Slide 1
Magnetic bands match across ocean ridges
Ocean-floor surveys reveal corresponding magnetic bands on opposite sides of spreading ridges.
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Survey an ocean ridge and the seafloor can reveal alternating magnetic bands, with corresponding patterns on opposite sides. The symmetry asks for an explanation. Why should rocks separated by a ridge preserve such similar sequences of magnetic information?
Slide 2
New crust moves away from the ridge
New oceanic crust forms at a spreading ridge and travels outward as more material arrives. Rocks made at similar times move in opposite directions.
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At a spreading ridge, new oceanic crust forms and moves outward as more material is added. Rock created at similar times travels in opposite directions. That process can preserve corresponding bands on either side, like two expanding records with a shared starting line.
Slide 3
The cooling crust records the magnetic field
As that rock cools, its magnetic minerals can lock in Earth’s magnetic-field direction. Later reversals leave different records in newly formed rock.
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As rock cools, magnetic minerals can preserve the direction of Earth's magnetic field. The field has reversed many times in geological history. Rocks formed during different polarity intervals therefore carry contrasting magnetic records, even though the solid seafloor itself is not striped in color.
Slide 4
Independent dates help test the matching pattern
Scientists compare those seafloor patterns with independently dated magnetic polarity reversals. Agreement supports the account of crust moving out from a ridge.
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Comparing marine magnetic patterns with dated polarity reversals links the bands to a sequence in time. Agreement between these records supports seafloor spreading. The argument becomes stronger when the spatial pattern and the dating evidence fit the same account of newly formed crust moving outward.
Slide 5
A record formed in the rock reveals later movement
Magnetic records acquired as crust formed let scientists reconstruct later seafloor movement. The bands preserve a history of motion.
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The stripes preserve information acquired as the seafloor formed, allowing later observers to reconstruct movement. Reversals do not happen at regular intervals, and ridges do not all spread equally fast. A tidy diagram explains the principle without supplying a universal geological clock.