How can you tell rocks apart?
In this investigation, students will explore how to use the physical characteristics of rocks to group and identify the rocks.
For each student group:
- rock samples with stick-on numbers from 1 - 6 (at least two sedimentary, igneous and metamorphic rocks; see the Digging Deeper section at the end of the investigation for specific examples)
- magnifiers for each student or pair of students
- pencils to record observations
- Rock Data Table (handout)
- Rock Identification Sheet (handout)
This investigation is considered generally safe to do with students. Please also review the investigation for your specific setting, materials, students, and conventional safety precautions.
Remind students of the rocks that they studied in their first investigation. What was the same about the rocks? What was different? What characteristics did they think would be useful in identifying their rocks? Make a list of their ideas to refer to during the investigation.
After the scene is set, introduce your students to the investigation question: “How can you tell rocks apart?”
Tell your students that they will be investigating this question and at the end of their investigations they will be able to provide reliable answers.
Have your students brainstorm ideas about how this investigation question could be investigated.
- Design an experiment that could be used to test the investigation question.
- What materials would be needed?
- What would you have to do?
- What would be measured?
- How long would the experiment take?
Here are some initial questions that your students can discuss, in pairs, groups and as a whole class:
- What characteristics can you observe in rock samples?
- How can you tell different rocks apart?
Have your students report out their ideas and make a list of them. Add to your class list called “Questions we have about rocks.” By the end of the investigation, more of these questions will probably be answered.
- If you have not already done so, divide your class into groups of about four students with each group sitting around its table or work area.
- Before your students begin, tell them how much time they will have to complete their investigation. (Group learning strategies often call for appointing a group time keeper who keeps the group on track.)
- Provide your students with the following tools for investigation:
- Hand lenses (enough for everyone)
- Samples of igneous, metamorphic and sedimentary rocks
- Pencils to record observations
- Rock Data Table: Copy Master 1 Word Document (38 KB) | Copy Master 1 Adobe PDF (10 KB)
- Tell your students that they will be examining their rocks, recording their observations about the rocks in their Rock Data Table. Circulate while they work to monitor progress and answer questions.
- When they finish, ask students to use the data in their tables to make three groups of rocks. The rocks in each group should have similar characteristics. When they have done this, ask groups to volunteer to share their groupings and their reasons for them.
- Give students copies of the Rock Identification Sheet. Ask them to use the sheets and their Rock Data Tables to identify the groups in which their rocks belong. If they feel confident about this, they can try to identify the specific names of the rock samples. Copy Master 2 Word Document (1.20 MB) | Copy Master 2 Adobe PDF (126 KB)
- When students are finished, hold a whole class discussion about how it is possible to use the characteristics of rocks to group them. Make a list of their ideas on a flipchart for later.
Give each group a new set of rocks (you can switch rocks from group to group) and ask the students to group the rocks based upon their characteristics. Ask them to give a reason for why they placed each rock in its group.
Complete this investigation by asking your students to reflect on this question and how their answers may have changed as a result of this investigation. For example, sedimentary rocks typically have grains that one can see, whereas igneous rocks don’t. Crystals in metamorphic rocks are often arranged in bands.
The following passage provides more detailed information related to this investigation that you may choose to explain to your students.
There are three main categories of rocks, which are defined by how the rocks are formed.
Sedimentary rock is often found in layers. One way to tell if a rock sample is sedimentary is to see if it is made from grains. Some samples of sedimentary rocks include limestone, sandstone, coal and shale.
Igneous rocks form when magma from inside the Earth moves toward the surface, or is forced above the Earth’s surface as lava and ash by a volcano. Here it cools and crystallizes into rock. Look for crystals in igneous rocks. Examples of igneous rocks are gabbro, granite, pumice and obsidian.
Metamorphic rocks are rocks that have become changed by intense heat or pressure while forming. One way to tell if a rock sample is metamorphic is to see if the crystals within it are arranged in bands. Examples of metamorphic rocks are marble, schist, gneiss, and slate.