Grade Level: 9 – 12

Purpose:
The crust is made of them. Dig down anywhere and you will hit them. Igneous rocks are the foundation of Illinois and no story of geology is complete without a study of them. The purpose of this lesson is to show how igneous rocks form, what makes them different from one another, and how to identify them.

Suggested Goals:
Students will learn how igneous rocks originate by growing crystals and creating a drawing showing where each variety of rock forms. They will learn to identify the various igneous rocks by using a chart that describes each variety.

Targeted Objectives:
By the completion of this activity, students should:

1. Be able to explain why igneous rocks are important.
2. Be able to explain how each variety of igneous rock forms.
3. Be able to explain what characteristics are used to identify igneous rocks.
4. Be able to tell one common igneous rock from another.
5. Be able to identify intrusive formations where igneous rocks form within the earth.

Background:
Igneous rocks are rocks that harden from molten magma. Some of them form within the earth and are called intrusive while others pour out onto the surface and are called extrusive. Where they form is crucial in their identification. Those that form within the earth cool slowly due to the earth’s heat. During that interval, which can last years, minerals in the magma migrate and are able to join with similar molecules to form crystals. The more time that the magma remains a liquid, the larger the crystals will become. Which minerals are present and how large the crystals become, are two important factors in the identification of igneous rocks.

Some magma, which is called lava when it reaches the earth’s surface, is shot high into the air from pressure and gasses that accompany many eruptions. The air cools the magma and hardens it while it still contains the gasses that were present in the magma as bubbles. The resulting rock is filled with an infinity of holes which were once gas bubbles in the airborne lava. It would be like suddenly freezing the foam that flows over the side of a soda bottle and it is also like the holes that can be seen in a piece of bread that were once bubbles of carbon dioxide.

If lava flows into the ocean or is the uppermost crust in a flow of lava, there is no time for even the smallest of crystals to form. The resulting rock becomes like glass. Walking on newly hardened lava sounds much like walking on broken glass as your feet break filaments of the rock called obsidian.

Lava that remains in the earth forms what are called intrusive formations. The source of the lava is called the batholith. The deeper in the crust the larger the batholith usually becomes. A laccolith is a smaller, but never-the-less, huge formation that may resemble a mushroom. It begins as a column of magma working its way toward the surface but for some reason it stops rising and the magma collects in a lake of molten rock. Both laccoliths and batholiths can form mountains. The Sierra Nevada mountains are the result of a batholith being uncovered by erosion. Laccoliths can also be uncovered to form mountains or they can push the overlying rocks up creating folded mountains above them.

Two smaller intrusive formations are dikes and sills. A dike is a sheet of lava that cuts through the rocks around it much like a shovel cutting through dirt does but from the opposite direction. A sill is a sheet of lava that stops rising toward the surface and spreads out between two layers of strata within the earth. Usually dikes are vertical and sills are horizontal but not always.

Texture (crystal size), color (which minerals are present), and presence or absence of holes are used to identify igneous rocks. The following diagram is useful in identifying the most common igneous rocks.

Small crystals refer to speck size crystals or smaller. Basalt is easy to remember because it is black with what looks like specks of salt in it so the name can help students remember what it looks like.

It is thought that modern magmas contain more of the dark minerals because the earth’s crust is getting thicker so the magmas that reach the surface are rich in heavier elements; minerals such as iron and magnesium that come from deeper in the earth.

Activity 1: Crystal Growing

Materials:

• Salol (Phenyl Salicylate)
• 10 metal teaspoons or 10 GLASS microscope slides
• Ice
• Magnifiers (hand lenses)
• Safety glasses
• Heat source such as a candle
• Matches
• Copies of the procedure sheet that follows (available in printable form in the PDF download version of this lesson)

Preparation:
Read the CAUTIONS on the bottle of salol. Salol is a very safe compound when used correctly but be sure to read the warnings before beginning class. Salol is a compound that grows crystals in minutes. It can be used to grow crystal repeatedly even in the limited time available in a class period. Give each group a pinch of the compound to use in their crystal growing activity. Groups of two or three students work best. Place the salol in a teaspoon or on a glass slide. I have found that the spoons work much better if they are available. If you are using glass slides be aware that there will be a carbon build-up on the bottom that should be removed between periods. It can easily be wiped off with a paper towel. Give each group a candle, an ice cube, a tiny quantity of salol other than that on the spoon/slide, and a magnifier. Have them wear their safety goggles.

