Magnetic Reversals

First published: 2018

Brief summary

USGS overview of magnetic-field reversals and paleomagnetic evidence.

Article

Earth’s magnetic field has reversed polarity many times, so that magnetic north and south exchange positions. These reversals are recorded when lava cools or sediment is deposited and magnetic minerals preserve the direction of the ambient field.

The reversals do not occur at fixed intervals. The geological record contains periods with frequent changes and long intervals of stable polarity, and the last complete reversal occurred about 780,000 years ago.

During a reversal, the field weakens and becomes more complex before returning with the opposite polarity. The transition generally unfolds over hundreds to thousands of years rather than occurring instantaneously.

Sequences of normal and reversed polarity provide a magnetostratigraphic time scale. Matching polarity patterns between rocks and the calibrated reversal chronology supports geological dating, correlation and measurements of sea-floor spreading.

Source details and credits

• Source / publisher: USGS
• Source type: Educational article
• URL type: WWW
• Credits: USGS
• URL: https://www.usgs.gov/programs/geomagnetism/science/magnetic-field-reversals