A geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with geographic north and geographic south). The Earth's field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite. These periods are called chrons.
Reversal occurrences are statistically random, with some periods lasting as little as 200 years. There have been 183 reversals over the last 83 million years. The latest, the Brunhes–Matuyama reversal, occurred 780,000 years ago, and may have happened very quickly, within a human lifetime.
A brief complete reversal, known as the Laschamp event, occurred only 41,000 years ago during the last glacial period. That reversal lasted only about 440 years with the actual change of polarity lasting around 250 years. During this change the strength of the magnetic field weakened to 5% of its present strength. Brief disruptions that do not result in reversal are called geomagnetic excursions.
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 by Motonori Matuyama in the 1920s; he 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.
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.
The next major advance in understanding reversals came when techniques for radiometric dating were developed in the 1950s. 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 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 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.
In 1963, Frederick Vine and Drummond Matthews provided a simple explanation by combining the seafloor spreading theory of Harry Hess with the known time scale of reversals: new sea floor is magnetized in the direction of the then-current field. Thus, sea floor spreading from a central ridge will produce pairs of magnetic stripes parallel to the ridge. Canadian L. W. Morley independently proposed a similar explanation in January 1963, but his work was rejected by the scientific journals Nature and Journal of Geophysical Research, and 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 were symmetrical and matched the pattern in the north Atlantic's Reykjanes ridge. 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.
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 which soon led to the development of the theory of plate tectonics. The relatively constant rate at which the 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.
Because no existing unsubducted sea floor (or sea floor thrust onto continental plates) is more than about (Ma) old, 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. These rocks can preserve a record of the field if it is not later erased by chemical, physical or biological change.
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 ~index fossils are seldom available.) 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
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 million years (and therefore 183 reversals).
The rate of reversals in the Earth's magnetic field has varied widely over time. million years. In a period of 3 million 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., 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 3
A superchron is a polarity interval lasting at least 10 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.
The Cretaceous Normal (also called the Cretaceous Superchron or C34) lasted for almost 40 million years, from about , 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.
The Kiaman Reverse Superchron lasted from approximately the late Carboniferous to the late Permian, or for more than 50 million years, from around . The magnetic field had reversed polarity. The name "Kiaman" derives from the Australian village of Kiama, where some of the first geological evidence of the superchron was found in 1925.
The Ordovician is suspected to have hosted another superchron, called the Moyero Reverse Superchron, lasting more than 20 million years (485 to 463 million years ago). Thus far, 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, causing the geomagnetic signal to be attenuated between the seabed and the surface.
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 theory.
There is no rate of reversals, as they are statistically random. 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 attempt to determine reversal rates.
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. 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 processes with long-ranging correlations between events in time. The data are also consistent with a deterministic, but chaotic, process.
Most estimates for the duration of a polarity transition are between 1,000 and 10,000 years, but some estimates are as quick as a human lifetime. Studies of 15-million-year-old lava flows on Steens Mountain, Oregon, indicate that the Earth's magnetic field is capable of shifting at a rate of up to 6 degrees per day. 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 remove variations with periods less than a few months. A variety of possible rock magnetic 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 degrees. The reversal was dated to approximately 15 million years ago. In August 2018, researchers reported a reversal lasting only 200 years.
The magnetic field of the Earth, and of other planets that have magnetic fields, is generated by dynamo action in which convection of molten iron in the planetary core generates electric currents which in turn give rise to magnetic fields. In simulations of planetary dynamos, reversals often emerge spontaneously from the underlying dynamics. For example, Gary Glatzmaier and collaborator Paul Roberts of UCLA ran a numerical model of the coupling between electromagnetism and fluid dynamics in the Earth's interior. Their simulation reproduced key features of the magnetic field over more than 40,000 years of simulated time and the computer-generated field reversed itself. Global field reversals at irregular intervals have also been observed in the laboratory liquid metal 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 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.
Some scientists, such as Richard A. Muller, think 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 by the action of plate tectonics at subduction zones or the initiation of new mantle plumes from the core-mantle boundary. Supporters of this hypothesis 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. There is no evidence for a reversal connected with the impact event that caused the Cretaceous–Paleogene extinction event.
