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. 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. Image File history File links Download high-resolution version (220x768, 28 KB)Late Cenozoic geomagnetic time scale. ... Image File history File links Download high-resolution version (220x768, 28 KB)Late Cenozoic geomagnetic time scale. ... The magnetosphere shields the surface of the Earth from the charged particles of the solar wind. ...

History

Over very long periods, geomagnetic reversal seems to have occurred with a frequency of 1 to 5 events per million years; however, this duration is highly variable. 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. FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ... The Cretaceous long normal was the long period of stability in the Earths magnetic field when no field reversals occurred. ... Magnetic field lines shown by iron filings In physics, a magnetic field is a solenoidal vector field in the space surrounding moving electric charges and magnetic dipoles, such as those in electric currents and magnets. ... The Brunhes-Matuyama Reversal was a geologic event approximately 780,000 years ago when the Earths 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. 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. The Dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a magnetic field. ... The NASA Columbia Supercomputer. ... It has been suggested that this article or section be merged into Chaos. ... A liquid will usually assume the shape of its container A liquid is one of the main states of matter. ... Hot metal work from a blacksmith In chemistry, a metal (Greek: Metallon) is an element that readily loses electrons to form positive ions (cations) and has metallic bonds between metal atoms. ... This article is about Earth as a planet. ...

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. The Sun (Latin: Sol) is the star at the center of the Solar System. ... It has been suggested that this article or section be merged with Schwabe-Wolf cycle. ...

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. Richard Muller Richard A. Muller (January 6, 1944 -) of San Francisco, California, USA, is a physicist who works at the University of California, Berkeley and Lawrence Berkeley National Laboratory. ... Earth cutaway from core to exosphere. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone. ... A lava lamp illustrates the basic concept of a mantle plume. ... The core-mantle boundary lies between the Earths silicate mantle and its iron-nickel core. ... Artists impression of a major impact event. ... A geomagnetic excursion, like a geomagnetic reversal, is a significant change in the Earths 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. 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 lead 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 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. Ferromagnetism is the phenomenon by which materials, such as iron, in an external magnetic field become magnetized and remain magnetized for a period after the material is no longer in the field. ... In physics, a ferrimagnetic material is one in which the magnetic moment of the atoms on different sublattices oppose as in antiferromagnetism but the opposing moments are unequal and a spontaneous magnetization remains. ... For other uses, see Volcano (disambiguation). ... Age of oceanic crust Oceanic crust is the part of Earths lithosphere which underlies the ocean basins. ... Lawrence Morley (Canada), along with Vine and Matthews (UK), contributed significantly to geology by relating the magnetic properties of ocean crust to the processes involved in the theory of plate tectonics. ... Frederick J. Vine (born in 1939) is a marine geologist and geophysicist and was a key contributor to the theory of plate tectonics. ... Drummond Hoyle Matthews (February 5, 1931–July 20, 1997) was a British marine geologist and geophysicist and a key contributor to the theory of plate tectonics. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... Age of oceanic crust. ... A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. ... Ophiolites are sections of the oceanic crust and the subjacent upper mantle that have been uplifted or emplaced to be exposed within continental crustal rocks. ... Annum is a Latin noun meaning year. ... Two types of sedimentary rock: limey shale overlaid by limestone. ... A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure and specific physical properties. ... In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism. ...

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 (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".

Image File history File links Download high-resolution version (379x607, 12 KB) Summary Reproduced from Lowrie (1997) Fundamentals of Geophysics Record of Normal and Reversed geomagnetic polarity from 0-160Mya. ... Image File history File links Download high-resolution version (379x607, 12 KB) Summary Reproduced from Lowrie (1997) Fundamentals of Geophysics Record of Normal and Reversed geomagnetic polarity from 0-160Mya. ...

The changing frequency of geomagnetic reversals over time

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 ago, 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 Superchron"

Another long period of time during which where no magnetic pole reversals. This period lasted approximately from the Late Carboniferous to the end Permian.

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 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. Image File history File links Approximate times of some of the most commonly reported geomagnetic excursions of the Brunhes Normal Polarity Chron. ... Image File history File links Approximate times of some of the most commonly reported geomagnetic excursions of the Brunhes Normal Polarity Chron. ... The Van Allen radiation belts and the point of the South Atlantic Anomaly. ...

No one knows if field decay will continue in the future. 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 [1]. Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... Binomial name †Homo erectus (Dubois, 1892) Synonyms † Pithecanthropus erectus † Sinanthropus pekinensis † Javanthropus soloensis † Meganthropus paleojavanicus Homo erectus (Latin: upright man) is an extinct species of the genus Homo. ... The Dodo, shown here in a 1651 illustration by Jan Savery, is an often-cited[1] example of modern extinction. ...

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.[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.g. radio, satellite communications) may be vulnerable to technological disruptions in the event of a full field reversal. Lightning strikes during a night-time thunderstorm. ... Electromagnetism is the physics of the electromagnetic field: a field which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles. ...

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.[3] For alternate meanings, see Cray (disambiguation). ...

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