Earth and Environmental Science 11–12 · Year 11
Classifying rocks as igneous, sedimentary or metamorphic with a dichotomous key
Module 1: Earth’s Resources
The idea
Texture (crystal or grain size, layering, foliation) and mineral content record how a rock formed, which is what a rock classification key reads.
Safety card
Hazards
- dilute acid
- sharp obsidian edges
Controls
- safety glasses
- handle obsidian by the rounded faces
Note
Chemicals are used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1 (general information for all staff, including the CSIS 1.7 risk assessment requirement), read the Safety Data Sheet for each chemical and complete a RiskAssess risk assessment before the lesson.
What you need
- rock set of ten: granite, basalt, pumice, obsidian, sandstone, limestone, shale, conglomerate, marble, slate (plus gneiss or schist if available)
- hand lens, steel nail, dropping bottle of 1 mol/L hydrochloric acid
- millimetre ruler for grain size
- printed dichotomous key covering texture (glassy, fine, coarse, clastic, foliated) and reaction with acid
How to do it
- Decide whether the specimen is made of interlocking crystals, cemented grains, or is glassy or frothy.
- Measure the typical crystal or grain size with the ruler and hand lens (record as under 1 mm, 1 to 5 mm, over 5 mm).
- Check for layering (sedimentary bedding) or foliation (aligned metamorphic minerals).
- Test with one drop of acid; record fizzing for limestone and marble.
- Follow the key to a rock name, then place each specimen on a rock-cycle diagram at the process that formed it.
What you should see
Granite and basalt both interlock but differ in crystal size (coarse against fine); pumice floats and obsidian is glassy; sandstone and conglomerate show cemented grains sorted by size; limestone and marble both fizz but marble has interlocking crystals; slate splits along foliation. The learner places each specimen on the rock-cycle diagram and names the texture or test that decided it.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A rock is one mineral.
- Metamorphic rocks have melted (they recrystallise below melting).
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES11-8EES11/12-3EES11/12-4EES11/12-5
- Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-11-02
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.
- www.nsw.gov.au/education-and-training/nesa/curriculum/science/earth-and-environmental-science-stage-6-2017
- www.earthlearningidea.com/PDF/134_Building_stones.pdf
- www.earthlearningidea.com/PDF/182_Rock_cycle_game.pdf
- www.ga.gov.au/education/classroom-resources/our-changing-earth-the-rock-cycle
- education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf