Earth’s magnetic field feels steady and dependable, but over geological time it is anything but fixed.
The magnetic north and south poles have swapped places many times in the planet’s history in events known as geomagnetic reversals.
These flips do not happen overnight. Normally, the magnetic field weakens, becomes unstable, and then gradually re-establishes itself with the poles reversed, a process thought to take around 10,000 years.
Now, a new study suggests that some of these reversals took far longer than scientists once believed.
Researchers from University of Utah, working with colleagues in France and Japan, have found evidence that about 40 million years ago, Earth’s magnetic field took as long as 70,000 years to complete a reversal.
The findings were published in Communications Earth & Environment and challenge long-standing assumptions about how Earth’s magnetic system behaves.
Over the past 170 million years, scientists estimate that Earth’s magnetic field has flipped more than 500 times.
These reversals are recorded in rocks and ocean sediments, where tiny magnetic minerals act like frozen compasses, locking in the direction of the magnetic field at the time they formed.
By reading these signals, researchers can reconstruct Earth’s magnetic past.
The new discovery came from sediment cores drilled from the floor of the North Atlantic Ocean during a 2012 expedition focused on climate change during the Eocene Epoch, a warm period that lasted from about 56 to 34 million years ago.
The drilling project retrieved long cylinders of layered sediment, each layer representing thousands of years of slow accumulation.
As paleomagnetists, the scientists measured both the direction and strength of magnetization preserved in these sediments.
Most of the time, they see relatively sharp transitions between one magnetic polarity and the other. But in one thick section of the cores, something unusual stood out. Instead of a quick flip, the magnetic field appeared to remain unstable over a long vertical stretch of sediment.
After collecting extra samples at very fine intervals, the team confirmed that this was not an artifact of sediment mixing or erosion. It was a genuine record of a magnetic field that lingered in a weakened, confused state for tens of thousands of years. They identified two slow reversals in this period, one lasting about 18,000 years and another stretching to roughly 70,000 years.
Why does this matter? Earth’s magnetic field acts as a shield, protecting the planet from harmful radiation from space. During a reversal, when the field is weaker and disorganized, more solar and cosmic radiation can reach the atmosphere and surface. Extended periods of reduced shielding could have influenced atmospheric chemistry, climate, and even the evolution of life by increasing radiation exposure, especially at high latitudes.
Although the discovery surprised researchers, it fits with predictions from computer models of Earth’s geodynamo—the churning motion of molten iron in the outer core that generates the magnetic field. These models suggest that while many reversals are relatively quick, some should be slow and drawn out, potentially lasting more than 100,000 years.
Until now, scientists simply had not found clear geological evidence of such extreme cases. This study shows that Earth’s magnetic field has always been more variable and unpredictable than once assumed, and that its slowest flips may have left lasting marks on the planet’s environment and history.
