Science 7–10 · Year 8

Density of granite, basalt and pumice by water displacement

Science understanding: Earth and space sciences (NSW Stage 4 focus area: Change)

Practical, model not builtLow risk

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The idea

Rocks of the same size differ in mass because their minerals and pore spaces differ, and the density difference between continental and oceanic rock is what lets one plate sink beneath another.

What you need

1 specimen each of granite, basalt, sandstone and pumice per group: 60 to 80 g of granite, basalt and sandstone, so that each displaces at least 20 cm cubed, and 20 to 40 g of pumice, which at a density below 1 g/cm cubed also displaces at least 20 cm cubed
1 electronic balance (0.1 g)
1 250 mL measuring cylinder (2 mL graduations) or a displacement can with a 100 mL cylinder
1 length of thread, paper towel
water at room temperature

How to do it

Dry each specimen and record its mass three times; use the mean.
Half fill the cylinder, read the level at the bottom of the meniscus, lower the specimen on the thread, tap out bubbles and read the new level; the difference is the volume. For pumice, hold it under with a pin and read quickly before it soaks.
Compute density = mass / volume in g/cm cubed for each rock; repeat the volume reading twice and use the mean.
Rank the rocks by density and place pumice in water to confirm it floats.
Compare the class means of granite, typical of continental crust, and basalt, which forms oceanic crust, and use the difference to explain why, where an oceanic and a continental plate converge, the oceanic plate is the one that sinks beneath the other.

What you should see

Pumice returns a density below 1.0 g/cm cubed and floats (its vesicles are gas-filled). Basalt usually returns a higher density than granite when bubbles are tapped off and the same balance is used, although with a 2 mL reading error on a small specimen the two can overlap; sandstone sits between or below granite depending on its porosity and how much water it soaks up. Repeated volume readings agree within 2 mL, which for a 20 cm cubed specimen is a 10 percent spread, which is why no specimen is smaller than that: a 20 g granite chip displaces only 7.4 cm cubed, where the same 2 mL is 27 percent and the granite and basalt means cannot be separated at all. The learner knows it worked when the class means rank pumice lowest and basalt highest and every group's pumice value is under 1.0.

What changes

What you change: rock type
What you measure: density
What you keep the same: dry specimens
same balance and cylinder
water temperature
bubbles removed

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Heavier means denser; a large pumice is heavier than a small basalt chip yet less dense.
All rocks sink.
Density changes when you cut a rock in half; the ratio stays the same.

Safety card

Low riskLearners carry it out

Hazards

water spills
glass cylinder breakage

Controls

work over a tray
plastic cylinder if available

Note

No hazardous chemical and no flame or heating apparatus: a generic classroom risk assessment (CSIS 1.7 or RiskAssess) covers trips, spills, warm lamps and sharp edges.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC4-CHG-01 SC4-WS-01 SC4-WS-04 SC4-WS-05
Australian Curriculum v9AC9S8U04AC9S8U03AC9S8I03AC9S8I04

Sources

The pages the author read to write this activity.

All Concept Studio activities