Geomagnetic reversal
Encyclopedia
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
such that the positions of magnetic north and magnetic south are interchanged. The Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
'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 in the opposite direction. These periods are called chrons. The time spans of chrons are randomly distributed with most being between 0.1 and 1million 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 excursion
Geomagnetic excursion
A geomagnetic excursion, like a geomagnetic reversal, is a significant change in the Earth's magnetic field. Unlike reversals however, an excursion does not permanently change the large scale orientation of the field, but rather represents a dramatic, typically short-lived decrease in field...
s.
History
In the early 20th century geologists first noticed that some volcanic rocks were magnetized opposite to the direction of the local Earth's field. The first estimate of the timing of magnetic reversals was made in the 1920s by Motonori MatuyamaMotonori Matuyama
was a Japanese geophysicist who was the first to surmise that the Earth's magnetic field had undergone reversals in the past. The era of reversed polarity preceding the current Brunhes era of normal polarity is called the Matuyama reversed chron and the boundary between them is called the...
, who observed that rocks with reversed fields were all of early Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
age or older. At the time, the Earth's polarity was poorly understood and the possibility of reversal aroused little interest.
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
Continental drift
Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
. 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.
The next major advance in understanding reversals came when techniques for radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
were developed in the 1950's. Allan Cox
Allan V. Cox
Allan Verne Cox was an American geophysicist. His work on dating geomagnetic reversals, with Richard Doell and Brent Dalrymple, made a major contribution to the theory of plate tectonics. Allan Cox won numerous awards, including the prestigious Vetlesen Prize, and was the president of the American...
and Richard Doell
Richard Doell
Richard Doell was a distinguished American scientist known for developing the time scale for geomagnetic reversals with Allan V. Cox and Brent Dalrymple. This work was a major step in the development of plate tectonics...
, at the United States Geological Survey
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
, wanted to know whether reversals occurred at regular intervals, and invited the geochronologist Brent Dalrymple
Brent Dalrymple
G. Brent Dalrymple is an American geologist, author of The Age of the Earth and Ancient Earth, Ancient Skies, and National Medal of Science winner....
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
Australian National University
The Australian National University is a teaching and research university located in the Australian capital, Canberra.As of 2009, the ANU employs 3,945 administrative staff who teach approximately 10,000 undergraduates, and 7,500 postgraduate students...
. A group led by Neil Opdyke at the Lamont-Doherty Geological Observatory showed that the same pattern of reversals was recorded in sediments from deep-sea cores.
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
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
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
Drummond Matthews
Drummond Hoyle Matthews FRS was a British marine geologist and geophysicist and a key contributor to the theory of plate tectonics...
provided a simple explanation by combining the seafloor spreading
Seafloor spreading
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics....
theory of Harry Hess
Harry Hammond Hess
Harry Hammond Hess was a geologist and United States Navy officer in World War II.Considered one of the "founding fathers" of the unifying theory of plate tectonics, Rear Admiral Harry Hammond Hess was born on May 24, 1906 in New York City...
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
Lawrence Morley
Lawrence Morley, Ph.D. is a Canadian geophysicist. He is best known for his studies on the magnetic properties of ocean crust and their effect on plate tectonics.-Biography:Morley worked with Britons Fred Vine and Drummond Matthews...
independently proposed a similar explanation in January 1963, but his work was rejected by the scientific journals Nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...
and Journal of Geophysical Research
Journal of Geophysical Research
The Journal of Geophysical Research is a peer-reviewed scientific journal published by the American Geophysical Union 80 times per year. It contains original research on the physical, chemical, and biological processes that contribute to the understanding of the Earth, Sun, and solar system...
, 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
Pacific-Antarctic Ridge
The Pacific–Antarctic Ridge is a divergent tectonic plate boundary located on the seafloor of the South Pacific Ocean, separating the Pacific Plate from the Antarctic Plate...
were symmetrical and matched the pattern in the north Atlantic's Reykjanes
Reykjanes
Reykjanes or Reykjanesskagi is a peninsula and a volcanic system situated at the south-western end of Iceland, near the capital of Reykjavík....
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.
Observing past fields
Past field reversals can be and have been recorded in the "frozen" ferromagneticFerromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
(or more accurately, ferrimagnetic
Ferrimagnetism
In physics, a ferrimagnetic material is one in which the magnetic moments of the atoms on different sublattices are opposed, as in antiferromagnetism; however, in ferrimagnetic materials, the opposing moments are unequal and a spontaneous magnetization remains...
