Rock Identification Key Worksheet

Name:

Date:

Various rock samples and magnifying glass

🔍 Part 1: Understanding Dichotomous Keys

What is a Dichotomous Key? A dichotomous key is a tool that helps scientists identify objects by asking a series of yes/no questions about their characteristics. Each question leads you down a different path until you reach the correct identification.

1. A dichotomous key uses what type of questions to identify objects?

Multiple choice questions

Yes/no questions

Essay questions

Math problems

2. Which rock characteristics are most useful for identification? (Check all that apply)

Texture (rough, smooth, glassy)

Color

Hardness

Weight

Crystal size

3. The word "dichotomous" means _____________ or split into _____________ parts.

4. Why do geologists use dichotomous keys instead of just looking at pictures?

🪨 Part 2: Rock Identification Practice

Use this simplified rock key to answer the questions below:

Step 1: Is the rock made of visible crystals? If YES → go to Step 2. If NO → go to Step 4.

Step 2: Are the crystals large (bigger than rice grains)? If YES → Granite. If NO → go to Step 3.

Step 3: Is the rock very hard and light-colored? If YES → Quartzite. If NO → Gneiss.

Step 4: Does the rock have layers? If YES → Shale. If NO → go to Step 5.

Step 5: Is the rock very hard and dark? If YES → Basalt. If NO → Limestone.

5. A rock has large, visible crystals that are bigger than rice grains. Using the key above, this rock is:

Granite

Basalt

Shale

Limestone

6. A rock has no visible crystals, no layers, and is very hard and dark. What is it?

Quartzite

Basalt

Granite

Gneiss

7. Trace the path through the key for a rock with small crystals that is very hard and light-colored. Write the step numbers you followed: _____ → _____ → _____ → _____