The seafloor records a field that keeps reversing

Molten rock rising at a mid-ocean ridge cools into new crust. Magnetic minerals align with Earth's field as the rock forms, then preserve that orientation. Because the field has reversed many times, seafloor spreading produces roughly mirrored bands of normal and reversed magnetisation on either side of a ridge.

The bands are not perfectly tidy barcodes; spreading rates vary and geology disturbs records. Combined with radiometric dating, however, their symmetry supplied powerful evidence that ocean floor is created and carried away. A planet-scale motion became legible because cooling stone retained the direction a compass would once have pointed.