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Case file 13 / Science & Discovery

The Ocean-Floor Tape

Symmetric magnetic stripes test an idea about moving continents.

Seafloor spreading · 10 min · 4 evidence exhibits · 3 reasoning steps

Follow the record.
Question the explanation.

01 / The briefing

Begin with the question.

Coastlines can suggest a puzzle, but a visual resemblance alone does not explain a planet. Alfred Wegener assembled geological, fossil, and ancient-climate evidence for continental movement, yet a convincing physical account remained difficult. Decades later, ocean-floor measurements offered tests that did not depend on how neatly a modern map could be cut apart. Magnetic patterns on opposite sides of mid-ocean ridges and the ages of oceanic rocks could be compared with predictions of seafloor spreading. Your task is to decide whether the combined observations are better explained by a stationary floor or by new crust forming and moving outward. This case does not present modern plate tectonics as identical to every detail of Wegener’s proposal. It follows how a broad idea gained a much stronger mechanism and new independent evidence.

The timeline

  1. Wegener publicly presents continental drift.

  2. The Origin of Continents and Oceans develops his argument.

  3. Ocean-floor mapping and magnetic measurements expand the evidence available.

  4. Vine and Matthews, and independently Morley, connect magnetic stripes with reversals and seafloor spreading.

A researched reconstruction for learning. These reasoning steps simplify a complex record; no score can capture the full history.

02 / The evidence

What does the record show?

Read each exhibit before settling on an explanation. The numbered source links below every exhibit lead to the record behind it.

Exhibit 01

More than a jigsaw

Matching fossils, geological features, and ancient-climate evidence motivated continental reconstructions. Shoreline resemblance alone was not the full argument.

Source record [s1]

Exhibit 02

A magnetic record

As volcanic oceanic rock forms and cools, it can preserve the direction of Earth’s magnetic field. The field has reversed repeatedly.

Source record [s2]

Exhibit 03

Stripes across the ridge

Alternating magnetic bands occur in corresponding patterns on opposite sides of spreading ridges. The arrangement is a spatial record that a formation-and-motion model must explain.

Source record [s2]

Exhibit 04

Ages provide another test

Rocks near a spreading ridge are youngest, with older rocks farther away. This adds an age relationship independent of simply noticing stripes.

Source record [s3]

03 / Your reasoning

Follow the strongest explanation.

Choose an interpretation, then compare it with the evidence. You can revise your choice. The goal is understanding, not a perfect score.

Reasoning step 01

Reasoning step 1

Which evidence most improves on the observation that two coastlines look similar?

Evidence-based explanation

Independent matches constrain a reconstruction. Adjusting the display can improve appearance without strengthening the geological explanation.

Consider: Exhibit 01, Exhibit 02, Exhibit 03, Exhibit 04. Follow their source links to check the record.

Reasoning step 02

Reasoning step 2

If new crust forms at a ridge and moves outward while magnetic polarity reverses, what pattern should result?

Evidence-based explanation

Both sides acquire the field orientation when they form, then move away. The pattern is predicted by combining formation, motion, and reversals.

Consider: Exhibit 01, Exhibit 02, Exhibit 03, Exhibit 04. Follow their source links to check the record.

Reasoning step 03

Reasoning step 3

Which age pattern is the strongest additional check?

Evidence-based explanation

Continuous creation at the ridge predicts an outward age sequence. This is a new test rather than a restatement of magnetic symmetry.

Consider: Exhibit 01, Exhibit 02, Exhibit 03, Exhibit 04. Follow their source links to check the record.

04 / The findings

The chain behind the outcome.

Seafloor spreading explained the magnetic pattern as a record written during crust formation and carried outward by motion. Rock ages supplied a separate check: new oceanic crust should be young at the ridge and older farther away. These findings helped transform a debate centered on continental resemblance into a theory supported by measurable processes. Modern plate tectonics includes moving plates containing oceanic as well as continental material, with creation and recycling of crust. It is not simply the assertion that continents somehow plow through an unchanging ocean floor. The case illustrates why a strong explanation does more than accommodate a known pattern. It predicts another kind of observation and survives that test too. A visually compelling clue can begin an investigation, but converging evidence from different measurements makes the conclusion substantially harder to explain away.

Take the method with you

  • Visual resemblance is a starting clue, not a complete mechanism.
  • Combine independent records rather than counting repeated versions of one observation.
  • A good model predicts both patterns and ages.

You can read findings at any time. Exploring every reasoning step marks this case as complete in your notebook.

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The open record

Sources & further reading

Read the original record and its scope before relying on a conclusion. Source references support the exhibit summaries; Faultory’s reasoning prompts are editorial reconstructions.

  1. s1
  2. s2
  3. s3

Written and curated for Faultory by Yunus Emre Vurgun, with AI-assisted research and drafting. Sources and interpretations can need correction. Read our editorial standards or report an issue.