Geomagnetic reversal: Difference between revisions
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THE EARTH HAS MAGNETS |
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{{Redirect|Magnetic reversal|switching of a magnet|Magnetization reversal}} |
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{{Redirect|Polarity reversal |a seismic anomaly |Polarity reversal (seismology)}} |
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[[File:Geomagnetic polarity late Cenozoic.svg|right|thumb|285px|Geomagnetic polarity during the late [[Cenozoic Era]]. Dark areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed.]] |
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A '''geomagnetic reversal''' is a change in the [[Earth's magnetic field]] such that the positions of magnetic north and magnetic south are interchanged. The [[Earth]]'s field has alternated between periods of ''normal'' polarity, in which the direction of the field was the same as the present direction, and ''reverse'' polarity, in which the field was the opposite. These periods are called ''chrons''. The time spans of chrons are randomly distributed with most being between 0.1 and 1{{nbsp}}million years{{cn|date=September 2012}} with an average of 450,000 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. However,a study published in 2012 by a group from the German Research Center for Geosciences suggests that a brief complete reversal occurred only 41,000 years ago during the last ice age. The reversal lasted only about 440 years with the actual change of polarity lasting around 260 years. During this change the strength of the magnetic field dropped to 5% of its present strength.<ref> [http://www.sciencedaily.com/releases/2012/10/121016084936.htm] </ref> Brief disruptions that do not result in reversal are called [[geomagnetic excursion]]s. |
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== History == |
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In the early 20th century geologists first noticed that some volcanic rocks were magnetized opposite to the direction of modern day Earth's field. The first estimate of the timing of magnetic reversals was made in the 1920s by [[Motonori Matuyama]], who observed that rocks with reversed fields were all of early [[Pleistocene]] age or older. At the time, the Earth's polarity was poorly understood and the possibility of reversal aroused little interest.<ref name=Cox1973>{{cite book|last=Cox|first=Allan|authorlink=Allan V. Cox|coauthors= |title=Plate tectonics and geomagnetic reversal|publisher=W. H. Freeman|year=1973|location=San Francisco, California|pages=138–145, 222–228|isbn=0-7167-0258-4}}</ref><ref name=Glen>{{cite book|last=Glen|first=William|title=The Road to Jaramillo: Critical Years of the Revolution in Earth Science|publisher=[[Stanford University Press]]|year=1982|isbn=0-8047-1119-4}}</ref> |
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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. In the absence of reliable methods for obtaining absolute ages for rocks, it was thought that reversals occurred approximately every million years.<ref name=Cox1973 /><ref name=Glen/> |
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The next major advance in understanding reversals came when techniques for [[radiometric dating]] were developed in the 1950s. [[Allan V. Cox|Allan Cox]] and [[Richard Doell]], at the [[United States Geological Survey]], wanted to know whether reversals occurred at regular intervals, and invited the geochronologist [[Brent Dalrymple]] to join their group. They produced the first magnetic-polarity time scale in 1959. As they accumulated data, they continued to refine this scale in competition with Don Tarling and Ian McDougall at the [[Australian National University]]. A group led by Neil Opdyke at the [[Lamont–Doherty Earth Observatory|Lamont-Doherty Geological Observatory]] showed that the same pattern of reversals was recorded in sediments from deep-sea cores.<ref name=Glen/> |
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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 were plotted on a map did it become apparent that remarkably regular and continuous magnetic stripes appeared on the ocean floors.<ref name=Cox1973 /><ref name=Glen/> |
