Minds on Science Gazette

Volume 9
Planning Activity: Earth Science

Designing Units & Courses

Don't Take It for Granite: Rock Classification

In this activity students use a simple dichotomous key to classify rocks as either igneous, metamorphic, or sedimentary.

Objectives

  • To identify the properties of rocks.
  • To classify rocks according to their physical properties.

Concepts

  • Rock
  • Igneous
  • MetamorphicSedimentary
  • Mineral
  • Interlocking crystals (minerals)
  • Noninterlocking crystals (minerals)

Materials

box of rocks: granite, obsidian, pumace, basalt, limestone, shale, sandstone, conglomerate, slate, schist, gneiss, small bottle dilute hydrochloric acid, hand lens.

Procedure

1. Observing one rock at a time, use the classification key to determine whether the rock is igneous, metamorphic or sedimentary.

2. Make a chart of your results including the rock sample, specific properties, and the class it belongs to.

3. Use reference books to determine the name of each of the rock specimens.

Rock Classification Key

1a. If the rock is made up of minerals that you can see, go to 2a.

1b. If the rock is not made up of visible minerals, go to 5a.

2a. If the rock is made up of minerals that interlocking ("melted together"), go to 3a.

2b. If the rock is made up of minerals that are noninterlocking ("glued together"), go to 6a.

3a. If the minerals in the sample are of the same kind, the rock is metamorphic.

3b. If the minerals in the sample are of two or more different types, go to 4a.

4a. If the minerals in the sample are distributed in a random pattern (not lined up), the rock is igneous.

4b. If the minerals in the sample are not distributed randomly but show a preferred arrangement or banding (lined up), the rock is metamorphic.

5a. If the rock is either glassy or frothy (has small holes), it is igneous.

5b. If the rock is made up of strong, flat sheets that look as though they will split off into slatelike pieces, it is metamorphic.

6a. If the rock is made of silt, sand, or pebbles cemented together (it may have fossils), it is sedimentary.

6b. If the rock is not made of silt, sand, or pebbles but contains a substance that fizzles when dilute hydrochloric acid is poured on it, it is sedimentary.

Applications to Science Teaching

1. Design a concept map that includes all the concepts listed in the concept list above. Make sure you show how the concepts are linked together. Use your map to answer this question: What prerequisite concepts do students need to know before they can do the rock classification activity? Add these concepts to the map.

2. What central or focus cognition will this activity help students understand?

3. What is the basis for this statement: In this activity students will be involved in concept learning, not propositional learning?