Magnetic anomaly striping and plate tectonic chronology
Seafloor spreading creates a natural tape-recorder of Earth's magnetic field. As magma solidifies at the ridge, magnetic minerals align with the prevailing field direction. When the field reverses (on average every 200,000-300,000 years), the new crust records the opposite polarity. The Vine-Matthews-Morley hypothesis (1963) correctly predicted that oceanic crust would show symmetric magnetic anomaly patterns on either side of ridges. By dating these stripes using radiometric methods and matching them to the Geomagnetic Polarity Time Scale, geologists can calculate plate velocities and reconstruct ocean basin history with remarkable precision — giving us the absolute timeline of plate tectonics.