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In 1963 [[Frederick Vine]] and [[Drummond Matthews]] provided a simple explanation by combining the [[seafloor spreading]] theory of [[Harry Hammond Hess|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.<ref name=vine1963>{{cite journal|title=Magnetic Anomalies over Oceanic Ridges|journal= Nature|year=1963|first=Frederick J.|last=Vine|coauthors=Drummond H. Matthews|volume=199|issue=4897|pages=947–949|doi= 10.1038/199947a0|bibcode=1963Natur.199..947V}}</ref> Canadian [[Lawrence Morley|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 remained unpublished until 1967, when it appeared in the literary magazine ''[[Saturday Review (US magazine)|Saturday Review]]''.<ref name= Cox1973/> The [[Morley–Vine–Matthews hypothesis]] was the first key scientific test of the seafloor spreading theory of continental drift.<ref name=Glen/> |
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Beginning in 1966, [[Lamont–Doherty Earth Observatory|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.<ref name=Cox1973 /><ref name=Glen/> |
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==Observing past fields== |
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[[File:Geomagnetic polarity 0-169 Ma.svg|thumb|left|upright=0.6|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.]] |
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Past field reversals can be and have been recorded in the "frozen" [[Ferromagnetism|ferromagnetic]] (or more accurately, [[Ferrimagnetism|ferrimagnetic]]) minerals of consolidated sedimentary deposits or cooled [[Volcano|volcanic]] flows on land. |
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The past record of geomagnetic reversals was first noticed by observing the magnetic stripe "anomalies" on the [[Oceanic crust|ocean floor]]. [[Lawrence Morley|Lawrence W. Morley]], [[Fred Vine|Frederick John Vine]] and [[Drummond Matthews|Drummond Hoyle Matthews]] made the connection to seafloor spreading in the [[Morley-Vine-Matthews hypothesis]]<ref name= vine1963/><ref>{{cite journal|last=Morley|first=Lawrence W.|coauthors=A. Larochelle|year=1964|title=Paleomagnetism as a means of dating geological events|series=Special|journal=Geochronology in Canada|publisher=Royal Society of Canada|volume=Publication 8|pages=39–50}}</ref> which soon led to the development of the theory of [[plate tectonics]]. The relatively constant rate at which the [[Seafloor spreading|sea floor]] spreads results in substrate "stripes" from which past magnetic field polarity can be inferred from data gathered from towing a [[magnetometer]] along the sea floor. |
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Because no existing unsubducted sea floor (or [[ophiolites|sea floor thrust onto continental plates]]) is more than about {{mya|180|million years}} ([[Ma (unit)|Ma]]) old, other methods are necessary for detecting older reversals. Most [[sedimentary rock]]s incorporate tiny amounts of iron rich [[mineral]]s, whose orientation is influenced by the ambient magnetic field at the time at which they formed. These rocks can preserve a record of the field if it is not later erased by [[Diagenesis|chemical, physical or biological change]]. |
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Because the magnetic field is global, similar patterns of magnetic variations at different sites may be used to correlate age in different locations. In the past four decades much paleomagnetic data about seafloor ages (up to ~{{mya|250| Ma}}) has been collected and is useful in estimating the age of geologic sections. Not an independent dating method, it depends on "absolute" age dating methods like radioisotopic systems to derive numeric ages. It has become especially useful to metamorphic and igneous geologists where [[index fossil]]s are seldom available. |
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== Geomagnetic polarity time scale == |
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{{Further2|[[Magnetostratigraphy]]}} |
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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{{nbsp}}million years.<ref name=Cande>{{cite journal|doi=10.1029/94JB03098|last=Cande|first=S. C.|last2=Kent|first2=D. V.|title=Revised calibration of the geomagnetic polarity timescale for the late Cretaceous and Cenozoic|journal=[[Journal of Geophysical Research]]|volume=100|pages=6093–6095|year=1995|bibcode=1995JGR...100.6093C}}</ref><ref>{{cite web|url=http://deeptow.whoi.edu/gpts.html|title=Geomagnetic Polarity Timescale|publisher=[[Woods Hole Oceanographic Institution]]|work=Ocean Bottom Magnetometry Laboratory|accessdate=March 23, 2011}}</ref> |
