The Earth's field has alternated between periods of normal polarity, when the direction of the field was the same as the present, and reverse polarity, when the field was the opposite. These periods are called chrons.
There is no pattern to these changes, which seem to take place at random. Chrons last from between 0.1 and 1 million years (see diagram) with an average of 450,000 years. Most reversals take between 1,000 and 10,000 years to happen.
The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago; and may have happened very quickly, within a human lifetime. A brief complete reversal, known as the Laschamp event, occurred only 41,000 years ago during the last glacial period. That reversal lasted only about 440 years with the actual change of polarity lasting around 250 years. During this change the strength of the magnetic field weakened to 5% of its present strength. Brief disruptions that do not result in reversal are called geomagnetic excursions.
Records of the past[change | change source]
The past record of geomagnetic reversals was first noticed by observing the magnetic stripe reversals on the ocean floor. This soon led to the development of the theory of plate tectonics. The relatively constant rate at which the sea floor spreads causes "stripes" in the basalt. From these past magnetic fields polarity can be inferred. Data is got by towing a magnetometer along the sea floor.
No existing unsubducted sea floor is more than about 180 million years old, so other methods are used for detecting older reversals. Most sedimentary rocks have tiny amounts of iron rich minerals. Their orientation reflects the magnetic field when they formed. The rocks keep that record unless they get changed by some later process.
Superchrons[change | change source]
A superchron is a polarity interval lasting at least 10 million years. There are two well-established superchrons, the Cretaceous Normal and the Kiaman.
The Cretaceous Normal (also called the Cretaceous Superchron or C34) lasted for almost 40 million years. Between the Cretaceous Normal and the present, the frequency has generally increased slowly.
The Kiaman Reverse Superchron lasted from the late Carboniferous to the late Permian. That is more than 50 million years, from 312 to 262 million years ago (mya). The magnetic field had reversed polarity. The name "Kiaman" derives from the Australian village of Kiama, where some of the first geological evidence of the superchron was found in 1925.
Causes[change | change source]
The magnetic field of the Earth, and of other planets that have magnetic fields, is caused by dynamo action of molten iron in the planetary core. This convection (movement) generates electric currents which in turn give rise to magnetic fields. In simulations of planetary dynamos, reversals occur from the underlying dynamics. For example, Gary Glatzmaier and collaborator Paul Roberts of UCLA ran a numerical model of the coupling between electromagnetism and fluid dynamics in the Earth's interior. Their simulation reproduced key features of the magnetic field over more than 40,000 years of simulated time and the computer-generated field reversed itself. Global field reversals at irregular intervals have also been observed in a laboratory liquid metal experiment VKS2.
Effects on life[change | change source]
As far as we know, there is no effect on life. Studies have been done to see if reversals relate in any way to extinction events. Statistical analysis shows no evidence for a correlation between reversals and extinctions.
References[change | change source]
- Leonardo Sagnotti et al (2014). "Extremely rapid directional change during Matuyama-Brunhes geomagnetic polarity reversal". Geophys. J. Int. 199 (2): 1110–1124. doi:10.1093/gji/ggu287.
- Ice age polarity reversal was global event: Extremely brief reversal of geomagnetic field, climate variability, and super volcano. Sciencedaily.com (2012-10-16).
- Vine, Frederick J. & Drummond H. Matthews 1963. Magnetic anomalies over oceanic ridges. Nature 199 (4897): 947–949.
- Morley, Lawrence W. & Larochelle A. 1964. "Paleomagnetism as a means of dating geological events". Geochronology in Canada. Special (Royal Society of Canada) Publication 8: 39–50.
- Merrill, Ronald T.; McElhinny, Michael W.; McFadden, Phillip L. (1998). The magnetic field of the Earth: paleomagnetism, the core, and the deep mantle. Academic Press. ISBN 978-0-12-491246-5.
- Courtillot, Vincent (1999). Evolutionary catastrophes: the science of mass extinctions. Cambridge: Cambridge University Press. pp. 110–11. ISBN 978-0-521-58392-3. Translated from the French by Joe McClinton.
- Glatzmaier, Gary A. & Roberts, Paul H. 1995. "A three dimensional self-consistent computer simulation of a geomagnetic field reversal". Nature 377: pp. 203–209.
- Glatzmaier, Gary; Roberts, Paul. "When North goes South".
- Berhanu M. et al. "Magnetic field reversals in an experimental turbulent dynamo". Europhysics Letters 77: p. 59001.
- Plotnick, Roy E. (1980). "Relationship between biological extinctions and geomagnetic reversals". Geology 8 (12): 578. doi:10.1130/0091-7613(1980)8<578:RBBEAG>2.0.CO;2.
- Glassmeier, Karl-Heinz & Vogt, Joachim 2010. Magnetic polarity transitions and biospheric effects. Space Science Reviews 155 (1-4): 387–410.