) minerals of consolidated sedimentary deposits or cooled volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
flows on land.
The past record of geomagnetic reversals was first noticed by observing the magnetic stripe "anomalies" on the ocean floor
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
. Lawrence W. Morley
Lawrence Morley
Lawrence Morley, Ph.D. is a Canadian geophysicist. He is best known for his studies on the magnetic properties of ocean crust and their effect on plate tectonics.-Biography:Morley worked with Britons Fred Vine and Drummond Matthews...
, Frederick John Vine
Fred Vine
Frederick John Vine is a British marine geologist and geophysicist and was a key contributor to the theory of plate tectonics.-Early life:...
and Drummond Hoyle Matthews
Drummond Matthews
Drummond Hoyle Matthews FRS was a British marine geologist and geophysicist and a key contributor to the theory of plate tectonics...
made the connection to seafloor spreading in the Morley-Vine-Matthews hypothesis
Morley-Vine-Matthews hypothesis
The Vine–Matthews–Morley hypothesis, also known as the Morley–Vine–Matthews hypothesis was the first key scientific test of the seafloor spreading theory of continental drift and Plate tectonics....
which soon led to the development of the theory of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
. The relatively constant rate at which the sea floor
Seafloor spreading
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics....
spreads results in substrate "stripes" from which past magnetic field polarity can be inferred from data gathered from towing a magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
along the sea floor.
Because no existing unsubducted sea floor (or sea floor thrust onto continental plates
Ophiolites
An ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks...
) is more than about (Ma) old, other methods are necessary for detecting older reversals. Most sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s incorporate tiny amounts of iron rich mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
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 chemical, physical or biological change
Diagenesis
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 and metamorphism. These changes happen at relatively low temperatures and pressures...
.
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 ~) 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
Index fossil
Index fossils are fossils used to define and identify geologic periods . They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil...
s are 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 83million years.Changing frequency over time
The rate of reversals in the Earth's magnetic field has varied widely over time. , the field reversed 5 times in a million years. In a 4-million-year period centered on , there were 10 reversals; at around , 17 reversals took place in the span of 3million years. In a period of 3million years centering on , 13 reversals occurred. No fewer than 51 reversals occurred in a 12-million-year period, centering on . 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.Superchrons
A superchron is a polarity interval lasting at least 10million 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.The Cretaceous Normal (also called the Cretaceous Superchron or C34) lasted for almost 40million years, from about , including stages of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period from the Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
through the Santonian
Santonian
The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...
. 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.
The Kiaman Reverse Superchron lasted from approximately the late Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
to the late Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
, or for more than 50million years, from around . The magnetic field had reversed polarity. The name "Kiaman" derives from the Australian village of Kiama
Kiama, New South Wales
-Transport:The town is served by Kiama Station on the South Coast Line. It is served by road in the form of the Princes Highway and the Kiama Bypass.-Attractions:...
, where some of the first geological evidence of the superchron was found in 1925.
The Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
is suspected to host another superchron, called the Moyero Reverse Superchron, lasting more than 20million years (485 to 463million years ago) . But until now this possible superchron has only been found in the Moyero river section north of the polar circle in Siberia. Moreover, the best data from elsewhere in the world do not show evidence for this superchron.
Certain regions of ocean floor, older than , 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 and , and these sections of ocean floor are especially deep, so the signal is attenuated between the floor and the surface.
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 theoryDynamo theory
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
. 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
Poisson process
A Poisson process, named after the French mathematician Siméon-Denis Poisson , is a stochastic process in which events occur continuously and independently of one another...
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. A random reversal pattern with inhibition can be represented by a gamma process. In 2006, a team of physicists at the University of Calabria
University of Calabria
The University of Calabria is a state-run university in Italy.Located in Arcavacata di Rende, a suburb of Cosenza, the university was founded in 1972...
found that the reversals also conform to a Lévy distribution, which describes stochastic process
Stochastic process
In probability theory, a stochastic process , or sometimes random process, is the counterpart to a deterministic process...
es with long-ranging correlations between events in time. The data are also consistent with a deterministic, but chaotic, process.
Duration
Most estimates for the duration of a polarity transition are between 1,000 and 10,000 years. However, studies of lava flows on Steens MountainSteens Mountain
Steens Mountain is a large fault-block mountain in the southeastern part of the U.S. state of Oregon. Located in Harney County, it stretches some and rises from an elevation of about above the Alvord Desert to its peak at...
