Science 7–10 · Year 8
Isostasy: pieces of wood floating in water and in syrup
Science understanding: Earth and space sciences (ACARA AC9S8U03 through the mountain ranges formed where plates converge; NSW Stage 4 focus area: Change)
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The idea
Crust floats on the denser mantle the way wood floats on water: a thicker or lighter piece stands higher and has a deeper root, which is why continents stand above the ocean floor and why crust thickened where plates converge rises as mountain ranges with deep roots.
What you need
- 2 pieces of wood of the same cross-section (about 2 cm square) but different lengths, each drilled lengthwise to slide on a stiff wire
- 2 stiff wires fixed to the base of a 250 mL beaker with adhesive putty
- water coloured with food dye; 250 mL golden syrup for the second run
- 1 ruler; 1 electronic balance and the water-displacement method to measure the wood's density
How to do it
- Measure each piece's mass and dimensions and compute its density.
- Predict, then slide both pieces onto the wires in the water-filled beaker; measure the height of each piece above the surface and the depth below.
- Compute the fraction submerged for each piece and compare with the density ratio wood over water.
- Repeat with the beaker filled with syrup to the same level; measure again and explain the change.
- Add a small mass to the top of one piece and measure how far it sinks; relate this to a mountain range or an ice sheet loading the crust.
What you should see
Both pieces float with the same fraction submerged, equal to the wood's density over the fluid's (Archimedes' principle), so the longer piece stands higher above the water and reaches deeper below it, the Earthlearningidea outcome that the floating depth is proportional to the height. A piece of density 0.50 g/cm cubed floats half out of water; at 0.70 g/cm cubed only 30 percent stands above. In syrup, which is denser than water, both pieces float higher, as the sheet predicts. A load pushes a piece down by the load's mass divided by (cross-section x fluid density). The learner knows it worked when the submerged fraction is the same for both pieces in a given fluid and lower in syrup than in water.
What changes
- What you change
- length of the wood and fluid density
- What you measure
- height above the surface and depth below
- What you keep the same
- cross-section and type of wood
- fluid level
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Mountains are piled on top of a rigid crust; they are balanced by a root pushed down into the mantle.
- The mantle is liquid because the crust floats on it; it is solid rock that flows slowly under sustained load.
- A heavier piece sinks to the bottom; wood of any size floats with the same fraction submerged.
Safety card
Hazards
- wire ends
- sticky syrup spills
Controls
- bend wire tips over
- work on a tray, wash pieces in warm water
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-01SC4-WS-02SC4-WS-04
- Australian Curriculum v9AC9S8U03AC9S8I01AC9S8I04
Sources
The pages the author read to write this activity.