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Activities

Classifying Rocks I

Objectives:

  1. Demonstrate that classifying rocks can be accomplished without knowing formal rock names or the three categories of rocks.

  2. Show how easily accessible beach materials or road gravel can be used to demonstrate classification.

  3. Discuss the philosophy of classification schemes.
Time Needed: One or two periods (45 to 50 minutes each period) Materials: Four or five specimens of each rock to be classified.

Six to twelve different rock types; ideally several from each three category (igneous, metamorphic and sedimentary).

Procedure:
Part 1
  1. Divide the class into four or five small groups, each with a table to work on.

  2. Distribute the rocks.

  3. Explain the directions: You have 20 minutes to classify these rocks in any way that you would like. The group does the classification and chooses one of its members to describe why and how they did their classification to the other groups.

  4. Tell the groups to begin the work. Monitor the progress of each group by observing and discussing choices and reasons. Do not give advice.

  5. At the end of the allotted time, stop the groups and ask one group to explain their classification system. Call on each group in turn.

  6. Encourage and validate the oral responses. There are no wrong answers at this point.

  7. At the close of the oral presentations, ask the class what groups had similar conclusions. Look for comparisons and contrasts in classifying.

  8. Ask the class what they have learned with this exercise.
Part 2
  1. Choose a sample of disintegrating sandstone and granite.

  2. Ask each group to describe the differences between the two samples. Use a large group discussion mode to determine responses.

  3. If a response has not dealt with the particles and now they are held together, ask each group to report on this.

  4. Discuss the findings.

  5. At this point, each group will have realized that the sandstone crumbles and granite does not. Some people will tell you that the sandstone has individual particles that are "bound" to each other by "cement." Granite is different.

  6. Now you can name the rocks and discuss why they are different, building on the groups's collective insights.

  7. Because they are held together differently, these rocks are also formed differently. Granite has interlocking crystals and sandstone has cemented grains. The strength of the sandstone is only as strong as the cement whereas the granite is inherently strong because crystals are interlocking. This is due to crystallization from a liquid. Two different classes or rocks are now defined: (1) rocks that form from the cementing of particles (sedimentary) and (2) rocks that form by cooling from a liquid (igneous).

  8. Lastly, you can introduce another type of rock that is lineated or foliated. This means that crystals are lined up in a uniform orientation.

  9. Now three different visual classes of rocks are defined. If you were lucky, one or more of the groups would have defined them. Give them names at this point: (1) IGNEOUS, formed by crystallization from a liquid, (2) SEDIMENTARY, formed by cementing of grains (Or precipitation of grains). and (3) METAMORPHIC, formed by the addition of heat and pressure applied to pre-existing rocks.
Related Activities:
Before
  1. Collect rocks and bring them to class.
  2. Discuss what a geologist is and what he/she does.
After
  1. Discuss and classify the rocks brought in by the students.
  2. Give groups mystery rocks.
  3. Talk about rocks elsewhere: Nova Scotia, Canada, Russia, Australia, Mars, the Moon.

Prepared by:
Howard Donohoe, Nova Scotia Department of Natural Resources

   

    Last Modified: 2004-12-10