Procedure:
In this activity you are going to do an amazing thing, you are going to grow crystals. This process can take years to happen in nature but you will grow them quickly with a chemical called salol. If you touch the chemical be sure to wash your hands before leaving the classroom. Your teacher will give you a small amount of salol.

Part 1:
You will need to melt the salol over a candle. Do not hold it over the flame very long because it will get so hot that no crystals will grow for a long time. Slide it over the flame and wait to see how much melts. Continue to do this until almost all of the salol has melted, then set it down on the table and do not disturb it. The crystals should begin forming in just a few seconds. If you melted ALL of it, the crystals will not be able to begin forming since they need something to attach to. You will need to drop some solid grains of salol, called seed crystals, into the melted salol. If these melt, wait a few moments and try again until the crystals begin to grow. Move the solid salol from side to side to let the light show the crystals better. One of the neat things about salol is that it can be melted over and over again to grow new crystals. If you do not like the ones you got the first time, try again.

Observations:
[Have students record their observations from Part 1.]

Part 2:
Now we are going to repeat the experiment, but once you see crystals beginning to form, place the spoon/slide on an ice cube to harden.

Observations:
[Have students record their observations from Part 2.]

Questions:

1. Which crystals were larger? No ice or ice? Why? [Students should notice that the crystals were largest in the salol with no ice. They are bigger because the molecules had more time to lock into place. Once the salol hardens all molecular migration ceases and no more molecules can increase the size of the seed crystals.]
2. If crystals formed underground where it is warm a very long time, would you expect large or small crystals to form? Why? [The largest crystals would form underground where the magma stays liquid longer and there is more time for the molecules to join.]

Activity 2: Drawing Igneous Rock Formation

Materials:

• blank copy paper
• crayons, colored pencils, or markers

Procedure:
Before proceeding to the next activity, discuss the terms batholith, laccolith, dike, and sill.

In the following activity, your students will create a drawing showing where each major variety of igneous rock would form. They will also draw a batholith, laccolith, dike, sill and a volcano so that they will become more familiar with those terms.

Draw the following diagram on the board and then give each student a blank piece of copy paper and have them create a masterpiece picture compete with labels and color.

Questions:

1. What do the polygons in the batholith represent? [Crystals]
2. Why are the crystals larger in the batholith than in the pipe? [The crystals in the batholith form farther under ground where they stay melted longer and have more time to form.]
3. Why are there no crystals in the obsidian? [It is made from lava that flows into water and hardens instantly. There is no time for even tiny crystals to form.]
4. What kind of rock would form on the left side of the volcano? [Basalt]
5. If we could see farther underground, what would we expect to see? [Even larger crystals in granite]
6. What kind of rock is made of pink feldspar with large crystals? [Granite]
7. What kind of rock is likely to form in a sill, dike, or laccolith? [Granite]
8. How do pumice and scoria form? [They are shot out of a volcano and are filled with gas bubbles.]

Activity 3: Rock Identification

Purpose:
In the final activity, students will identify several common igneous rocks.

Materials:

• Ten rock kits (boxes, baggies or baskets), containing pieces of
  • Granite
  • Pumice
  • Scoria
  • Obsidian
  • Basalt
  • Rhyolite
  • Gabbro
• 10 Magnifiers or hand lenses
• Copies of the procedure sheet that follows (available in printable form in the PDF download version of this lesson)

Procedure:
Look at each of the specimens in your kit. Two of the specimens will have holes. Place them where you feel they belong on the chart. Another of the specimens is glassy. Find its place. Decide the texture (crystal size) and the coloring and place each rock where you believe it should go.

