Waco Gem and Mineral Club, Waco, Tx

10.3 Igneous Rocks


10.3 Igneous Rocks

a) Demonstrating effects of cooling and gases

Igneous rocks form from molten magma from inside the earth that cools and solidifies as it nears or reaches the surface. To show kids how a hot, liquid substance can become rock hard when it cools, here are a couple of easy demonstrations.

A. Fast cooling versus slow cooling. As molten magma cools, crystals form. If the magma cools very slowly, those crystals have a chance to grow large. This is what happens in granite, an intrusive igneous rock that generally forms deep underground and takes an extremely long time to cool. If magma cools more quickly, crystals don‘t have a chance to grow as large, so the resulting rock has a smaller crystal structure. This is seen in basalt, an extrusive igneous rock formed from magma that rose to the surface of the earth where it cooled more quickly in the air. Sometimes magma cools super-fast, and when that happens, crystals may not have a chance to form at all, as seen in smooth obsidian.

If you have some available, use specimens of granite, basalt, and obsidian to illustrate this difference in rock texture and crystal size. Have kids examine each closely with a magnifying glass to see the differences and have them use their sense of touch to feel the different textures. You can illustrate how crystals grow to different sizes depending upon how quickly they cool with the following experiment:


 Cooking pan with a half-cup of water Two and one-half cups of sugar
 Hotplate or stovetop Empty bowl chilling inside a larger bowl half-filled with ice cubes
 Spoon and ladle  


1. Bring the water to a boil and slowly stir in sugar until you form hot syrup.

2. Ladle just a small bit of your syrup into the empty chilled bowl to cool quickly as a thin film on the bottom of the bowl.

3. Leave the rest of your syrup in the original cooking pan to cool slowly.

Once both mixtures have cooled, you should observe that the mixture in the chilled bowl is very clear and smooth, with only tiny sugar crystals having formed, whereas the mixture that cooled more slowly in the hot pan is coarser and lumpier and cloudy or milky looking. Similarly, magma that cooled quickly as lava on the surface of the earth tends to have smaller crystals and a more finely grained texture whereas granite, which cooled much more slowly as magma deep beneath the earth, tends to have large crystals and a very lumpy texture.

B. Quick cooling and the effects of gas bubbles. If magma cools super-fast, no crystals may form at all, and you end up with volcanic glass, or obsidian. While we usually think of volcanic glass as being smooth and shiny, as you’ll see in this demonstration, a little gas can make a big difference in texture and appearance.


 3 cups of sugar Spoonful of baking soda
 3/4 cup of light corn syrup Cooking pan & wooden spoon
 3 tablespoons of white vinegar Candy thermometer
 1/3 cup of water Cookie sheet or shallow brownie pan
 Butter or margarine Stove or hotplate


1. Grease the cookie sheet or shallow brownie pan with your butter or margarine and chill it in a refrigerator or over ice cubes.

2. Stir your sugar, corn syrup, vinegar, and water together in a cooking pan over high heat. Stirring constantly, cook to 302° F (150° C) on the candy thermometer, or 'hard crack' stage. (Some candy thermometers will have this spot marked and labeled.) The ingredients should end up forming a hot, syrupy liquid.

3. Pour the thick syrup onto the chilled, greased cookie sheet or brownie pan and smooth it into a thin layer.

4. When the syrup mixture cools, it will become a hard lump. (In this case, it‘s a hard-candy lump that should be edible.)

Likewise, hot, soft, liquid molten magma solidifies into a hard igneous rock when it cools. In this instance, you will have created a smooth, clear rock with a texture somewhat like obsidian. Obsidian is lava that cooled very quickly, so quickly that crystals didn‘t have a chance to grow, thus resulting in smooth volcanic glass.

Another volcanic rock that cools to a glassy state is pumice, but unlike smooth obsidian, pumice is rough and porous. It‘s shot through with thousands of tiny bubbles from gases. These gases whipped up a volcanic froth that cooled quickly in the air. To illustrate this effect, you can follow the very same recipe outlined above but with the following twist. After pouring just half of your syrup into one chilled and greased pan or cookie sheet, set your cooking pan down and quickly stir a spoonful of baking soda into the remaining half of your mixture. The baking soda will react with the vinegar to release carbon dioxide bubbles throughout your mixture, which you should now pour into a second chilled and greased pan.

When both mixtures have cooled, shatter both into smaller pieces and have your kids compare pieces side-by-side along with specimens of obsidian and pumice.

b) Making a Volcano

The classic earth science project is making a model volcano that erupts with fluid lava. Here‘s how!


 A 2-foot square sheet of poster board or plywood Red & yellow food coloring
 A small can (empty tomato paste or small mushroom cans work well) Dishwashing liquid
 Newspaper, foil, or wire mesh Measuring cups
 Plaster of Paris, mixing bowl, spoon Plastic film canister with a snap top
 Water Alka-Seltzer or denture cleanser
 Paint, spray adhesive, sand, laquer (optional) Water
 Baking soda Newspapers or dry cloth


1. On your poster board or plywood base, make a mound or cone shape from damp and wadded newspapers, wadded foil, wire mesh, or other suitable material.

2. At the very top, wrap this material around a small can or bottle.

3. Mix plaster of Paris (two parts plaster, one part water), and coat your mound with it, leaving the can open at the top. Then set it aside to let the plaster dry.

4. You can either use the volcano the way it is, or you and your kids can paint the volcano whatever colors you prefer or, for a realistic touch, apply a spray adhesive and sprinkle your volcano with sand (or glitter, for an artistic touch).

5. If you plan on re-using the volcano many times, you should coat the finished work with a lacquer so that it may be easily wiped clean.