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=== Changing frequency over time === |
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The rate of reversals in the Earth's magnetic field has varied widely over time. {{mya|72|million years ago (Ma)}}, the field reversed 5 times in a million years. In a 4-million-year period centered on {{mya|54| Ma}}, there were 10 reversals; at around {{mya|42| Ma}}, 17 reversals took place in the span of 3{{nbsp}}million years. In a period of 3{{nbsp}}million years centering on {{mya|24| Ma}}, 13 reversals occurred. No fewer than 51 reversals occurred in a 12-million-year period, centering on {{mya|15}}. Two reversals occurred during a span of 50,000 years. These eras of frequent reversals have been counterbalanced by a few "superchrons" – long periods when no reversals took place.<ref>{{cite journal|date=2001-03-02|title=When the Compass Stopped Reversing Its Poles|journal=[[Science (journal)|Science]]|publisher=[[American Association for the Advancement of Science]]|volume=291|issue=5509|pages=1714–1715|url=|doi=10.1126/science.291.5509.1714 |last=Banerjee|first=Subir K.}}</ref> |
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=== Superchrons === |
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A ''superchron'' is a polarity interval lasting at least 10{{nbsp}}million years. There are two well-established superchrons, the Cretaceous Normal and the Kiaman. A third candidate, the Moyero, is more controversial. The Jurassic Quiet Zone in ocean magnetic anomalies was once thought to represent a superchron, but is now attributed to other causes. |
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The ''Cretaceous Normal'' (also called the ''Cretaceous Superchron'' or C34) lasted for almost 40{{nbsp}}million years, from about {{mya|120|83}}, including stages of the [[Cretaceous]] period from the [[Aptian]] through the [[Santonian]]. The frequency of magnetic reversals steadily decreased prior to the period, reaching its low point (no reversals) during the period. Between the Cretaceous Normal and the present, the frequency has generally increased slowly.<ref name=Merrill>{{cite book|last=Merrill|first= Ronald T.|last2=McElhinny|first2=Michael W.|last3=McFadden|first3=Phillip L.|title=The magnetic field of the earth: paleomagnetism, the core, and the deep mantle|publisher=[[Academic Press]]|year=1998|isbn=978-0-12-491246-5}}</ref> |
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The ''Kiaman Reverse Superchron'' lasted from approximately the late [[Carboniferous]] to the late [[Permian]], or for more than 50{{nbsp}}million years, from around {{mya|312|262}}.<ref name=Merrill/> The magnetic field had reversed polarity. The name "Kiaman" derives from the Australian village of [[Kiama, New South Wales|Kiama]], where some of the first geological evidence of the superchron was found in 1925.<ref>{{cite book|last=Courtillot|first=Vincent|authorlink=Vincent Courtillot|title=Evolutionary Catastrophes: the Science of Mass Extinctions|location=Cambridge|publisher=[[Cambridge University Press]]|year=1999|pages=110–11|isbn=978-0-521-58392-3}} Translated from the French by Joe McClinton.</ref> |
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The [[Ordovician]] is suspected to host another superchron, called the ''Moyero Reverse Superchron'', lasting more than 20{{nbsp}}million years (485 to 463{{nbsp}}million years ago) . But until now this possible superchron has only been found in the Moyero river section north of the polar circle in Siberia.<ref name=Pavlov>{{cite journal|last=Pavlov|first=V.|last2=Gallet|first2=Y.|title=A third superchron during the Early Paleozoic|journal=Episodes|publisher=International Union of Geological Sciences|volume=28|issue=2|pages=78–84}}</ref> Moreover, the best data from elsewhere in the world do not show evidence for this superchron.<ref name=McElhinny>{{cite book|last= McElhinny|first=Michael W.|last2=McFadden|first2=Phillip L.|title=Paleomagnetism: Continents and Oceans|publisher=[[Academic Press]]|year=2000|isbn=0-12-483355-1}}</ref> |
