Geologists have discovered that strong changes in the polarity of the Earth's magnetic field can occur in a very short time - just a century and a half is enough for the poles to turn more than 100 degrees. Scientists came to this conclusion as a result of radioisotope and paleomagnetic analysis of stalagmites about 100 thousand years old, found in one of the caves in southern China. In addition, such a rapid transition may be the beginning of a long period of geodynamic anomaly, in which the Earth's magnetic field changes its direction quite often and quickly over the course of a millennium, scientists write in the Proceedings of the National Academy of Sciences.
At the beginning of the 20th century, geologists discovered that the south and north magnetic poles of the Earth changed places several times during the life of our planet. The process of changing the poles is of a stochastic nature and therefore does not have a pronounced period - between two successive changes of positions, both several tens of millennia and several million years can occur. At the same time, during the life of a person on Earth, such an inversion has never happened.
It is believed that the last of these upheavals occurred about 773 thousand years ago, but after that the so-called "geomagnetic excursions" - periods of magnetic field instability, in which the direction of the magnetic field temporarily rotates through an angle from 60 to 180 degrees, after which quite quickly returns to its original position. The field strength during such events in some areas can be reduced to almost zero. As in the case of complete inversion of the magnetic field, these periods of instability are associated with turbulent processes occurring in the liquid-metal shell of the Earth's core, but an unambiguous explanation for this effect has not yet been proposed.
To study in more detail the processes that occur during such a geomagnetic excursion, geologists from Taiwan, China and Australia, led by Chuan-Chou Shen from the National Taiwan University, proposed to study stalagmites from Sanxing Cave in southern China.
Location of Sanxing Cave, where stalagmites were collected for paleomagnetic research, and photographs of two stalagmites examined.
Analysis of the content of uranium and thorium isotopes in stalagmites made it possible to date the samples with an accuracy of 10 years: it turned out that the age of the rocks is from 91 to 107 thousand years. Further paleomagnetic studies showed that during this interval a rather large number of magnetodynamic anomalies were observed on Earth, in which the duration and frequency varied greatly. The amplitude of the reversal of the magnetic field during this period sometimes exceeded 100 degrees, while these rotations showed a pronounced asymmetry - relative to the normal direction, the polarity in the Eastern Hemisphere rotated mainly to the south and east.
It turned out that if in the period from 92 to 98 thousand years ago, some periods of anomalies stretched for millennia, then 103-106 thousand years ago, the duration of geomagnetic excursions was somewhat less - several centuries. Scientists also managed to measure the duration of the reversal of the magnetic field. It turned out that the Earth's magnetic field took only about 144 years to turn a hundred degrees - this is much less than the time that scientists previously considered sufficient for such a process.
The tilt of the Earth's magnetic axis in the period from 99 to 96 thousand years ago (left) and the diagram of the maximum displacement of the planet's magnetic pole during this period (right).
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According to geologists, the data they obtained made it possible for the first time to estimate with such accuracy the temporal characteristics of geomagnetic excursions with a sharp change in the polarity of the Earth's magnetic field. The authors of the work note that during periods of such anomalies, the planet's magnetic field is always weakened, because of which cosmic radiation can penetrate deeper into the atmosphere. The results of the work showed that a change in magnetic polarity can occur very quickly, and if this happens, it will affect satellite systems, the biosphere and human society.
In order to more accurately study the dynamics of the Earth's magnetic field, scientists conduct not only geological research, but also use modern computational methods. So, with the help of computer modeling, geologists were able to show that the change in the magnetic polarity of our planet, most likely, occurs as a result of the impact of dynamo waves inside the core. It is interesting that sometimes, when studying such complex problems, not only modern approaches come to the rescue, but also the rituals of people who lived on Earth more than a thousand years ago. So, by measuring the magnetization of sand particles that were formed when setting fire to cattle corrals in southern Africa, scientists have reconstructed the picture of the distribution of the Earth's magnetic field in 1000-1600 AD.
Alexander Dubov