You now have a dormant volcano. Here‘s how to make it active and ready to erupt in ways that will simulate two types of volcanic eruptions.

A. Lava flow eruption. Some volcanic eruptions are relatively mild. Rather than a single, massive explosion, they issue a flow of hot, basaltic lava, like we see with lava flows on the Hawaiian Islands or with the extinct cinder cone volcanoes and lava fields in the American West. Here‘s how to simulate this sort of eruption:

1. Place your volcano on newspapers or a drop cloth.

2. Fill the can at the top of your volcano one-third with baking soda.

3. In a separate cup, mix one-third cup of vinegar with a couple drops of red and yellow food coloring and two drops of liquid detergent.

4. Pour this mixture into your volcano with the baking soda to creating a sudden eruption and lava flow!

If you have specimens available, show kids samples of basalt, pahoehoe, or a’a. These are the sorts of igneous rocks formed in a lava flow like the one you‘ve just demonstrated.

B. Explosive eruption. Other volcanic eruptions involve massive, violent explosions, like that which blew the top off of Mount Saint Helens in 1980. Here‘s how to simulate this sort of eruption:

1. Fill a plastic film canister three-fourths with water or vinegar.

2. Drop in an effervescent tablet (Alka-Seltzer or denture cleanser work well).

3. Quickly snap on the canister lid, give it a hard shake, and quickly place the canister into the mouth of your volcano, with the lid of the canister pointing up.

4. Keep kids back from the volcano as they wait. After just a second or two, the lid of the canister will pop several feet into the air along with a quick squirt of foamy water. (This works very quickly if using vinegar along with Alka-Seltzer, but may take a bit longer if using water with a denture cleanser tablet.)

If you have specimens available, show kids samples of rhyolite and andesite. These are the sorts of igneous rocks formed during an explosive eruption.

Making a plaster volcano can be time-consuming and involved and may require several days to complete in stages. It requires time for the plaster to dry, for decorating or painting the plaster, then for coating the volcano with a protective layer of lacquer and allowing that to dry. Here‘s an easier way for individual kids or pairs or teams of kids to make small erupting volcanoes of their own much more quickly.


 Square-foot sheet of stiff cardboard (1 for each child or team of kids) Red & yellow food coloring
 Test tubes or small bottles (1 for each volcano being made) Dishwashing liquid
 Clay or Play-Doh Measuring cups
 Baking soda Newspapers or drop cloths


1. With the cardboard as a base, kids position a small bottle or test tube in the middle.

2. Kids fill their bottles/tubes half full of baking soda, then pile modeling clay around the bottle in the shape of a volcano cone, leaving the top of the bottle open.

3. Mix vinegar with drops of red and yellow food coloring and a drop or two of dishwashing liquid.

4. Pour your vinegar solution into the baking soda to watch the volcano erupt!

c) Collecting igneous rocks

Following are igneous rocks kids may be able to collect if they live in the right area of the country, or that they may be able to purchase from rock dealers or to trade through the mail via the AFMS Patricia Egolf Rock Pals program as a club project with kids in other AFMS/FRA clubs who live in areas where igneous rocks are common:

Andesite is a gray to black volcanic rock with a high silica content that commonly erupts as thick, sticky lava flows from stratovolcanoes, such as those in the Andes Mountains, which gave this igneous rock its name.

Basalt is generally a hard, dense, heavy, dark gray or black rock formed from magma that flowed out of a volcano or vent in thick streams or sheets. Basalt can come in a variety of forms. A’a (pronounced ah-ah is variety that cooled with a jagged, rough and rubbly surface. Pahoehoe (pronounced pah-hoi-hoi cooled with a glassy smooth hummocky or ropy texture.

Gabbro is a dark (often black), coarse-grained, intrusive igneous rock chemically equivalent to basalt but that cooled deep beneath the Earth‘s surface, resulting in large crystal structures within the rock that sparkle in the light.

Granite cooled from magma deep under the earth and as a result usually has large mineral crystals all grown together. Depending on the type of granite, these minerals might include quartz, feldspar, mica, olivine, etc.

Obsidian is a heavy, smooth, and shiny volcanic glass rich in iron and magnesium that cooled very quickly during an eruption, so quickly that crystals didn‘t have time to grow, thus resulting in glass. Chemically, it‘s often identical to pumice, which makes it terrific to use for compare-and-contrast with pumice.

Pegmatite is a very coarse-grained igneous granite consisting of quartz, feldspar, and mica and commonly also containing large gemstone crystals such as tourmaline, aquamarine, and kunzite. Pegmatites form as a magma that cools quickly after intruding as a dike or sill into other rock.

Pumice is formed from magma that shoots out during a particularly violent, explosive eruption. Gases dissolved in liquid magma expand rapidly during the eruption, making pumice extremely frothy (like froth created when you shake a soda can and open it). Millions of tiny gas bubbles leave cavities shot through pumice, making it extremely light—so light that it can often float on water!

Rhyolite is often a banded light-colored, fine-grained rock that formed when thick, sticky lava flowed for relatively short distances.

Scoria is similar to basalt, but whereas basalt usually flows in a thick, fluid layer from a volcano, scoria is shot into the air as a cinder during explosive eruption events. Thus, like Swiss cheese, it‘s peppered with holes from gas bubbles, making it much lighter than basalt.

Tuff is volcanic ash and cinder that settles while still quite hot and becomes welded and compacted into layers of coarse, often lightweight rock that‘s usually white or gray or cream in color.

Note: Kids can use this activity toward satisfying requirements for other badges, too: Rocks & Minerals (Activity 1.4) and Collecting (Activity 5.1).