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Certain regions of ocean floor, older than {{mya|160|Ma}}, have low-amplitude magnetic anomalies that are hard to interpret. They are found off the east coast of North America, the northwest coast of Africa, and the western Pacific. They were once thought to represent a superchron called the ''Jurassic Quiet Zone'', but magnetic anomalies are found on land during this period. The geomagnetic field is known to have low intensity between about {{mya|130|Ma}} and {{mya|170|Ma}}, and these sections of ocean floor are especially deep, so the signal is attenuated between the floor and the surface.<ref name=McElhinny/> |
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=== Statistical properties of reversals === |
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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.<ref>{{cite journal|last=Phillips|first=J. D.|last2=Cox|first2=A.|title=Spectral analysis of geomagnetic reversal time scales|journal=[[Geophysical Journal International|Geophysical Journal of the Royal Astronomical Society]]|volume=45|pages=19–33|year=1976|bibcode = 1976GeoJI..45...19P |doi = 10.1111/j.1365-246X.1976.tb00311.x }}</ref> 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]].<ref name=Merrill/> Finally, as mentioned above, the rate of reversals changes over time. |
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The randomness of the reversals is inconsistent with periodicity, but several authors have claimed to find periodicity.<ref>e.g., {{cite journal|last=Raup|first=D. M.|title=Magnetic reversals and mass extinctions|journal=[[Nature (journal)|Nature]]|volume= 314|pages=341–343|year=1985|doi=10.1038/314341a0|bibcode = 1985Natur.314..341R }}</ref> However, these results are probably artifacts of an analysis using sliding windows to determine reversal rates.<ref name="Lutz">{{cite journal|last=Lutz|first=T. M.|title=The magnetic reversal record is not periodic|journal=[[Nature (journal)|Nature]]|volume=317|pages=404–407|year=1985|doi=10.1038/317404a0|bibcode = 1985Natur.317..404L }}</ref> |
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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 tens of thousands of years after a reversal. This could be due to an inhibition in the underlying mechanism, or it could just mean that some shorter polarity intervals have been missed.<ref name=Merrill/> 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 process]]es with long-ranging correlations between events in time.<ref>{{cite web|url= http://physicsworld.com/cws/article/news/2006/mar/21/geomagnetic-flip-may-not-be-random-after-all |title=Geomagnetic flip may not be random after all|work=physicsworld.com|date=March 21, 2006|last=Dumé|first=Belle|accessdate=December 27, 2009}}</ref><ref>{{cite journal|last=Carbone|first= V.|last2=Sorriso-Valvo|first2=L.|last3=Vecchio|first3=A.|last4=Lepreti|first4=F.|last5=Veltri|first5=P.|last6=Harabaglia|first6=P.|last7=Guerra| first7 = I.|title=Clustering of Polarity Reversals of the Geomagnetic Field|journal=[[Physical Review Letters]]|volume=96|issue= 12|page=128501|doi=10.1103/PhysRevLett.96.128501|bibcode=2006PhRvL..96l8501C|arxiv = physics/0603086 }}</ref> The data are also consistent with a deterministic, but chaotic, process.<ref>{{cite journal|last=Gaffin|first=S.|title=Analysis of scaling in the geomagnetic polarity reversal record|journal=[[Physics of the Earth and Planetary Interiors]]|volume=57|pages=284–289|year=1989|bibcode = 1989PEPI...57..284G |doi = 10.1016/0031-9201(89)90117-9 }}</ref> |
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==Character of transitions== |
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===Duration=== |
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Most estimates for the duration of a polarity transition are between 1,000 and 10,000 years.<ref name=Merrill/> However, studies of lava flows on [[Steens Mountain]], Oregon, indicate that the magnetic field could have shifted at a rate of up to 6 degrees per day about 15{{nbsp}}million years ago.<ref name="nature.com">{{cite journal|title=New evidence for extraordinarily rapid change of the geomagnetic field during a reversal | url=http://www.nature.com/nature/journal/v374/n6524/abs/374687a0.html|journal=Nature|date=20 April 1995| doi= 10.1038/374687a0|last1=Coe|first1=R. S.|last2=Prévot|first2=M.|last3=Camps|first3=P.