Earth and Environmental Science · 35 minutes
Suggested grades 9–12 · Unit 1: Earth Evidence and Energy · Lesson 1 of 2
Read a Seafloor Age Pattern
Evaluate an idealized age transect as evidence for a plate-motion explanation.
Mr. S Teaches · Original starter lesson · HS-EARTH-01 · Grade range is planning guidance.Watch, notice, discuss
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The Plate Tectonics Revolution: Crash Course Geography #19
By CrashCourse
Locate the seafloor evidence in this broader plate-tectonics account, then return to the age-distance graph.
Open on YouTube: The Plate Tectonics Revolution: Crash Course Geography #19 (opens a new tab) ↗Magnetic Striping and Seafloor Spreading
By Science and Nature with Thomas Stevenson
Compare magnetic stripes with the lesson’s age pattern as a separate line of evidence; neither diagram alone gives a full spreading history.
Open on YouTube: Magnetic Striping and Seafloor Spreading (opens a new tab) ↗What idea from the video helps you plot ages against distance and defend a qualified spreading interpretation with two data comparisons? Identify one useful example and one limit of the video's model or explanation.
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Learning objective
Plot ages against distance and defend a qualified spreading interpretation with two data comparisons.
Before you begin
- Use signed coordinates and unit conversions.
Gather your materials
- Paper or notebook
- Pencil; calculator optional for numerical work; all task data supplied below
Language to know
- oceanic crust
- Rock forming much of the ocean floor.
- mid-ocean ridge
- A region where new oceanic crust forms at a spreading boundary.
- transect
- A line along which observations are sampled.
Understand the concept
Original model data: A ridge lies at distance 0 km with rock age near 0 million years. At −100 and +100 km, rocks are 5 million years old; at −200 and +200 km, they are 10 million years old. The symmetrical pattern is consistent with new crust forming near the ridge and moving outward. A single idealized transect does not establish every plate’s history, and real age determinations have uncertainty. The model does not claim that all Earth rocks follow this simple pattern.
Teach it step by step
- Translate the prose measurements into a distance–age table and graph.
- Compare same-distance points on opposite sides, citing both paired ages.
- Calculate a model half-spreading rate using 100 km over 5 million years.
- Write an evidence-based interpretation and identify an additional transect or dating check needed to test it.
Make the thinking visible
Worked example or model response
100 km/5 million years=20 km per million years, equivalent to about 2 cm/year on one side. The full separation rate of the two sides in this symmetric model would be about 4 cm/year. Confusing these rates doubles or halves the intended quantity.
Guided practice
Together predict the model age at +300 km if the same average rate continued, then label it a prediction rather than a supplied measurement.
Independent practice
Plot the graph and write a five-sentence argument for the spreading interpretation. Explain why an ancient continental rock far away would require additional geological context rather than automatic placement on this line.
Quick assessment
Does one symmetric pattern prove that spreading rates never change?
Answer key & assessment criteria
Answers and teaching guidance
- Guided: The extrapolated age is 15 million years; it is a conditional prediction, not an observed datum.
- Independent: Require symmetrical evidence, increasing age with distance, correct rate distinction and a limit involving dating uncertainty or nonconstant history.
- Quick check: No. The supplied averages and idealization do not establish constant rates for every period or boundary.
Look for: Assess the stated objective using the concrete criteria and explanations in the answer key. Invite an alternative supported solution; distinguish a model’s prediction from an established observation.
Make it work for your learner
More support
Provide axes and two completed paired points before calculating the rate.
Extension challenge
Add hypothetical age uncertainties of ±1 million years and explain how they affect confidence in an exact rate.
Verified references · Selected contributions
Standards connections
Selected NGSS high school-band contribution; full performance requires additional evidence and instruction.
Subject-content standards
These records identify the part of a subject standard practiced here. Course placement is instructional guidance; the source’s category or band remains authoritative.
Contributes to NGSS performance expectation
HS-ESS1-5
NGSS · High school band
Evaluate crust-age evidence in a plate-tectonic explanation.
Read the official performance expectation ↗A selected idealized oceanic-crust transect supports plate-tectonic reasoning. Continental age evidence, dating methods and complex plate histories require additional sources.
Verified 2026-11-04. Selected contribution, not full attainment.
Supporting literacy connections
These records apply to the specified reading, writing or communication task. They do not establish subject-content alignment.
Supporting Common Core literacy connection
RST.9-10.7
Common Core · Grades 9–10
Translate technical prose into visuals and explain mathematical or visual information.
Read the official standard ↗The activity explicitly analyzes the provided technical text, table or model. It contributes selected literacy practice rather than complete grade-band reading coverage or independent subject-content alignment.
Verified 2026-11-04. Selected contribution, not full attainment.