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· A geomagnetic reversal is a change in the Earth's magnetic fieldsuch that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods ofnormal polarity, in which the direction of the field was the same as the present direction, andreverse polarity, in which the field was in the opposite direction. These periods are called chrons. The time spans of chrons are randomly distributed with most being between 0.1 and 1 million years. Most reversals are estimated to take between 1,000 and 10,000 years. The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago. Brief disruptions that do not result in reversal are called geomagnetic excursions. |
History
Three decades later, when Earth's magnetic field was better understood, theories were advanced suggesting that the Earth's field might have reversed in the remote past. Most paleomagnetic research in the late 1950s included an examination of the wandering of the poles and continental drift. Although it was discovered that some rocks would reverse their magnetic field while cooling, it became apparent that most magnetized volcanic rocks preserved traces of the Earth's magnetic field at the time the rocks had cooled. At first it was thought that reversals occurred approximately every million years, but by the 1960s it had become apparent that the timing of magnetic reversals was erratic.
During the 1950s and 1960s information about variations in the Earth's magnetic field was gathered largely by means of research vessels. But the complex routes of ocean cruises rendered the association of navigational data with magnetometer readings difficult. Only when data was plotted on a map, did it become apparent that remarkably regular and continuous magnetic stripes appeared on the ocean floors.
In 1963 Frederick Vine and Drummond Matthews provided a simple explanation by combining the seafloor spreading theory of Harry Hess with the known time scale of reversals: if new sea floor is magnetized in the direction of the field, then it will change its polarity when the field reverses. Thus, sea floor spreading from a central ridge will produce magnetic stripes parallel to the ridge.Canadian L. W. Morley independently proposed a similar explanation in January 1963, but his work was rejected by the scientific journalsNature and Journal of Geophysical Research, and remained unpublished until 1967, when it appeared in the literary magazine Saturday Review.The Morley–Vine–Matthews hypothesis was the first key scientific test of the seafloor spreading theory of continental drift.
Beginning in 1966, Lamont–Doherty Geological Observatory scientists found that the magnetic profiles across the Pacific-Antarctic Ridge were symmetrical and matched the pattern in the north Atlantic's Reykjanes ridges. The same magnetic anomalies were found over most of the world's oceans, which permitted estimates for when most of the oceanic crust had developed.
Geomagnetic polarity since the middle Jurassic. Dark areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed.
Past field reversals can be and have been recorded in the "frozen" ferromagnetic (or more accurately, ferrimagnetic) minerals of consolidated sedimentary deposits or cooledvolcanic flows on land.
Originally, however, the past record of geomagnetic reversals was first noticed by observing the magnetic stripe "anomalies" on the ocean floor. Lawrence W. Morley, Frederick John Vine and Drummond Hoyle Matthews made the connection to seafloor spreading in the Morley-Vine-Matthews hypothesiswhich soon led to the development of the theory of plate tectonics. Given that the sea floor spreads at a relatively constant rate, this results in broadly evident substrate "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from towing a magnetometer along the sea floor.
However, because no existing unsubducted sea floor (or sea floor thrust onto continental plates, such as in the case ofophiolites) is much older than about 180 million years (Ma) in age, other methods are necessary for detecting older reversals. Most sedimentary rocks incorporate tiny amounts of iron richminerals, whose orientation is influenced by the ambient magnetic field at the time at which they formed. Under favorable conditions, it is thus possible to extract information of the variations in magnetic field from many kinds of sedimentary rocks. However, subsequent diagenetic processes after burial may erase evidence of the original field.
Because the magnetic field is present globally, finding similar patterns of magnetic variations at different sites is one method used to correlate age across different locations. In the past four decades great amounts of paleomagnetic data about seafloor ages (up to ~250 Ma) have been collected and have become an important and convenient tool to estimate the age of geologic sections. It is not an independent dating method, but is dependent on "absolute" age dating methods like radioisotopic systems to derive numeric ages. It has become especially useful to metamorphic and igneous geologists where the use ofindex fossils to estimate ages is seldom available.
Geomagnetic polarity time scale
Through analysis of seafloor magnetic anomalies and dating of reversal sequences on land, paleomagnetists have been developing a Geomagnetic Polarity Time Scale (GPTS). The current time scale contains 184 polarity intervals in the last 83 million years.
Changing frequency of geomagnetic reversals over time
Statistical properties of reversals
Several studies have analyzed the statistical properties of reversals in the hope of learning something about their underlying mechanism. The discriminating power of statistical tests is limited by the small number of polarity intervals. Nevertheless, some general features are well established. In particular, the pattern of reversals is random. There is no correlation between the lengths of polarity intervals.There is no preference for either normal or reversed polarity, and no statistical difference between the distributions of these polarities. This lack of bias is also a robust prediction of dynamo theory. Finally, as mentioned above, the rate of reversals changes over time.
The randomness of the reversals is inconsistent with periodicity, but several authors have claimed to find periodicity.However, these results are probably artifacts of an analysis using sliding windows to determine reversal rates.
Most statistical models of reversals have analyzed them in terms of a Poisson process or other kinds of renewal process. A Poisson process would have, on average, a constant reversal rate, so it is common to use a non-stationary Poisson process. However, compared to a Poisson process, there is a reduced probability of reversal for some tens of thousands after a reversal. This could be due to some inhibition in the underlying mechanism, or it could just mean that some shorter polarity intervals have been missed. A random reversal pattern with inhibition can be represented by a gamma process. In 2006, a team of physicists at the University of Calabria found that the reversals also conform to a Lévy distribution, which describes stochastic processes with long-ranging correlations between events in time.The data are also consistent with a deterministic, but chaotic, process.
