Michigan Rocks



Learning Goals

Instructional Sequence



Rock Types

Lesson Number: 3

Main Ideas & Objectives:
MI-2: There are three basic types of rocks; each rock type forms under certain conditions and tells a story about what the Earth was like when that rock was formed.
O-3: Interpret the environment that a particular rock formed in based on rock type.

Grade Levels:
Grades 4-8. Students in lower grades (4,5) will need additional support for activity 3.3. Activities 3.1 and 3.2 could be used in grade 3.

Lesson Purpose:
This lesson answers the question, "What can rocks tell us?" The lesson builds classifying and observational skills. Students learn to interpret features of rocks that are clues to their rock type and environment of formation.

Activity Description:
3.1 Classifying Rocks - Students work in groups to classify rock samples from a box of rocks based on their commonalities and differences.
3.2 Rock Types - Groups share their rock classifications. Teacher helps class construct a list of characteristics for the three basic rock types and the environment of formation for each rock type.
3.3 Using Dichotomous Keys - Students use a dichotomous key to identify rocks and decide what environment the rocks formed in.

Background Information:
There are three basic types of rocks. Each type of rock forms under particular conditions. As a result, each rock type provides a clue to the conditions present when that rock was formed. Each rock type can be identified based on several observable characteristics.

Rock Type Observable Characteristics Where Formed Example
1. Igneous      
Volcanic (Extrusive) Crystals so small you can't see them with the unaided eye. Sometimes there are larger, visible crystals in a very fine-grained matrix. These rocks were once liquid magma that erupted from volcanoes. They cooled very quickly, which is why the crystals are usually very small. Basalt – usually dark gray to black, fine-grained volcanic rock; sometimes has gas bubbles (vesicles)
Intrusive Interlocking Crystals These rocks were once liquid magma, but they did not erupt from volcanoes. Instead, they cooled slowly underground. The crystals had time to grow large. We see them because erosion has stripped off and removed all of the rock above it. Granite – pinkish, whitish igneous rock with interlocking crystals of quartz and feldspar. Sometimes includes mica.
2. Sedimentary      
Clastic Made up of smaller rocks cemented together. Sometimes has fossils. Usually has layers. These rocks formed when loose sediment (rocks, sand) were deposited by water, compacted, and cemented together. These rocks form along beaches, by rivers, or under the water in lake or oceans.

Conglomerate – composed of pieces pebble-size or larger

Sandstone - composed of sand-size pieces

Shale - composed of pieces smaller than fine sand

Chemical Usually a light gray, sometimes with crystals, sometimes with shells, sometimes just massive. These rocks are also deposited in water. However, they form as a chemical reaction in the water that leaves a chemical deposit, usually on an ocean bottom. Limestone – a whitish or grayish rock made of the mineral calcite.
3. Metamorphic    

  Usually has interlocking crystals and layers (called foliation) These rocks formed when igneous, sedimentary, or other metamorphic rocks are heated and/or squished, forming a new rock type. This usually happens during mountain building events (regional metamorphism), but also happens during igneous activity (contact metamorphism).

Quartzite – metamorphosed sandstone

Slate – metamorphosed shale.

Schist – metamorphosed sedimentary rocks that had lots of clay.

Gneiss – metamorphosed granite.

Once a person has identified the rock type, the specific name of the rock is determined by the minerals in the rocks. Identifying the minerals in the rocks is beyond the scope of this lesson.

Materials Needed:
For each group –
Hand lenses
Rocks Dichotomous Key
Box of Rocks:

Granite* Gabbro Rhyolite Basalt*
Pumice Scoria Obsidian Limestone* (clastic)
Limestone (chemical) Conglomerate* Sandstone* Shale*
Slate* Mica Schist * Gneiss* Marble

* indicates minerals used in Lesson 4 on Michigan Geology Map

Optional Resources

Detailed Procedures:
Activity 3.1 – Classifying Rocks

  1. Provide each group with a box of rocks and hand-lenses.
  2. Have each group make a list of ways that they could group the rocks. Students may group the rocks according to their criteria. For example, all rocks made up of smaller rocks might be in one group, and all rocks with fossils might be in another group. Allow students to use color, but suggest that they find other ways to group the rocks as well. Students can re-arrange their groups if they desire.
  3. Have students record their groups and the criteria used to classify the rocks into these groups.

Activity 3.2 – Rock Types

  1. Lead a whole class discussion centered around student observations. Make a whole-class list of different ways to group rocks.
  2. Introduce the three rock types: igneous, sedimentary, and metamorphic. Explain how each rock type forms. Some students in the group may be able to provide this information.
  3. Have students find the following rocks from their box. Explain the rock type of these rocks and have students suggest what characteristics of these rocks they could use to group these rocks together.
    a. Igneous –Granite, Basalt
    b. Sedimentary - Sandstone, Shale, Conglomerate, Limestone
    c. Metamorphic – Slate, Schist, Gneiss, Quartzite

Activity 3.3 – Using Rock Dichotomous Keys

  1. Provide a model for how to use a dichotomous key to identify rocks. If you had students use a key to identify minerals, they may already know how to use a dichotomous key.
    a. Explain that a dichotomous key helps you identify rocks by giving two choices for a given rock characteristic.
    b. Give an example using sandstone. Model the process outloud, explaining the choices that fit each step.
    c. Identify what conditions this rock formed under – since it is sandstone, it formed from sand either along a beach or a river.
    d. Model the process again as a whole-class exercise using granite. For each choice ask for student answers to guide the process.
    e. Identify the conditions for the formation of granite.
  2. Write the numbers of three rocks from the box on the board. Have students work in groups to identify the rocks listed, using the dichotomous key. Students should also identify the environment that the rocks formed under. Have students record their answers in a table.
Rock Name Rock Type How We Know Where It Formed

Teacher Key

Rock Name Rock Type How We Know Where It Formed
Ex: Sandstone Sedimentary Made of well-sorted, rounded sand grains cemented together Started out as sand along a river or beach. Sand was compacted and cemented together to form hard rock.
Granite Igneous visible angular crystals or grains are interlocking Cooled from magma underground.
Gneiss Metamorphic visible angular interlockying grains or crystals. Shows foliation (similar to layers) Started out as a granite, but was squeezed and heated, rearranging the minerals.
Shale Sedimantary visible layers, splits along uneven planes Started out as clay in a river, lake, or ocean. Clay was compacted and cemented together to form rock.
  1. Share group answers to find out if all groups successfully identified each rock type and environment.
  2. Suggested rocks to use include: conglomerate, gneiss, granite, shale

Management Details:

  1. All members of the group should have a job.
  2. Avoid telling students they are wrong if they mis-identify a rock or give a geologically-incorrect answer. Try to figure out how students came to their ideas and if necessary, use guided questions to help them revise their ideas.
  3. During group discussions, be sure to call on all students, not just students who have their hands raised. Give all students an opportunity to participate in the discussion and group work.
  4. Remind students to put the rocks back in the correct places in the boxes.


  1. Make sure groups are heterogeneous. Help students help each other.
  2. Allow all students to touch and feel the rock specimens.

Students ability to use and/or develop a Dichotomous Key for rocks indicates that students are able to:

  • Classify rocks according to physical characteristics (grain shape, mineral alignment)
  • Use a dichotomous key and/or
  • Develop a dichotomous key for rocks

Using This Activity

  1. Do these activities match your learning goal?
  2. How will you establish a purpose?
  3. How will you elicit student ideas?
  4. What are the inquiry elements of this lesson? Are they sufficient?
  5. What modifications do you need to make to this activity to meet the learning needs of your students?