Earth and Environmental Science 11–12 · Year 11

Sea-floor spreading and magnetic stripes: a magnetised-pin model and the spreading-rate calculator

Module 2: Plate Tectonics

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

New crust forms at a ridge and records the magnetic field of its day, so on each side of the ridge a stripe’s width equals the half spreading rate multiplied by the length of that polarity interval.

What you need

a card with rows of pins pushed through it, hidden in the gap between two benches or two piles of books as the ridge (Earthlearningidea Magnetic stripes)
a bar magnet and a good magnetic compass
printed polarity time scale with the Brunhes–Matuyama reversal at 0.774 Ma (ICS chart v2023/09, base of the Chibanian)
ruler, calculator

How to do it

Pull up a few centimetres of card symmetrically and magnetise the pins that appear by stroking them with the North end of the bar magnet toward their points (a ‘normal’ field).
Pull up more card until the next set of pins, pointing the other way, appears and magnetise them the same way to model a reversed field; repeat for one or two more sets.
Put the magnet well away, lay the card flat and move the compass across it; record where the needle swings and check the pattern against the centreline.
In the calculator, enter the full spreading rate and read the distance to the 0.774 Ma boundary; compare the Mid-Atlantic and East Pacific values.
Explain why the pattern is symmetrical about the ridge and why a ship’s magnetometer could map it.

What you should see

The compass needle swings back and forth across the card and the changes are symmetrical about the centreline, as Earthlearningidea describes. Distance to the Brunhes–Matuyama boundary = half rate × 0.774 Ma: for the Mid-Atlantic Ridge, spreading at about 2.5 cm/yr (USGS This Dynamic Earth), the boundary sits 9.68 km from the axis; for the East Pacific Rise near Easter Island, spreading at more than 15 cm/yr, it sits more than 58.05 km out, so the same 0.774 Myr of history is at least six times wider on the fast ridge.

What changes

What you change: spreading rate (calculator)
What you measure: distance from the ridge axis to the 0.774 Ma boundary
What you keep the same: same reversal sequence
card pulled symmetrically
same magnet and stroke direction

Common misconceptions

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

The sea floor is the oldest part of the crust (none is older than about 180 Ma).
Magnetic reversals happen at regular intervals.

Safety card

Low riskLearners carry it out

Hazards

sharp pin points

Controls

handle the pinned card by its edges
count pins in and out

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

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 2028EES11-9 EES11/12-1 EES11/12-2 EES11/12-3 EES11/12-4 EES11/12-5
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; currentINS11-11
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities