Earth’s Interior & Plate Tectonics — Extension · Earth & Space Science, Grade 9
Connect Earth’s interior, moving plates, crustal evidence, and natural hazards.
Test a Seafloor-Spreading Model
Investigate
Does ocean-crust age change with distance from a spreading ridge?
Plan three teaching sessions: define the model and comparison; run or analyze repeated supplied/collected records; then graph, explain, revise and submit the notebook. These independent home-study routes do not certify the school’s mandated investigations or laboratory minutes.
Materials
- Graph paper
- Calculator
- Supplied distance and age data
Before you begin
This is a paper-and-data investigation. Use supplied illustrative data or teacher-approved observations. Do not enter hazardous sites to collect evidence.
Plan a fair test
Compare signed distance from the ridge with crust age. Sample on both sides using the same dating method. The supplied ages idealize constant spreading and omit uncertainty.
Method
- Predict the youngest location under a spreading model.
- Plot distance on the horizontal axis and age on the vertical axis.
- Compare ages at equal distances on opposite sides.
- Calculate one-side motion from distance divided by age; convert km per million years to cm per year.
- Calculate the rate at which two opposite sites separate.
- Describe a different observation that would further test the model.
Open the investigation data (illustrative)
Illustrative data created for this activity; these are not measurements from an actual site or an official reference table.
| Distance from ridge (km) | Crust age (million years) |
|---|---|
| -120 | 6 |
| -60 | 3 |
| 0 | 0 |
| 60 | 3 |
| 120 | 6 |
Analyze and explain
- How does the pattern support a spreading model?
- Calculate one-side speed and total separation speed.
- Would these data show whether the rate was constant at every instant?
- Identify the system boundary, units and conditions held comparable.
- Explain how the graph or figure supports the mechanism rather than merely repeat its trend.
- State one uncertainty or scale limitation and design a follow-up comparison that could challenge the explanation.
What a strong explanation includes
- Uses youngest-at-ridge and two-sided age evidence
- Shows conversion from km/Ma to cm/year
- Distinguishes one-side speed from total separation speed
- Acknowledges idealized data and proposes another test
- Keep observations separate from model-generated or supplied illustrative records.
- Explain a revision using the evidence and a stated model limitation.
Parent / teacher guide: discuss after the investigation
Age increases symmetrically away from the ridge. On one side, 120 km / 6 Ma = 20 km/Ma = 2 cm/year; opposite sides separate at 4 cm/year in this ideal model. The pattern supports new crust forming at the ridge. Magnetic-stripe symmetry and independent motion measurements would add evidence. Average ages cannot establish the speed at every instant. A complete notebook also specifies the boundary, measurement units and matched conditions. Describe a mechanism connecting the pattern to the claim, then state a limitation and an additional comparison. Supplied illustrative outcomes are not a claim about your collected results; differences must be interpreted rather than corrected to match the example.
Compare this model with the learner’s actual evidence. Different measurements can support a different conclusion; check the reasoning and limitations, not matching wording.
Extend the chapter investigation by evaluating its evidence and proposing a better test.
Problem 1
Answer and explanation
Age increases symmetrically away from the ridge. On one side, 120 km / 6 Ma = 20 km/Ma = 2 cm/year; opposite sides separate at 4 cm/year in this ideal model. The pattern supports new crust forming at the ridge. Magnetic-stripe symmetry and independent motion measurements would add evidence. Average ages cannot establish the speed at every instant. A complete notebook also specifies the boundary, measurement units and matched conditions. Describe a mechanism connecting the pattern to the claim, then state a limitation and an additional comparison. Supplied illustrative outcomes are not a claim about your collected results; differences must be interpreted rather than corrected to match the example.
Problem 2
Answer and explanation
Age increases symmetrically away from the ridge. On one side, 120 km / 6 Ma = 20 km/Ma = 2 cm/year; opposite sides separate at 4 cm/year in this ideal model. The pattern supports new crust forming at the ridge. Magnetic-stripe symmetry and independent motion measurements would add evidence. Average ages cannot establish the speed at every instant. A complete notebook also specifies the boundary, measurement units and matched conditions. Describe a mechanism connecting the pattern to the claim, then state a limitation and an additional comparison. Supplied illustrative outcomes are not a claim about your collected results; differences must be interpreted rather than corrected to match the example.
Problem 3
Answer and explanation
Age increases symmetrically away from the ridge. On one side, 120 km / 6 Ma = 20 km/Ma = 2 cm/year; opposite sides separate at 4 cm/year in this ideal model. The pattern supports new crust forming at the ridge. Magnetic-stripe symmetry and independent motion measurements would add evidence. Average ages cannot establish the speed at every instant. A complete notebook also specifies the boundary, measurement units and matched conditions. Describe a mechanism connecting the pattern to the claim, then state a limitation and an additional comparison. Supplied illustrative outcomes are not a claim about your collected results; differences must be interpreted rather than corrected to match the example.
Problem 4
Answer and explanation
Age increases symmetrically away from the ridge. On one side, 120 km / 6 Ma = 20 km/Ma = 2 cm/year; opposite sides separate at 4 cm/year in this ideal model. The pattern supports new crust forming at the ridge. Magnetic-stripe symmetry and independent motion measurements would add evidence. Average ages cannot establish the speed at every instant. A complete notebook also specifies the boundary, measurement units and matched conditions. Describe a mechanism connecting the pattern to the claim, then state a limitation and an additional comparison. Supplied illustrative outcomes are not a claim about your collected results; differences must be interpreted rather than corrected to match the example.
Problem 5
Answer and explanation
Age increases symmetrically away from the ridge. On one side, 120 km / 6 Ma = 20 km/Ma = 2 cm/year; opposite sides separate at 4 cm/year in this ideal model. The pattern supports new crust forming at the ridge. Magnetic-stripe symmetry and independent motion measurements would add evidence. Average ages cannot establish the speed at every instant. A complete notebook also specifies the boundary, measurement units and matched conditions. Describe a mechanism connecting the pattern to the claim, then state a limitation and an additional comparison. Supplied illustrative outcomes are not a claim about your collected results; differences must be interpreted rather than corrected to match the example.
Problem 6
Answer and explanation
Age increases symmetrically away from the ridge. On one side, 120 km / 6 Ma = 20 km/Ma = 2 cm/year; opposite sides separate at 4 cm/year in this ideal model. The pattern supports new crust forming at the ridge. Magnetic-stripe symmetry and independent motion measurements would add evidence. Average ages cannot establish the speed at every instant. A complete notebook also specifies the boundary, measurement units and matched conditions. Describe a mechanism connecting the pattern to the claim, then state a limitation and an additional comparison. Supplied illustrative outcomes are not a claim about your collected results; differences must be interpreted rather than corrected to match the example.