|volume=374|issue=6524|page=687|bibcode=1995Natur.374..687C}}</ref> This was initially met with skepticism from paleomagnetists. Even if changes occur that quickly in the core, the mantle, which is a [[semiconductor]], is thought to act as a low-pass filter, removing variations with periods less than a few months. A variety of possible [[rock magnetism|rock magnetic]] mechanisms were proposed that would lead to a false signal.<ref name=Merrill2010/> However, paleomagnetic studies of other sections from the same region (the Oregon Plateau flood basalts) give consistent results.<ref>{{cite journal |last1=Prévot |first1=M. |first2=E. |last2=Mankinen |first3=R. |last3=Coe |first4=C. |last4=Grommé |year=1985 |title=The Steens Mountain (Oregon) Geomagnetic Polarity Transition 2. Field Intensity Variations and Discussion of Reversal Models |journal=[[J. Geophys. Res.]] |volume=90 |issue=B12 |pages=10417–10448|bibcode = 1985JGR....9010417P |doi = 10.1029/JB090iB12p10417 }}</ref><ref>{{cite journal|last1=Mankinen|first1=Edward A.|last2=Prévot |first2=Michel |last3=Grommé |first3=C. Sherman |last4=Coe |first4=Robert S.|title=The Steens Mountain (Oregon) Geomagnetic Polarity Transition 1. Directional History, Duration of Episodes, and Rock Magnetism|journal=Journal of Geophysical Research|date=1 January 1985|volume=90|issue=B12|page=10393|doi=10.1029/JB090iB12p10393 |bibcode=1985JGR....9010393M}}</ref> It appears that the reversed-to-normal polarity transition that marks the end of Chron C5Cr ({{ma|16.7}}) contains a series of reversals and excursions.<ref>{{cite journal|last1=Jarboe |first1=Nicholas A. |last2=Coe |first2=Robert S. |last3=Glen |first3=Jonathan M.G. |title=Evidence from lava flows for complex polarity transitions: the new composite Steens Mountain reversal record |journal=[[Geophysical Journal International]] |volume= 186 |issue=2 |pages=580–602 |year=2011 |doi=10.1111/j.1365-246X.2011.05086.x|bibcode = 2011GeoJI.186..580J }}</ref> In addition, geologists Scott Bogue of Occidental College and Jonathan Glen of the US Geological Survey, sampling lava flows in [[Battle Mountain, Nevada]], found evidence for a brief, several year long interval during a reversal when the field direction changed by over 50°. The reversal was dated to approximately 15{{nbsp}}million years ago.<ref name="po09062010">{{cite web|last=Edwards|first=Lin|title=Evidence of Second Fast North-South Pole Flip Found|date=6 September 2010|url=http://phys.org/news202971192.html}}</ref><ref>{{cite journal|last=Bogue|first=S.W.|title=Very rapid geomagnetic field change recorded by the partial remagnetization of a lava flow|journal= Geophys. Res. Lett.|volume= 37|page= L21308| doi=10.1029/2010GL044286|date=10 November 2010 |bibcode = 2010GeoRL..3721308B }}</ref> |
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===Magnetic field=== |
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The magnetic field will not vanish completely, but many poles might form chaotically in different places during reversal, until it stabilizes again.<ref name="NASA_inconstant">{{cite web |url=http://www.webcitation.org/5nDzbvFyX |title=Earth's Inconstant Magnetic Field |accessdate=01-07-11}}</ref><ref name=Glatzmaier>{{cite web|url=http://es.ucsc.edu/~glatz/geodynamo.html|title=The Geodynamo|last1=Glatzmaier|first1=Gary}}</ref> |
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==Causes== |
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[[File:NASA 54559main comparison1 strip.gif|thumb|350px|NASA computer simulation using the model of Glatzmaier and Roberts.<ref name=selfconsistent/> The tubes represent [[Magnetic field#Magnetic field lines|magnetic field lines]], blue when the field points towards the center and yellow when away. The rotation axis of the Earth is centered and vertical. The dense clusters of lines are within the Earth's core.<ref name=Glatzmaier/>]] |
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The magnetic field of the Earth, and of other planets that have magnetic fields, are generated by [[dynamo theory|dynamo action]] in which convection of molten iron in the planetary core generates electric currents which in turn give rise to magnetic fields.<ref name=Merrill/> Most scientists believe that reversals are an inherent aspect of this process. In [[Computational physics|simulations]], it is observed that magnetic field lines can sometimes become tangled and disorganized through the [[Chaos (physics)|chaotic]] motions of [[liquid]] [[metal]] in the [[Earth's Core|Earth's core]]. For example, Gary Glatzmaier and collaborator Paul Roberts of [[UCLA]] have made a numerical model of the electromagnetic, fluid dynamical processes of Earth's interior. Their simulation reproduced key features of the magnetic field over more than 40,000 years of simulated time. Additionally, the computer-generated field reversed itself.