Chemical composition of igneous rock can often be estimated just from looking at the rock. Geologists look at the proportions of light-colored and dark-colored minerals in an igneous rock to estimate the chemical makeup of rock. Light-colored or felsic, minerals have more silica in them. Silica is one of the most abundant elements on Earth and is the chief component of quartz. Felsic minerals are most often colorless, white, gray or pink but can be any number of colors. The dark, or mafic, minerals are richer in iron and magnesium. Mafic minerals are chiefly black, brown, dark gray and sometimes green.
The mineral proportions of the rock are what allows geologists to classify rocks chemically. Depending on the proportion of light minerals to dark minerals, igneous rocks can be broken into four main types: felsic, intermediate, mafic and ultramafic. The following list gives more information about igneous chemical categories. This information is provided for you, the teacher, to better understand igneous rocks.
- Felsic rocks are high in silica (65% +). They are usually light-colored. Some examples are: Rhyolite (extrusive) and granite (intrusive).
- Intermediate rocks have lower silica content (55-65%). They are darker than felsic rocks but lighter than mafic rocks. Some examples are: Andesite/dacite (extrusive) and diorite/granodiorite (intrusive).
- Mafic rocks have low silica content (45-55%). They are usually dark-colored and contain iron and magnesium. Some examples are: Basalt (extrusive) and gabbro (intrusive). Basalt is the rock that is produced at spreading ridges and makes up the sea floor.
- Ultramafic rocks have extremely low silica content (less than 45%) and contain large amounts of iron and magnesium. They are usually dark-colored, but high olivine content can lend green shades to the rock. Other rare colors can be found. An example of ultramafic rock is Peridotite (intrusive).
- Create colored tags for the students to wear. Each color represents a certain kind of mineral in a molten material (magma or lava). Hang colored pieces by a string to make things easier.
- Split the class into at least 4-5 groups that have identical tags. There should be enough for every student to have one.
- Spread the class out randomly. Have them as far away from each other as possible.
- After the students have spread out, yell "CRYSTALLIZE!" and have students hurry to find others with their same name tag.
- After a few seconds, yell "STOP!" and have students stop where they are.
- Look around the class. How many groups have formed? Are there any groups? Use the size of each "crystal" to discuss the texture of the new rock created.
- Repeat steps 3-5 with different time intervals.
- The goal is to create 'extrusive' rocks, where either no or few crystals were formed, 'intrusive' rocks where large crystals were formed, and for more advanced students, porphyritic or vesicular rocks were formed. Consider a 'large' crystal to be three or more students of like crystal color that have found each other.
- You may want to start each round by giving the students a scenario to think about. Perhaps they are lava that has just come from a sea floor spreading ridge or has exploded from a violent volcanic eruption. Perhaps they are magma that has eased its way into the sediments of the continental crust. Then allow them the appropriate time to "crystallize".
- During the activity you may want to pull students aside and allow them to see what is happening. A video camera looking down onto the room allows students to see the game after it has been played.
What type of igneous rocks were formed? How did cooling rate affect the size of crystals formed? Do you think this process really happens in magma? In lava?
For teachers interested in rock identification, the following is a suggestion of how you can set up a rock-ID activity. For information about how to identify igneous rocks, refer to the above definitions and the United States Geologic Survey's (USGS) web site for igneous rocks, .
For pictures of common rock-forming minerals, refer to . Using this information and the rocks contained in the Rocks and Minerals Discovery Chest, pass around the rocks to small groups and have them identify the rocks based on texture, mineral content and chemical composition.