Shortly 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. 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. 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 disappears entirely during reversals. 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, paleointensity measurements show that the magnetic field has not disappeared during reversals. Based on paleointensity data for the last 800,000 years, the magnetopause is still estimated to have been at about three Earth radii during the Brunhes-Matuyama reversal. Even if the internal magnetic field did disappear, the solar wind can induce a magnetic field in the Earth's ionosphere sufficient to shield the surface from energetic particles.
Hypotheses have also advanced toward linking reversals to mass extinctions. Many such arguments were based on an apparent periodicity in the rate of reversals, but 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.
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.
The 2012 phenomenon was a range of eschatological beliefs that cataclysmic or otherwise transformative events would occur on or around 21 December 2012. This date was regarded as the end-date of a 5,126-year-long cycle in the Mesoamerican Long Count calendar, and as such, festivities to commemorate the date took place on 21 December 2012 in the countries that were part of the Maya civilization (Mexico, Guatemala, Honduras, and El Salvador), with main events at Chichén Itzá in Mexico, and Tikal in Guatemala.Various astronomical alignments and numerological formulae were proposed as pertaining to this date. A New Age interpretation held that the date marked the start of a period during which Earth and its inhabitants would undergo a positive physical or spiritual transformation, and that 21 December 2012 would mark the beginning of a new era. Others suggested that the date marked the end of the world or a similar catastrophe. Scenarios suggested for the end of the world included the arrival of the next solar maximum, an interaction between Earth and the black hole at the center of the galaxy, or Earth's collision with a mythical planet called Nibiru.
Scholars from various disciplines quickly dismissed predictions of concomitant cataclysmic events as they arose. Professional Mayanist scholars stated that no extant classic Maya accounts forecast impending doom, and that the idea that the Long Count calendar ends in 2012 misrepresented Maya history and culture, while astronomers rejected the various proposed doomsday scenarios as pseudoscience, easily refuted by elementary astronomical observations.Barremian
The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 125.0 ± 1.0 Ma). It is a subdivision of the Early Cretaceous epoch (or Lower Cretaceous series). It is preceded by the Hauterivian and followed by the Aptian stage.Bernard Brunhes
Antoine Joseph Bernard Brunhes (3 July 1867 – 10 May 1910) was a French geophysicist known for his pioneering work in paleomagnetism, in particular, his 1906 discovery of geomagnetic reversal. The Brunhes–Matuyama reversal is named for him.
Brunhes was educated at the École Normale Supérieure in Paris, from which he graduated as an agrégé qualified in physics. Appointed at Université Lille Nord de France, he taught physics and electrical engineering at École centrale de Lille from 1893 to 1895.
In November 1900, he was appointed as head of the Puy-de-Dôme Observatory, built on an extinct volcano in the Auvergne region of France, where he worked until his death in 1910.
It was during his time at the observatory that he made the crucial observation that led to his discovery of geomagnetic reversal. In 1905, he found that rocks in an ancient lava flow at Pontfarin in the commune of Cézens (part of the Cantal département) were magnetised in a direction almost opposite to that of the present-day magnetic field. From this, he deduced that the magnetic North Pole of the time was close to the current geographical South Pole, which could only have happened if the magnetic field of the Earth had been reversed at some point in the past. He was correct, though it took another 50 years before his theory was fully accepted by the scientific community.Brunhes–Matuyama reversal
The Brunhes–Matuyama reversal, named after Bernard Brunhes and Motonori Matuyama, was a geologic event, approximately 781,000 years ago, when the Earth's magnetic field last underwent reversal. Estimations vary as to the abruptness of the reversal:
it might have extended over several thousand years, or much more quickly, perhaps within a human lifetime.The apparent duration at any particular location varied from 1,200 to 10,000 years depending on geomagnetic latitude and local effects of non-dipole components of the Earth's field during the transition.The Brunhes–Matuyama reversal is a Global Boundary Stratotype Section and Point (GSSP), selected by the International Commission on Stratigraphy as a marker for the beginning of the Middle Pleistocene, also known as the Ionian Stage. It is useful in dating ocean sediment cores and subaerially erupted volcanics. There is a highly speculative theory that connects this event to the large Australasian strewnfield.Cataclysmic pole shift hypothesis
The cataclysmic pole shift hypothesis is a fringe theory suggesting that there have been geologically rapid shifts in the relative positions of the modern-day geographic locations of the poles and the axis of rotation of the Earth, creating calamities such as floods and tectonic events.There is evidence of precession and changes in axial tilt, but this change is on much longer time-scales and does not involve relative motion of the spin axis with respect to the planet. However, in what is known as true polar wander, the solid Earth can rotate with respect to a fixed spin axis. Research shows that during the last 200 million years a total true polar wander of some 30° has occurred, but that no super-rapid shifts in the Earth's pole were found during this period. A characteristic rate of true polar wander is 1° or less per million years. Between approximately 790 and 810 million years ago, when the supercontinent Rodinia existed, two geologically rapid phases of true polar wander may have occurred. In each of these, the magnetic poles of the Earth shifted by approximately 55°--resulting from a large shift in the crust.Extinction event
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the rate of speciation. Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.