When you have finished call your teacher over to see if you are correct in your identification.

When your teacher has approved your identification, write a description of each rock!

• Pumice:
• Scoria:
• Granite (see ISM Geology Online picture of granite):
• Obsidian:
• Gabbro (see ISM Geology Online picture of gabbro):
• Rhyolite (see ISM Geology Online picture of felsite):
• Basalt (see ISM Geology Online picture of basalt):

Questions:

1. One of the rocks in your kit is so light that it floats. Which one is it? Why do you think that it floats? [Pumice. It is filled with holes.]
2. One of the rocks is made of crystals of the minerals feldspar, quartz, hornblende, and mica (see ISM Geology Online picture of mica and quartz). Usually the rock will be pink with black and white crystals in it. Which rock is it? [Granite]
3. Which rock formed the farthest under ground and how do you know? [Granite or Gabbro (depends on your specimens) because it has the largest crystals.]

Assessment:

1. Correct the rock drawing for accuracy and neatness.
2. On the final test for the unit, have your students match the names of the rocks with where they would go on the drawing.
3. Collect the crystal growing activity and correct the questions.
4. Give a lab grade for how well each group was able to identify the specimens.
5. Have a lab portion to the unit test in which the students identify the igneous rocks.
6. Give the following essay questions:
   1. What properties are used to identify igneous rocks?
   2. Explain why some rocks have large crystals and others have microscopic ones?
7. Use class discussion as an indicator of class understanding.

Extensions:

• Look for samples of igneous rocks in a stream bed. Identify them and make a collection.
• Create a poster of the drawing that shows where igneous rocks form and glue the rocks where they would go.
• Obtain specimens of other igneous rocks and have your students try to identify them.
• Grow crystals of sugar, salt, and alum or give your students extra credit for growing them at home and bringing them in.
• Obtain commercial crystal growing kits and grow beautiful crystals in your classroom.
• View a video about volcanoes or igneous rocks
• Examine ISM Geology Online picture of pictures for granite, peridotite, gabbro, diorite, basalt, felsite, and porphry.
• The GEMS booklet Stories in Stone has wonderful lessons on crystals. It contains patterns for making cardboard crystal shapes and detailed information about growing crystals.

Print Resources:

• Geology Underfoot in Illinois by Raymond Wiggers, Mountain Press Publishing Company, Missoula, Montana 1997. phone 800-234-5308
• The Audubon Society Field Guide to Rocks and Minerals available in most bookstores
• The Pocket Guide to Rocks and Minerals by Michael O’Donoghue, Parkgate Books, London 1991, 1999 Available in most bookstores
• Peterson Field Guide Rocks and Minerals by Frederick H. Pough Houghton Mifflin Company 1996 Boston and New York available in most bookstores
• Stories in Stone by Kevin Cuff, GEMS (Great Explorations in Math and Science), Lawrence Hall of Science, 1995

Web Resources:

• The Illinois Geologic Survey
  http://www.isgs.uiuc.edu/isgshome/isgshome.html
• See ISM Geology Online pictures for granite, peridotite, gabbro, diorite, basalt, felsite, and porphry.
  http://geologyonline.museum.state.il.us/geogallery/
• Mineral Identification Project
  http://www.ironorchid.com/minerals/

Lesson Specifics:
The lesson requires about two or three class periods. It is desirable to have groups of two or three for the lab portions

Illinois State Board of Education Goals and Standards:

• 11.A.4: Collect, organize and analyze data accurately and precisely.
• 12.E.4b: Describe how rock sequences and fossil remains are used to interpret the age and changes in the Earth.
• 12.E.5: Analyze the processes involved in naturally occurring short-term and long-term Earth events (e.g., floods, ice ages, temperature, sea-level fluctuations).