<ref name=selfconsistent>{{cite article|last=Glatzmaier|first=Gary A.|last2=Roberts|first2=Paul H.|title=A three dimensional self-consistent computer simulation of a geomagnetic field reversal|journal=[[Nature]]|volume=377|pages=203-209}}</ref><ref>{{cite web|url=http://www.psc.edu/science/glatzmaier.html|title= When North goes South|last1=Glatzmaier|first1=Gary|last2=Roberts|first2=Paul}}</ref> Global field reversals at irregular intervals have also been observed in the laboratory [[liquid]] [[metal]] experiment [[VKS2]].<ref name=VKSreversals>{{cite article|last=Berhanu|first=M.|last2=Monchaux|first2=R.|last3=Fauve|first3=S.|last4=Mordant|first4=N.|last5=Petrelis|first5=F.|last6=Chiffaudel|first6=A.|last7=Daviaud|first7=F.|last8=Dubrulle|first8=B.|last9=Marie|first9=L.|last10=Ravelet|first10=F.|last11=Bourgoin|first11=M.|last12=Odier|first12=P.|last13=Pinton|first13=J.-F.|last14=Volk|first14=R.|title=Magnetic field reversals in an experimental turbulent dynamo|journal=[[EPSL]]|volume=77|pages=59001}}</ref> |
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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 [[Solar cycle|reversals]] every 9–12 years. However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas reversals on Earth seem to occur during periods of low Earth field strength.<ref name=Coe>{{cite journal|doi= 10.1098/rsta.2000.0578|last=Coe|first=Robert S.|last2=Hongré|first2=Lionel|last3=Glatzmaier|first3=Gary A.|title=An Examination of Simulated Geomagnetic Reversals from a Palaeomagnetic Perspective|journal=[[Philosophical Transactions of the Royal Society A: Physical, Mathematical and Engineering Sciences]]|volume=358|pages=1141–1170|year=2000}}</ref> |
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===External triggers=== |
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Some scientists, such as [[Richard A. Muller]], believe that geomagnetic reversals are not spontaneous processes but rather are triggered by external events that directly disrupt the flow in the Earth's core. Proposals include [[impact events]]<ref name= muller86>{{cite journal|last=Muller|first=Richard A.|last2=Morris|first2=Donald E.|title=Geomagnetic reversals from impacts on the Earth|journal=[[Geophysical Research Letters]]|volume=13|issue=11|pages=1177–1180|year=1986|doi=10.1029/GL013i011p01177|bibcode=1986GeoRL..13.1177M}}</ref><ref name=muller2002>{{cite journal|title=Avalanches at the core-mantle boundary|journal=[[Geophysical Research Letters]]|year=2002|first=Richard A.|page=1935|last=Muller|volume=29|issue=19|doi=10.1029/2002GL015938|bibcode=2002GeoRL..29s..41M}}</ref> or internal events such as the arrival of continental slabs carried down into the [[Mantle (geology)|mantle]] by the action of [[plate tectonics]] at [[Subduction|subduction zone]]s or the initiation of new [[mantle plume]]s from the [[core-mantle boundary]].<ref name=McFadden>{{cite journal|doi=10.1016/0031-9201(86)90118-4|last=McFadden|first=P. L.|last2=Merrill|first2=R. T.|title=Geodynamo energy source constraints from paleomagnetic data|journal=[[Physics of the Earth and Planetary Interiors]]|volume=43|pages=22–33|year=1986|bibcode = 1986PEPI...43...22M }}</ref> 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 state or the other, such that half the recoveries become reversals. However, the proposed mechanism does not appear to work in a quantitative model, and the evidence from [[stratigraphy]] for a correlation between reversals and impact events is weak. Most strikingly, there is no evidence for a reversal connected with the impact event that caused the [[Cretaceous–Paleogene extinction event]].<ref>{{cite journal|last=Merrill|first=R. T.|coauthors=McFadden, P. L.|title=Paleomagnetism and the Nature of the Geodynamo|journal=Science|date=20 April 1990|volume=248|issue=4953|pages=345–350|doi=10.1126/science.248.4953.345|bibcode = 1990Sci...248..345M }}</ref> |
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==Effects on biosphere and human society== |
