnification.
 Conduct tests to analyze the size of crystals that form from a solution
under different cooling conditions.
 Using information gained in the lab; hypothesize about the conditions
under which each of three rocks formed.
 On the basis of their properties and origin, identify igneous rocks by
name.
 Read to learn more about the rock cycle.
Getting Started:
1. Brainstorm what you know and want to learn about crystals. Your teacher
will record your ideas.
2. Collect a plastic box for your group. Take from it one small cup of
magnesium sulfate crystals, three hand lenses, one loupe, and four sheets
of black construction paper. Place a few crystals on the black paper and
observe them closely.
3. Discuss your observations. Answer the following questions:
a. What is the shape of each crystal?
b. In what ways are the crystals the same or different from one
another? Why do you think this is so?
c. How are these crystals different from others you have seen?
d. How do you think the crystals formed?
4. Fold your black paper like a chute and pour the crystals back into the
small cup. Save the paper.
5. Remove the three igneous rock samples labeled #1, #1, and #5 from
your plastic box. Spend a few minutes examining the size of the crystals,
or texture, of each rock. Discuss your observations with your group using
the following questions as your guide:
a. What are the properties of the rocks?
b. What do you notice about the texture, or crystal size, of each rock?
c. How are the rocks alike and different?
d. On the basis of what you already know about crystals, how might
the size of the crystals reveal how each rock was formed?
6. Report to the class what you observed about the texture, or crystal size,
of each rock. Although each of these rocks is igneous, they look different
because their crystal size is different. This is an indication that they
formed under different conditions. You will investigate this concept during
Inquiry 22.1.
Inquiry 22.1: Investigating Crystallization
1. Look at the materials set up in front of the room. Predict what would
happen if you mixed the magnesium sulfate crystals with water and
heated the solution.
2. Before you begin, review the Safety tips with the class.
3. Watch as student volunteers create a magnesium sulfate solution.
4. Discuss with the class what you already know about solutions, solvents,
and solutes. Answer these questions:
a. What does it mean when we say that something “goes into
solution”?
b. In most cases, how does the temperature of the water affect the
solute’s ability to go into solution?
c. How does stirring the water affect the solute’s ability to go into
solution?
d. What is a “saturated” solution?
5. now answer these questions:
a. What would happen to the solution if it were allowed to cool?
b. Is there any way to get back the crystals that dissolved in the
water?
c. Do you think the rate at which the solution cools will affect the size
of crystals that form? Explain your prediction.
6. With your teacher, review Procedure Steps 7 through 11. Listen as your
teacher discusses how to cool one Petri dish quickly and another slowly.
One group will be selected to cool their dishes in two stages.
7. Collect one copy of Student Sheet 22.1a for your group. Your group is
responsible for recording your experimental design on the sheet before
you complete the inquiry. Write down the question you will try to answer
and predict what you think will happen. Then describe the materials and
procedures you will use, what you will keep the same in each setup, what
you will measure, and how you will measure it.
8. Have one member collect the solution from the hot pot area. Collect a
pipette and two Petri dishes. Very carefully, put about 10 to 20 mL of
solution in each Petri dish, just enough to cover the bottom of the dish, as
shown in Figure 22.1.
9. Add a “pinch” of crystals (approximately 10 granules) to each Petri dish,
as shown in Figure 22.2. These “seed” crystals will help other crystals
begin to grow.
10. Set out one of your Petri dishes in an area where it will cook quickly and
the other dish in an area where it will cool slowly. Use index cards to
label each dish either “Fast Cooling #1” or “Slow Cooling #2.” Put your
group number on the cards as well. Circle the number. (If your group is
testing the two-stage cooling, label your two Petri dishes “Two-Stage
Cooling #3.)
11. Clean up. Replace all rock samples and observation tools in the plastic
box. You will use them again during “Reflecting on What You’ve Done.”
If your teacher teaches another class, do the following:
a. Refill the small cups with magnesium sulfate.
b. Put out new Petri dishes and index cards.
c. Do not throw away the black paper. It will be used again in Lesson
23.
Reflecting on What You’ve Done:
1. Collect one copy of Student Sheet 22.1b. Discuss the sheet with your
teacher.
2. Notice that the Petri dishes are labeled #1, #2, or #3. Examine your own
and other groups’ petri dishes and record your observations on Student
Sheet 22.1b. Do not complete the last column yet. Use a metric ruler to
measure the crystal size, if possible. Explain why you think the crystals
are this size.
3. Discuss your observations of the various crystal sizes. Answer these
questions:
a. What observations did you make about the contents of each Petri
dish?
b. What observations did you make about the crystal size in each Petri
dish?
c. Did the rate of cooling (that is, how fast each Petri dish cooled)
affect crystal size? If so, how?
4. Collect your plastic box and remove the three igneous rock samples (#1,
#2, and #5), hand lenses, and loupe. Try to draw some conclusions
about how each of these rocks formed, given your observations of the
Petri dishes. Again, use your ruler and loupe to measure the crystals.
5. Your teacher will give your group scissors and two copies of Inquiry
Master 22.1, which provides information on each rock studied in this
lesson. Cut the masters into four tables so that each group member has
one. With your group, identify each of the rocks by name. Record your
identifications in the last column of Student Sheet 22.1b.
6. Apply the results of this investigation to the earth by answering these
questions:
a. Using what you learned from this investigation, how quickly or
slowly do you think igneous rocks form under the earth’s surface?
What type of rock might this form?
b. Using what you learned from this investigation, how quickly or
slowly do you think igneous rocks form on the earth’s surface?
What type of rock might this form?
c. If fast-cooling lava forms fine-grained volcanic rocks such as basalt,
why aren’t all volcanic rocks basalt?
d. On the basis of the evidence gathered from your investigation, how
do you think each rock sample formed?
7. Read “The rock Cycle,” on pages 250-251.
8. Look ahead to the next lesson, in which you will examine pieces of
volcanic rock called ash.
9. Play the “Comparing Catastrophic Events” game with your group by doing
the following:
a. Collect your group’s bag of activity cards. If your cards are not cut,
you will need to cut the four sheets of Comparing Catastrophic
Events Activity Cards into individual cards.
b. Shuffle all four sets of cards. Put the cards into one pile in the
center of your group. Each group member should have an Activity
Board.
c. To play the game, each student selects 10 activity cards from the
pile to place on his or her Activity Board in the appropriate cells.
The remaining cards should remain in a pile in the center of the
group.
d. Working clockwise, one student at a time selects a card from the
pile and places it on his or her board before play rotates to the next
student. (Refer to Table 8.1: Comparing Catastrophic Events, if
you need to.) Return any cards you cannot use to the discard pile.
e. The game ends when the first student fills one row on his or her
Comparing Catastrophic Events Activity Board. The cards must be
placed correctly to win. Use Table 18.1 as a key.
f. When you are finished, replace your group’s cards in the plastic
resealable bag. If it has not already been done for you, mark on
the outside of the bag the number of sets of cards in that bag.