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How often has Earth’s magnetic field reversed during geologic history?

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geomagnetic field The magnetic field of a bar magnet has a simple configuration known as a dipole field. Close to Earth's surface this field is a reasonable approximation of the actual field.

Earth’s magnetic field has undergone numerous reversals throughout geologic history, a process known as geomagnetic reversal. These reversals occur without a discernible pattern, meaning there is no regular periodicity to the events.12

The frequency of these reversals is highly variable:
* Average Interval: On average, the dipole component of the magnetic field reverses every 300,000 to 1,000,000 years. Some sources note a rough average of every 500,000 years.23
* Variability: The time between reversals can be as short as less than 40,000 years or as long as 35,000,000 years.2

These reversals are considered sudden on a geologic timescale, typically taking about 5,000 years to complete. During this transition, the dipole strength diminishes, and the field is believed to be maintained by a non-dipolar component at a reduced strength. Evidence for these historical shifts is primarily found in the magnetic orientation of rocks, particularly in the symmetrical strips of magnetized seafloor found along oceanic ridges, which act as a record of the Earth's changing polarity over time.213