magnetic-polarity time scale

The magnetic-polarity time scale is a geological tool used to date rocks by analyzing the Earth's past magnetic field reversals. The Earth's magnetic field has not always been oriented as it is today; its polarity has reversed numerous times throughout geologic history. These reversals, which appear to happen relatively quickly on a geologic timescale (around 5,000 years), are recorded in magnetic minerals within rocks.1234

Ocean floor basalts provide strong evidence for these reversals, showing symmetrical patterns of oppositely magnetized "stripes" spreading from mid-ocean ridges. This seafloor acts like a magnetic tape, capturing the alternating orientations of the Earth's magnetic field over time. By dating these magnetic stripes using radiometric methods, scientists have constructed a magnetic-polarity time scale that extends back approximately 260 million years. This scale is crucial for understanding Earth's geologic history.142