Flipping the Compass
Earth's magnetic field has reversed its polarity hundreds of times throughout geological history, with magnetic north and south swapping places in a process driven by turbulent convection in the liquid iron outer core. These reversals are recorded in the permanent magnetization of rocks — when lava cools past its Curie temperature, ferromagnetic minerals lock in the ambient field direction, creating a tape-recorder-like archive of past field behavior stretching back billions of years.
The Geomagnetic Polarity Timescale
The sequence of normal and reversed polarity intervals, calibrated by radiometric dating, forms the Geomagnetic Polarity Timescale (GPTS). This timescale is the foundation of magnetostratigraphy — correlating sedimentary sequences worldwide by their magnetic polarity pattern. The current normal polarity epoch (Brunhes) began 780,000 years ago. Before that, the Matuyama reversed epoch lasted about 2.6 million years, punctuated by brief normal events like the Jaramillo and Olduvai subchrons.
Anatomy of a Reversal
During a reversal, the axial dipole field weakens over several thousand years, the field becomes dominated by complex non-dipole components with multiple simultaneous magnetic poles, and then the dipole re-establishes in the opposite polarity. The transition field is about 10-20% of normal strength, and virtual geomagnetic poles wander across the globe in complex paths. No two reversals are identical, reflecting the chaotic nature of the geodynamo.
Consequences and Hazards
During a reversal, the weakened magnetosphere allows increased cosmic radiation and solar wind penetration to Earth's surface. Studies have linked some past reversals to changes in atmospheric chemistry, increased mutation rates, and even extinction events, though these connections remain debated. The current field has been weakening at ~5% per century, leading to speculation about an approaching reversal — but the geodynamo's behavior is inherently unpredictable on human timescales.