Pole reversal: Gradually instead of abruptly?

Last magnetic field reversal could have taken 22, 000 years

The next pole reversal of the Earth's magnetic field is determined. But how quickly such a polarity reversal has been controversial so far. © Petrovich9 / iStock
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Long chaos: The polarity reversal of the Earth's magnetic field is apparently slower and erratic than previously thought - this could also apply to a future pole reversal. Because new data suggest that the last umpolung lasted more than 22, 000 years, some 770, 000 years ago. During this time, the magnetic field weakened drastically several times, shifted its poles and then turned completely around, as researchers report in the journal "Science Advances".

The Earth's magnetic field is less stable than you might think: its poling has reversed completely several times in Earth's history, most recently around 770, 000 years ago. There are now signs that another pole reversal may be imminent. Thus, the magnetic field has weakened, its axis has shifted and an extensive magnetic anomaly has arisen under the South Pacific. In addition, the magnetic north pole is moving faster than expected today.

Folding or leveling?

But whether these symptoms are actually the harbingers of a pole reversal or just evidence of a temporary phase of weakness, is debatable. Equally disputed is the question of how fast a polarity reversal takes place. In 2014, researchers found evidence of a surprisingly rapid turnaround in the last pole reversal - it could have happened within just 100 years. But that would mean that the pole change can take place even within a person's lifetime - and that there may be little time for forewarning or adjustments.

Part of the lava samples came from the Haleakala volcano crater in Hawaii. dronepicr / CC-by-sa 2.0

However, this presupposes that the geophysicists are sure about the time course of the polarity reversals. But that is precisely not the case, because there are also studies, according to which the reversal lasts millennia. "Despite decades of research, the geometric structure, timing and duration of pole reversal are still puzzling, " said Brad Singer of the University of Wisconsin-Madison and his team.

Lava samples as eyewitnesses

To provide more clarity, the researchers have now collected and examined further data on the last polarity reversal and their time course. In addition to sediment and ice cores, they primarily analyzed lava samples from Chile, Tahiti, Hawaii, the Caribbean and the Canary Islands. They come from volcanic eruptions that occurred about 700, 000 to 800, 000 years ago. display

"Lava pipes are ideal witnesses to the magnetic field, " explains Singer. "Because they contain many iron-bearing minerals and when they cool, they conserve the direction and strength of the currently prevailing magnetic field." Based on such data, it is therefore particularly easy to see how the earth's magnetic field changes during a polarity reversal changed. For their study, the researchers dated their 40 lava samples with the included argon isotopes and determined the magnetization.

22, 000 years for a pole reversal

The result: The last pole reversal was not an easy, rapid turnaround, but dragged on for more than 22, 000 years. The actual polarity reversal was preceded by an extended period of instability that began 795, 000 years ago. During this time, the magnetic field weakened several times and there were even two brief excursions phases in which the polarity disappeared or shifted strongly.

Only after this preliminary phase followed the final pole change. "The actual Umpolungsprozess began 784, 000 years ago and ended in the final pole reversal 773, 000 years ago, " report Singer and his team. However, even this final phase of the upset took at least 4, 000 years. A similar long course was followed by an accompanying geodynamo simulation, with which the researchers checked the plausibility of their results.

What does this mean for the forecasts?

According to the scientists, their results speak for the fact that the polarity reversal of the earth's magnetic field is not an abrupt but a lengthy and complex process. "The integration of the lava, sediment and ice core data reveals the extraordinary complexity in the evolution of the magnetic field before and during the last pole reversal, " the researchers said. The new data is also one of the most detailed testimonials of the last umpolung.

But what does this mean for the possible pole reversal? The new findings indicate that a clear prognosis of future development could be even more difficult than expected. Because the magnetic turbulence before such a pole reversal make it even harder to distinguish temporary fluctuations from real harbingers. Whether, in the foreseeable future, polarization of the earth's magnetic field is actually imminent for us remains open. (Science Advances, 2019; doi: 10.1126 / sciadv.aaw4621)

Source: University of Wisconsin-Madison

- Nadja Podbregar