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Not long after the first geomagnetic polarity time scales were produced, scientists began exploring the possibility that reversals could be linked to extinctions. Most such proposals rest on the assumption that the Earth's magnetic field would be much weaker during reversals. Possibly the first such hypothesis was that high energy particles trapped in the [[Van Allen radiation belt]] could be liberated and bombard the Earth.<ref name=Glassmeier2010>{{cite journal|last=Glassmeier|first=Karl-Heinz|coauthors=Vogt, Joachim|title=Magnetic Polarity Transitions and Biospheric Effects|journal=Space Science Reviews|date=29 May 2010|volume=155|issue=1-4|pages=387–410|doi=10.1007/s11214-010-9659-6|bibcode = 2010SSRv..155..387G }}</ref><ref>{{cite journal|last=Uffen|first=Robert J.|title=Influence of the Earth's Core on the Origin and Evolution of Life|journal=[[Nature (journal){{!}}Nature]]|date=13 April 1963|volume=198|issue=4876|pages=143–144|doi=10.1038/198143b0|bibcode = 1963Natur.198..143U }}</ref> Detailed calculations confirm that, if the Earth's dipole field disappeared entirely (leaving the quadrupole and higher components), most of the atmosphere would become accessible to high energy particles, but would act as a barrier to them, and cosmic ray collisions would produce secondary radiation of [[beryllium-10]] or [[chlorine-36]]. An increase of beryllium 10 was noted in a 2012 German study showing a peak of beryllium-10 in Greenland ice cores during a brief complete reversal 41,000 years ago which led to the magnetic field strength dropping to an estimated 5% of normal during the reversal.<ref> [http://www.sciencedaily.com/releases/2012/10/121016084936.htm] </ref> There is evidence that this occurs both during secular variation<ref>{{cite journal|last=McHargue|first=L.R|coauthors=Donahue, D, Damon, P.E, Sonett, C.P, Biddulph, D, Burr, G|title=Geomagnetic modulation of the late Pleistocene cosmic-ray flux as determined by 10Be from Blake Outer Ridge marine sediments|journal=Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms|date=1 October 2000|volume=172|issue=1-4|pages=555–561|doi=10.1016/S0168-583X(00)00092-6|bibcode = 2000NIMPB.172..555M }}</ref><ref>{{cite journal|last=Baumgartner|first=S.|title=Geomagnetic Modulation of the 36Cl Flux in the GRIP Ice Core, Greenland|journal=Science|date=27 February 1998|volume=279|issue=5355|pages=1330–1332|doi=10.1126/science.279.5355.1330|bibcode = 1998Sci...279.1330B }}</ref> and during reversals.<ref>{{cite journal|last=Raisbeck|first=G. M.|coauthors=Yiou, F., Bourles, D., Kent, D. V.|title=Evidence for an increase in cosmogenic 10Be during a geomagnetic reversal|journal=Nature|date=23 May 1985|volume=315|issue=6017|pages=315–317|doi=10.1038/315315a0|bibcode = 1985Natur.315..315R }}</ref><ref>{{cite journal|last=Raisbeck|first=G. M.|coauthors=Yiou, F., Cattani, O., Jouzel, J.|title=10Be evidence for the Matuyama–Brunhes geomagnetic reversal in the EPICA Dome C ice core|journal=Nature|date=2 November 2006|volume=444|issue=7115|pages=82–84|doi=10.1038/nature05266|bibcode = 2006Natur.444...82R }}</ref> |
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Another hypothesis by McCormac and Evans assumes that the Earth's field would disappear entirely during reversals.<ref>{{cite journal|last=McCormac|first=Billy M.|coauthors=Evans, John E.|title=Consequences of Very Small Planetary Magnetic Moments|journal=[[Nature (journal)|Nature]]|date=20 September 1969|volume=223|issue=5212|pages=1255–1255|doi=10.1038/2231255a0|bibcode = 1969Natur.223.1255M }}</ref> They argue that the atmosphere of Mars may have been eroded away by the [[solar wind]] because it had no magnetic field to protect it. They predict that ions would be stripped away from Earth's atmosphere above 100 km. However, the evidence from paleointensity measurements is that the magnetic field does not disappear. Based on paleointensity data for the last 800,000 years,<ref>{{cite journal|last=Guyodo|first=Yohan|coauthors=Valet, Jean-Pierre|journal=Nature|date=20 May 1999|volume=399|issue=6733|pages=249–252|doi=10.1038/20420|bibcode = 1999Natur.399..249G }}</ref> the magnetopause is still estimated to be at about 3 Earth radii during the [[Brunhes-Matuyama reversal]].<ref name=Glassmeier2010/> Even if the magnetic field disappeared, the [[solar wind]] may induce a sufficient magnetic field in the Earth's [[ionosphere]] to shield the surface from energetic particles.<ref>{{cite journal|first=G. T.|last= Birk|first2=H.|last2=Lesch|first3=C.|last3=Konz|title=Solar wind induced magnetic field around the unmagnetized Earth|url= |doi=10.1051/0004-6361:20040154|journal=[[Astronomy & Astrophysics]]|year=2004|volume=420|issue=2|pages=L15–L18|bibcode=2004A&A...420L..15B|arxiv = astro-ph/0404580 }}</ref> |
