Geomagnetic reversal
A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged. These events, which are believed to last a few hundred to a few thousand years, often involve an extended decline in field strength followed by a rapid recovery after the new orientation has been established.
History
Through analysis of palaeomagnetic data, we now know that the field has reversed its orientation tens of thousands of times since its formation very early on in earth history. With the increasingly accurate Global Polarity Timescale (GPTS) it has become apparent that the rate at which reversals occur has varied considerably throughout the past. During some periods of geologic time (e.g. Cretaceous Long Normal), the Earth's magnetic field is observed to maintain a single orientation for tens of millions of years. Other events seem to have occurred very rapidly, with more than one reversal in 50,000 years. The last reversal was the Brunhes-Matuyama reversal approximately 780,000 years ago.
Causes
Scientific opinion is divided on what causes geomagnetic reversals. Many scientists believe that reversals are an inherent aspect of the dynamo theory of how the geomagnetic field is generated. In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core.
In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation. This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 7-15 years (see: solar cycle). However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength.
Present computational methods have used very strong simplifications in order to produce models that run to acceptable time scales for research programmes.
A minority opinion, held by such figures as Richard A. Muller, is that geomagnetic reversals are not spontaneous processes but rather triggered by external events which directly disrupt the flow in the Earth's core. Such processes may include the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones, the initiation of new mantle plumes from the core-mantle boundary, and possibly mantle-core shear forces resulting from very large impact events. Supporters of this theory hold that any of these events could lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field. Because the magnetic field is stable in either the present North-South orientation or a reversed orientation, they propose that when the field recovers from such a disruption it spontaneously chooses one or the other state, such that a recovery is seen as a reversal in about half of all cases. Brief disruptions which do not result in reversal are also known and are called geomagnetic excursions.
Observing past fields
Past field reversals can be and have been recorded in the "frozen" ferromagnetic (or more accurately, ferrimagnetic) minerals of solidified sedimentary deposits or cooled volcanic 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 hypothesis[1][2] which 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 simply 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 of ophiolites) 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 rich minerals, 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 have been accumulated about current seafloor ages (up to ~250 Ma) to such an extent that such data have become an important and convenient tool used to estimate the age of geologic sections in the field. It is, however, 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 of index fossils to estimate ages is seldom available.
The geomagnetic polarity time scale
The changing frequency of geomagnetic reversals over time
The rate of reversals in the Earth's magnetic field has varied widely over time. 72 million years ago (Ma), the field reversed 5 times in a million years. In a 4-million-year period centered on 54 Ma, there were 10 reversals; at around 42 Ma, 17 reversals took place in the span of 3 million years. In a period of 3 million years centering on 24 Ma, 13 reversals occurred. No less than 51 reversals occurred in a 12-million-year period, centering on 15 million years ago. These eras of frequent reversals have been counterbalanced by a few long periods when no reversals took place, as described below.
Until recently, it has generally been assumed that the frequency of geomagnetic reversals is random. However, it was shown in 2006 that the known reversals conform to a Lévy distribution. [1]
The Cretaceous Long Normal Superchron
A long period of time during which there were no magnetic pole reversals, the Cretaceous Long Normal (also called the Cretaceous Superchron or C34) lasted from about 120 to 83 million years ago. This time period included stages of the Cretaceous period from the Aptian through the Santonian.
An interesting trend can be seen when looking at the frequency of magnetic reversals approaching and following the Cretaceous Long Normal. The frequency steadily decreased prior to the period, reaching its low point (no reversals) during the period. Following the Cretaceous Superchron the frequency of reversals slowly increased over the next 80 million years, to the present.
The Jurassic Quiet Zone
The Jurassic Quiet Zone is a section of ocean floor which is completely devoid of the magnetic stripes that can be detected elsewhere. This could mean that there was a long period of polar stability during the Jurassic period similar to the Cretaceous Superchron. Another possibility is that as this is the oldest section of ocean floor, any magnetization that did exist has completely degraded by now. The Jurassic Quiet Zones exist in places along the continental margins of the Atlantic ocean as well as in parts the Western Pacific (such as just east of the Mariana Trench).