, Oregon, indicate that the magnetic field could have shifted at a rate of up to 6 degrees per day about 15million years ago. This was initially met with skepticism from paleomagnetists. Even if changes occur that quickly in the core, the mantle, which is a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
, is thought to act as a low-pass filter, removing variations with periods less than a few months. A variety of possible rock magnetic
Rock magnetism
Rock magnetism is the study of the magnetic properties of rocks, sediments and soils. The field arose out of the need in paleomagnetism to understand how rocks record the Earth's magnetic field. This remanence is carried by minerals, particularly certain strongly magnetic minerals like magnetite...
mechanisms were proposed that would lead to a false signal. However, paleomagnetic studies of other sections from the same region (the Oregon Plateau flood basalts) give consistent results. It appears that the reversed-to-normal polarity transition that marks the end of Chron C5Cr contains a series of reversals and excursions. 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 15million years ago.
Magnetic field
The magnetic field may not vanish completely, with many poles forming chaotically in different places during reversal, until it stabilizes again.Causes
The magnetic field of the Earth, and those planets that have magnetic fields are generated by dynamo actionDynamo theory
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
in which convection of molten iron in the planetary core generates electric currents which in turn give rise to magnetic fields. Most scientists believe that reversals are an inherent aspect of this process. In simulations
Computational physics
Computational physics is the study and implementation of numerical algorithms to solve problems in physics for which a quantitative theory already exists...
, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
in the 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. Global field reversals at irregular intervals have also been observed in the laboratory liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
experiment VKS2.
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
Solar cycle
The solar cycle, or the solar magnetic activity cycle, is a periodic change in the amount of irradiation from the Sun that is experienced on Earth. It has a period of about 11 years, and is one component of solar variation, the other being aperiodic fluctuations. Solar variation causes changes in...
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 field strength.
External triggers
Some scientists, such as Richard A. MullerRichard A. Muller
Richard A. Muller is a noted American professor of physics at the University of California, Berkeley. He is also a faculty senior scientist at the Lawrence Berkeley National Laboratory.-Career:...
, 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 or internal events such as the arrival of continental slabs carried down into the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
by the action of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
at subduction zone
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
s or the initiation of new mantle plume
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
s from the core-mantle boundary
Core-mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...
. 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
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
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-Tertiary extinction.
Future of the geomagnetic field
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 for no apparent 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.
The Earth's magnetic north pole
North Magnetic Pole
The Earth's North Magnetic Pole is the point on the surface of the Northern Hemisphere at which the Earth's magnetic field points vertically downwards . Though geographically in the north, it is, by the direction of the magnetic field lines, physically the south pole of the Earth's magnetic field...
is drifting from northern Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
towards Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
with a presently accelerating rate — 10 km per year at the beginning of the 20th century, up to 40 km per year in 2003, and since then has only accelerated. In the last decade magnetic north was shifting roughly one degree every five years.
Extinctions
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 field has much lower intensity during reversals. Possibly the first such hypothesis was that high energy particles trapped in the Van Allen radiation beltVan Allen radiation belt
The Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. It is believed that most of the particles that form the belts come from solar wind, and other particles by cosmic rays. It is named after its discoverer, James...
could be liberated and bombard the Earth. Detailed calculations confirm that, if the Earth's dipole field disappeared entirely (leaving the quadrupole and higher components), most of the atmosphere could be reached by high energy particles. However, the atmosphere would stop them. Instead there would be secondary radiation of or from collisions of cosmic rays with the atmosphere. There is evidence that this occurs both during secular variation and during reversals.
Another hypothesis by McCormac and Evans assumes that the Earth's field would disappear entirely during reversals. They argue that the atmosphere of Mars may have been eroded away by the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
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, the magnetopause is still estimated to be at about 3 Earth radii during the Brunhes-Matuyama reversal
Brunhes-Matuyama reversal
The Brunhes–Matuyama reversal, named after Bernard Brunhes and Motonori Matuyama, was a geologic event, approximately 780,000 years ago, when the Earth's magnetic field last underwent reversal. The reversal may have occurred slowly over several thousand years, or more quickly; opinions vary...
. Even if the magnetic field disappeared, the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
may induce a sufficient magnetic field in the Earth's ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
to shield the surface from energetic particles.
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. Statistical analysis shows no evidence for a correlation between reversals and extinctions.
Hypotheses have also been advanced linking reversals to mass extinctions. 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.
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.
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.