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.
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.
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.
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.
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.
so small you can't see them with the unaided eye. Sometimes
there are larger, visible crystals in a very fine-grained matrix.
rocks were once liquid magma that erupted from volcanoes. They
cooled very quickly, which is why the crystals are usually very
– usually dark gray to black, fine-grained volcanic rock;
sometimes has gas bubbles (vesicles)
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.
– pinkish, whitish igneous rock with interlocking crystals
of quartz and feldspar. Sometimes includes mica.
up of smaller rocks cemented together. Sometimes has fossils.
Usually has layers.
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.
– composed of pieces pebble-size or larger
- composed of sand-size pieces
- composed of pieces smaller than fine sand
a light gray, sometimes with crystals, sometimes with shells,
sometimes just massive.
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.
– a whitish or grayish rock made of the mineral calcite.
has interlocking crystals and layers (called foliation)
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).
– metamorphosed sandstone
– metamorphosed shale.
– metamorphosed sedimentary rocks that had lots of clay.
– metamorphosed granite.
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.
For each group –
Rocks Dichotomous Key
Box of Rocks:
||Mica Schist *
* indicates minerals used in Lesson 4 on Michigan Geology Map
Activity 3.1 – Classifying Rocks
Provide each group with a box of rocks and hand-lenses.
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.
Have students record their groups and the criteria used to classify
the rocks into these groups.
3.2 – Rock Types
Lead a whole class discussion centered around student observations.
Make a whole-class list of different ways to group rocks.
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.
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
3.3 – Using Rock Dichotomous Keys
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
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.
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.
||How We Know
||Where It Formed
||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.
||visible angular crystals or grains are interlocking
||Cooled from magma underground.
||visible angular interlockying grains or crystals. Shows foliation (similar to layers)
||Started out as a granite, but was squeezed and heated, rearranging the minerals.
||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.
group answers to find out if all groups successfully identified
each rock type and environment.
Suggested rocks to use include: conglomerate, gneiss, granite, shale
All members of the group should have a job.
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.
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.
Remind students to put the rocks back in the correct places in
Make sure groups are heterogeneous. Help students help each other.
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
Do these activities match your learning goal?
- How will you establish a purpose?
- How will you elicit student ideas?
What are the inquiry elements of this lesson? Are they sufficient?
What modifications do you need to make to this activity to meet
the learning needs of your students?