The Kiaman Long Reversed Superchron
Another long period of time during which where no magnetic pole reversals. This period lasted approximately from the late Carboniferous to the late Permian, from approximately 316 to 262 million years ago. The magnetic field was reversed compared to its present state. The name "Kiaman" derives from the Australian village of Kiama, where some of the first geological evidence of the superchron was found in 1925.[3]
Future of the present field
At present, the overall geomagnetic field is becoming weaker at a rate which would, if it continues, cause the dipole field to temporarily collapse by 2012. The South Atlantic Anomaly is believed by some to be a product of this. The present strong deterioration corresponds to a 10–15% decline over the last 150 years and has accelerated in the past several years; however, geomagnetic intensity has declined almost continuously from a maximum 35% above the modern value achieved approximately 2000 years ago. The rate of decrease and the current strength are within the normal range of variation, as shown by the record of past magnetic fields recorded in rocks.
The nature of Earth's magnetic field is one of heteroscedastic fluctuation. An instantaneous measurement of it, or several measurements of it across the span of decades or centuries, is not sufficient to extrapolate an overall trend in the field strength. It has gone up and down in the past with no apparent rhyme or reason. Also, noting the local intensity of the dipole field (or its fluctuation) is insufficient to characterize Earth's magnetic field as a whole, as it is not strictly a dipole field. The dipole component of Earth's field can diminish even while the total magnetic field remains the same or increases.
Because the magnetic field has never been observed to reverse by humans with instrumentation, and the mechanism of field generation is not well understood, it is difficult to say what the characteristics of the magnetic field might be leading up to such a reversal. Some speculate that a greatly diminished magnetic field during a reversal period will expose the surface of the earth to a substantial and potentially damaging increase in cosmic radiation. However, Homo erectus and their ancestors certainly survived many previous reversals. There is no uncontested evidence that a magnetic field reversal has ever caused any biological extinctions. A possible explanation is that the solar wind may induce a sufficient magnetic field in the Earth's ionosphere to shield energetic particles even in the absence of the Earth's normal magnetic field [2].
The Earth's magnetic north pole is drifting from northern Canada towards Siberia with a presently accelerating rate — 10km per year at the beginning of the 20th century, up to 40km per year in 2003.[3] It is also unknown if this drift will continue to accelerate.
Although the inspection of past reversals does not indicate biological extinctions, present society with its reliance on electricity and electromagnetic effects (e.g. radio, satellite communications) may be vulnerable to technological disruptions in the event of a full field reversal.
Glatzmaier and collaborator Paul Roberts of UCLA have made a numerical model of the electromagnetic, fluid dynamical processes of Earth's interior, and computed it on a Cray supercomputer. The results reproduced key features of the magnetic field over more than 40,000 years of simulated time. Additionally, the computer-generated field reversed itself.[4]
References
- ^ Vine, F. J., and D. H.Mathews (1963). "Magnetic anomalies over oceanic ridges." Nature 199: 947-949.
- ^ Morley, L. W., and A. Larochelle (1964). "Paleomagnetism as a means of dating geological events." Geochronology in Canada. Royal Society of Canada Special Publication 8, 39-50.
- ^ Courtillot, Vincent. Evolutionary Catastrophes: the Science of Mass Extinctions. Translated by Joe McClinton. Cambridge, Cambridge University Press, 1999; pp. 110-11.
Further reading
- Behrendt, J.C., Finn, C., Morse, L., Blankenship, D.D. "One hundred negative magnetic anomalies over the West Antarctic Ice Sheet (WAIS), in particular Mt. Resnik, a subaerially erupted volcanic peak, indicate eruption through at least one field reversal" University of Colorado, U.S. Geological Survey, University of Texas. (U.S. Geological Survey and The National Academies); USGS OF-2007-1047, Extended Abstract 030. 2007.
- Okada, M., Niitsuma, N., "Detailed paleomagnetic records during the Brunhes-Matuyama geomagnetic reversal, and a direct determination of depth lag for magnetization in marine sediments" Physics of the Earth and Planetary Interiors, Volume 56, Issue 1-2, p. 133-150. 1989.