Density of crust, mantle and core materials and what it tells us about the interior

The idea

Surface rocks are far less dense than the whole Earth, so the interior must hold much denser material, which the layered model reproduces.

What you need

• hand specimens about 50 to 150 g: granite, basalt, an olivine-rich rock (peridotite or a dunite chip) and a steel or iron bolt as the core stand-in
• electronic balance reading to 0.1 g
• measuring cylinder 250 mL (or 100 mL for small specimens) and a displacement (eureka) can
• water, paper towel, thread
• calculator or spreadsheet

How to do it

1. Weigh each dry specimen to 0.1 g.
2. Fill the displacement can to the spout, lower the specimen on a thread and collect the overflow in the measuring cylinder; read the volume to 1 mL. For small specimens read the rise in a part-filled cylinder instead.
3. Repeat the volume measurement three times per specimen and use the mean.
4. Calculate density = mass / volume in g/cm³ for each specimen.
5. Compare the four densities with the mean density of the Earth, 5.513 g/cm³ (NASA Earth fact sheet: mass 5.9722 × 10²⁴ kg, volumetric mean radius 6371.0 km).
6. In the layered-Earth calculator, enter a density for the crust, mantle, outer core and inner core and adjust them until the model matches 5.51 g/cm³.

What you should see

Earthlearningidea’s orange activity gives the whole Earth a relative density of 5.5 and the rocks of the crust an average of 3.0, so every rock specimen should measure well below the whole-Earth value while the steel or iron stand-in measures above it; the learner’s own densities are the result. Reading the volume to 1 mL limits a 20 mL specimen to about ±5 % in density. The mean density computed from the NASA mass and volumetric mean radius is 5.513 g/cm³, so a denser interior is required. With PREM radii (inner core 1221.5 km, core–mantle boundary 3480 km, Moho at 24.4 km depth) and layer densities of 12.9, 11.1, 4.5 and 2.8 g/cm³ (the orange sheet lists 12.6 to 13.0, 9.9 to 12.2, 3.3 to 5.7 and 3.0), the calculator returns 5.57 g/cm³, 1.0 % above 5.513, with the core occupying 16.3 % of the Earth’s volume.

What changes

What you change

rock type (granite, basalt, peridotite, iron)

What you measure

density in g/cm³

What you keep the same

• same balance and cylinder
• specimen fully submerged
• dry surface before weighing
• three volume readings averaged

Common misconceptions

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

• The Earth is solid rock all the way down.
• Density is the same thing as mass, so a bigger specimen is denser.
• The core must be huge to make the planet dense, whereas it is about a sixth of the volume.

Safety card

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-11-01
• Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-8INS11/12-3
• Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/earth-and-environmental-science-stage-6-2017
2. www.nsw.gov.au/education-and-training/nesa/curriculum/science/investigating-science-stage-6-2017
3. www.earthlearningidea.com/PDF/59_Oranges_and_Earth.pdf
4. www.earthlearningidea.com/PDF/Archimedes.pdf
5. www.ga.gov.au/education/classroom-resources/determining-density
6. nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
7. doi.org/10.1016/0031-9201(81)90046-7