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Hypotheses have also been advanced linking reversals to [[mass extinctions]].<ref>{{cite journal|last=Raup|first=David M.|title=Magnetic reversals and mass extinctions|journal=Nature|date=28 March 1985|volume=314|issue=6009|pages=341–343|doi=10.1038/314341a0|bibcode = 1985Natur.314..341R }}</ref> Many such arguments were based on an apparent periodicity in the rate of reversals; more careful analyses show that the reversal record is not periodic.<ref name="Lutz" /> |
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It may be, however, that the ends of superchrons have caused vigorous convection leading to widespread volcanism, and that the subsequent airborne ash caused extinctions.<ref name="mantleplumes">{{cite article|last=Courtillot|first=V.|last2=Olson|first2=P.|title=Mantle plumes link magnetic superchrons to phanerozoic mass depletion events|journal=[[Earth and Planetary Science Letters]]|year=2007|volume=260|pages=495-504|doi=10.1016/j.epsl.2007.06.003}}</ref> |
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Tests of correlations between extinctions and reversals are difficult for a number of reasons. Larger animals are too scarce in the fossil record for good statistics, so paleontologists have analyzed microfossil extinctions. Even microfossil data can be unreliable if there are hiatuses in the fossil record. It can appear that the extinction occurs at the end of a polarity interval when the rest of that polarity interval was simply eroded away.<ref name=Merrill2010>{{cite book|last=Merrill|first=Ronald T.|title=Our magnetic Earth : the science of geomagnetism|year=2010|publisher=The University of Chicago Press|location=Chicago|isbn=0-226-52050-1}}</ref> Statistical analysis shows no evidence for a correlation between reversals and extinctions.<ref>{{cite journal|last=Plotnick|first=Roy E.|title=Relationship between biological extinctions and geomagnetic reversals|journal=Geology|date=1 January 1980|volume=8|issue=12|page=578|doi=10.1130/0091-7613(1980)8<578:RBBEAG>2.0.CO;2|bibcode = 1980Geo.....8..578P }}</ref><ref name=Glassmeier2010/> |
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==References== |
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{{Reflist|2}} |
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==Further reading== |
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* Behrendt, J.C., Finn, C., Morse, L., Blankenship, D.D. "''[http://pubs.usgs.gov/of/2007/1047/ea/of2007-1047ea030.pdf 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. |
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* Okada, M., Niitsuma, N., ''"[http://adsabs.harvard.edu/abs/1989PEPI...56..133O 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. |
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==External links== |
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* [http://web.archive.org/web/20090316132328/http://www.agu.org/sci_soc/hoffman.html How geomagnetic reversals are related to intensity] |
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* [http://www.economist.com/node/9143913?story_id=9143913 "Look down, look up, look out!", ''The Economist'', May 10 2007] |
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* [http://www.newscientist.com/article/dn9148-ships-logs-give-clues-to-earths-magnetic-decline.html "Ships' logs give clues to Earth's magnetic decline", ''New Scientist'', May 11 2006] |
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* [http://www.physics.org/facts/frog-magnetic-field.asp Simple explanation of geomagnetic reversal, ]''physics.org'', accessed Nov 2 2012 |
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{{DEFAULTSORT:Geomagnetic Reversal}} |
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[[Category:Paleomagnetism]] |
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[[Category:Geophysics]] |
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[[Category:Earth]] |
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[[Category:Geomagnetic reversal|*]] |
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Revision as of 02:00, 23 May 2013
THE EARTH HAS MAGNETS