Recent geomagnetic reversals.

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. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see 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

In the early 20th century geologists first noticed that some volcanic rocks were magnetized in a direction opposite to what was expected. The first examination of the timing of magnetic reversals was done by Motonari Matuyama in the 1920s, who observed that there were rocks in Japan whose magnetic fields were reversed and those were all of early Pleistocene age or older. The Pleistocene ('plaɪstəsin is the epoch from 18 million to 10000 years BP covering the world's recent period At the time he published his proposal suggesting that the magnetic field had been reversed, the magnetic field itself was poorly understood so there was little interest in the possibility that it had reversed. ^[1]

Three decades later, theories existed of the cause of the magnetic field and some of these included the ability for the field to be reversed. Most paleomagnetic research in the late 1950s was examining the wandering of the poles and continental drift. Continental drift is the movement of the Earth 's Continents relative to each other Although it was discovered that some rocks would reverse their magnetic field while cooling, it became apparent that most magnetized volcanic rocks contained traces of the Earth's magnetic field at the time the rock cooled. At first it seemed that reversals happen every one million years, but during the 1960s it became apparent that the time between reversals is erratic. ^[1]

During the 1950s and 1960s research ships gathered information about variations in the Earth's magnetic field. Because of the complex routes of cruises, associating navigational data with magnetometer readings was difficult. A magnetometer is a scientific instrument used to measure the strength and/or direction of the Magnetic field in the vicinity of the instrument But when data was plotted on a map, it became apparent that there were remarkably regular and continuous magnetic stripes across the ocean floors. ^[1]

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 acquired the present magnetic field, spreading from a central ridge would produce magnetic stripes parallel to the ridge. Frederick John Vine (born June 17, 1939) is a marine geologist and Geophysicist and was a key contributor to the theory of Plate tectonics. Drummond Hoyle Matthews ( 5 February, 1931 &ndash 20 July, 1997) was a British marine geologist and geophysicist and a key contributor Harry Hammond Hess ( May 24, 1906 – August 25, 1969) was a Geologist and United States Navy officer in World War ^[2] Canadian L. W. Morley independently proposed a similar explanation in January 1963, but his work was rejected by the scientific journals Nature and Journal of Geophysical Research, and not published until 1967 in the literary magazine Saturday Review. Lawrence (Whitaker Morley PhD (1920- is a Canadian Geophysicist. Nature, in the broadest sense is equivalent to the natural world, physical universe, material world or material universe. Journal of Geophysical Research is the premier journal of the American Geophysical Union. Saturday Review (1924&ndash1986 was a weekly US -based Magazine. ^[1]

Further information: Morley-Vine-Matthews hypothesis

Starting in 1966, Lamont-Doherty Geological Observatory scientists found the magnetic profiles across the Pacific-Antarctic Ridge were symmetrical and matched the pattern in the north Atlantic's Reykjanes ridges. The Morley-Vine-Matthews hypothesis, also known as the Vine-Matthews-Morley hypothesis was the first key scientific test of the Seafloor spreading The Lamont-Doherty Earth Observatory (LDEO is a research institution specializing in the Earth sciences and is part of Columbia University. The Pacific-Antarctic Ridge (PAR is a divergent tectonic plate boundary located on the seafloor of the South Pacific Ocean, separating the Pacific Plate Reykjanes (pronounced /ˈreːkjəˌnɛs/) (or Reykjanesskagi) is a Peninsula and a volcanic system situated at the south-western end of Iceland The same magnetic anomalies were found over most of the world's oceans, and allowed estimation of the timing of the creation of most of the oceanic crust. ^[1]

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. Paleomagnetism is the study of the record of the Earth's magnetic field preserved in various Magnetic Minerals through time 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. 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 In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges Other events seem to have occurred very rapidly, with two reversals in a span of 50 ka. Annum is one form of the Latin noun meaning Year, not a form normally used for derivatives in modern languages the accusative singular The last reversal was the Brunhes-Matuyama reversal approximately 780 ka ago. The Brunhes-Matuyama Reversal was a geologic event approximately 780000 years ago when the Earth's magnetic field last underwent reversal.

