Earth's Magnetic Field Went Completely Haywire During Last Reversal and Took 22,000 Years to Get Back to Normal

The last reversal of Earth's magnetic field took far longer than was previously thought, scientists have discovered. By analyzing ancient volcanic rocks, researchers found the Matuyama-Brunhes reversal took about 22,000 years to complete, with the field starting to collapse about 795,000 years ago.

Earth's magnetic field is believed to be generated within the planet's iron core. It extends far out into space and helps protect the atmosphere from the solar wind—a stream of charged particles from our sun. Without the magnetic field, the particles would strip away the atmosphere, allowing harmful radiation through and eventually leaving Earth a lifeless, barren planet—as happened on Mars billions of years ago.

The magnetic field is dynamic—it moves around and gets stronger and weaker over time. It is also known to reverse, with the magnetic north and south pole flipping positions. This has happened many times over the planet's history—however, because of the timescales involved, little is known about what happens when a reversal takes place.

The last full magnetic field reversal took place about 780,000 years ago. It also came very close to reversing 42,000 years ago.

In a study published in Science Advances, a team led by Brad Singer, from the University of Wisconsin-Madison, looked at lava flows to trace back the last major reversal and find out how long it took.

Lava flows act as a time capsule of the planet, providing information on the position of Earth's magnetic field at the point it solidifies. Singer looked at these lava flows, and combined them with data from marine sediments from Antarctica to build up a picture of what was happening on Earth during the Matuyama-Brunhes reversal.

Findings showed the magnetic field started to collapse about 795,000 years ago. It became unstable and there were two partial reversals over the course of 18,000 years, before a full reversal that took about 4,000 years to complete.

This is far longer than the 9,000 years scientists previously thought it took for a full reversal to take place. Singer told Newsweek the Matuyama-Brunhes reversal is the "culmination of processes that began in the outer core dynamo about 22,000 years earlier."

He continued: "The 22,000 years between the onset of instability in the outer core dynamo was not a surprise. However, the complexity of the record between about 795,000 and 773,000 years ago is surprising. The reversal process is longer lived, and more complex, than previously imagined."

It has been suggested that Earth's magnetic field could be about to reverse. At the moment, the field is getting weaker, declining in strength by about 10 percent over the last 200 years. Singer said this would not be enough to measure in lava flows—during a decline the field would drop to about 10 percent of what it is today.

Singer said he and the team now plan to look at the timings of other reversals and excursions recorded in volcanic rocks.

James Channell from the University of Florida, who was not involved in the study, said it was an "important paper" because of the volcanic data it adds to the current records about the reversal. He said that while he agrees with the instability in the magnetic field reported, he questions the age ranges they propose because the volcanic records do not marry up nicely with those from sediments.

However, he said the paper raises a number of questions: "Is this pre-reversal instability a characteristic of all polarity reversals? As yet, there is no evidence of this from older reversals, such as those that bound the Jaramillo Subchron.

"Records of polarity reversals in marine sediments are used as a very important means of global correlation of climate and environmental records, therefore, a 22,000- to 30,000-year interval of instability prior to polarity reversals in general, would have important implications for the precision of global stratigraphic correlation."

magnetic field
Artist impression of Earth's magnetic field deflecting the solar wind. iStock