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. The dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a Magnetic field. 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. A computer is a Machine that manipulates data according to a list of instructions. Chaos in physics is often considered analogous to thermodynamic entropy Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001

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). The Sun (Sol is the Star at the center of the Solar System. The solar cycle, or the solar magnetic activity cycle, is the main source of periodic variation of all solar phenomena driving variations in Space weather. 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. Richard A Muller of San Francisco California, US, is a Physicist who works at the University of California Berkeley and Lawrence Berkeley 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. The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle. The core-mantle boundary (or CMB in the parlance of solid earth geophysicists lies between the Earth's Silicate mantle and its liquid Iron - Nickel See also Impact crater An impact event is the Collision of a large Meteoroid, Asteroid or Comet (generically 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. A geomagnetic excursion, like a Geomagnetic reversal, is a significant change in the Earth's magnetic field.

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. Ferromagnetism is the basic mechanism by which certain materials (such as Iron) form Permanent magnets and/or exhibit strong interactions with Magnets it In Physics, a ferrimagnetic material is one in which the Magnetic moment of the atoms on different sublattices are opposed as in Antiferromagnetism; however Plate tectonics and hotspots Divergent plate boundaries At the Originally, however, the past record of geomagnetic reversals was first noticed by observing the magnetic stripe "anomalies" on the ocean floor. Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins Lawrence W. Morley, Frederick John Vine and Drummond Hoyle Matthews made the connection to seafloor spreading in the Morley-Vine-Matthews hypothesis^[3][4] which soon led to the development of the theory of plate tectonics. Lawrence (Whitaker Morley PhD (1920- is a Canadian Geophysicist. Frederick John Vine (born June 17, 1939) is a marine geologist and Geophysicist and was a key contributor to the theory of Plate tectonics. Drummond Hoyle Matthews ( 5 February, 1931 &ndash 20 July, 1997) was a British marine geologist and geophysicist and a key contributor The Morley-Vine-Matthews hypothesis, also known as the Vine-Matthews-Morley hypothesis was the first key scientific test of the Seafloor spreading Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere 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. Seafloor spreading occurs at Mid-ocean ridges where new Oceanic crust is formed through volcanic activity and then gradually moves away from the ridge A magnetometer is a scientific instrument used to measure the strength and/or direction of the Magnetic field in the vicinity of the instrument 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. An Ophiolite is a section of the Earth's Oceanic crust and the underlying upper mantle that has been uplifted or emplaced to be exposed within Continental Annum is one form of the Latin noun meaning Year, not a form normally used for derivatives in modern languages the accusative singular 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. Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock) A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific 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. In Geology and Oceanography, diagenesis is any chemical physical or biological change undergone by a Sediment after its initial deposition and during

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. Index fossils (also known as guide fossils or zone fossils are Fossils used to define and identify geologic periods (or faunal stages

The geomagnetic polarity time scale

Record of Geomagnetic Polarity for ages 0-160 Ma. Reproduced from Lowrie(1997), "Fundamentals of Geophysics".

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.

It had generally been assumed that the frequency of geomagnetic reversals is random, and it was shown in 2006 that the known reversals conform to a Lévy distribution. In Probability theory and Statistics, the Lévy distribution, named after Paul Pierre Lévy, is a Continuous probability distribution for a non-negative

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 Marianas Trench). The Mariana Trench (or Mariana's Trench) is the deepest part of the world's Oceans and the deepest location on the surface of the Earth 's

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. ^[5]

Future of the present field

Geomagnetic variations since the last reversal.

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 3000–4000 AD. The South Atlantic Anomaly is believed by some to be a product of this. The South Atlantic Anomaly (or SAA is the region where Earth 's inner Van Allen radiation belt makes its closest approach to the planet's surface 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. In Statistics, a Sequence or a vector of Random variables is heteroskedastic, or heteroscedastic, if the random variables have different 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. For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on However, Homo erectus and their ancestors certainly survived many previous reversals. Homo erectus ( Latin: "upright man" is an extinct species of the genus Homo, believed to have been the first hominin There is no uncontested evidence that a magnetic field reversal has ever caused any biological extinctions. In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. A possible explanation is that the solar wind may induce a sufficient magnetic field in the Earth's ionosphere to shield the surface from energetic particles even in the absence of the Earth's normal magnetic field [1]. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation

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. The Earth's North Magnetic Pole is the wandering point on the Earth's surface at which the Earth's magnetic field points vertically downwards (i [2] 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. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of g. radio, satellite communications) may be vulnerable to technological disruptions in the event of a full field reversal. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light.

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. Cray Inc ( is a Supercomputer manufacturer based in Seattle Washington. 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. [3]

References

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.

External links

• How geomagnetic reversals are related to intensity
• The geodynamo
• "Look down, look up, look out!", The Economist, May 10th 2007
• "Ships' logs give clues to Earth's magnetic decline", New Scientist, May 11th 2006