Because most diversity and biomass on Earth is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life. Extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land animals.
The Great Oxygenation Event, which occurred around 2.45 billion years ago, was probably the first major extinction event. Since the Cambrian explosion five further major mass extinctions have significantly exceeded the background extinction rate. The most recent and arguably best-known, the Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. In addition to the five major mass extinctions, there are numerous minor ones as well, and the ongoing mass extinction caused by human activity is sometimes called the sixth extinction. Mass extinctions seem to be a mainly Phanerozoic phenomenon, with extinction rates low before large complex organisms arose.Gauss-Matuyama reversal
The Gauss-Matuyama Reversal was a geologic event approximately 2.58 million years ago when the Earth's magnetic field underwent reversal.
This event, which separates the Piacenzian from the Gelasian and marks the start of the Quaternary, is useful in dating sediments.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 change in field intensity, with a variation in pole orientation of up to 45° from the previous position. These events, which typically last a few thousand to a few tens of thousands of years, often involve declines in field strength to between 0 and 20% of normal. Excursions, unlike reversals, are generally not recorded around the entire globe. This is partially due to them not being recorded well within the sedimentary record, but also because they likely do not extend through the entire geomagnetic field. One of the first excursions to be studied was the Laschamp event, dated at around 40000 years ago. This event was a complete reversal of polarity, however, as it later turned out, though with the reversed field 5% of the normal strength. Since this event has also been seen in sites around the globe, it is suggested as one of the few examples of a truly global excursion.Geomagnetic pole
The geomagnetic poles are antipodal points where the axis of a best-fitting dipole intersects the surface of Earth. This theoretical dipole is equivalent to a powerful bar magnet at the center of Earth and comes closer than any other model to accounting for the magnetic field observed at Earth's surface. In contrast, the magnetic poles of the actual Earth are not antipodal; that is, the line on which they lie does not pass through Earth's center.
Owing to motion of fluid in the Earth's outer core, the actual magnetic poles are constantly moving. However, over thousands of years their direction averages to the Earth's rotation axis. On the order of once every half a million years, the poles reverse (north switches place with south).Jaramillo reversal
The Jaramillo reversal was a reversal and excursion of the Earth's magnetic field that occurred approximately one million years ago. In the geological time scale it was a "short-term" positive reversal in the then-dominant Matuyama reversed magnetic chronozone; its beginning is widely dated to 990,000 years before the present (BP), and its end to 950,000 BP (though an alternative date of 1.07 million years ago to 990,000 is also found in the scientific literature).The causes and mechanisms of short-term reversals and excursions like the Jaramillo, as well as the major field reversals like the Brunhes–Matuyama reversal, are subjects of study and dispute among researchers. One theory associates the Jaramillo with the Bosumtwi impact event, as evidenced by a tektite strewnfield in the Ivory Coast, though this hypothesis has been claimed as "highly speculative" and "refuted".Laschamp event
The Laschamp event was a short reversal of the Earth's magnetic field. It occurred 41,400 (±2,000) years ago during the last ice age and was first recognised in the late 1960s as a geomagnetic reversal recorded in the Laschamp lava flows in the Clermont-Ferrand district of France. The magnetic excursion has since been demonstrated in geological archives from many parts of the world. The period of reversed magnetic field was approximately 440 years, with the transition from the normal field lasting approximately 250 years. The reversed field was 75% weaker, whereas the strength dropped to only 5% of the current strength during the transition. This reduction in geomagnetic field strength resulted in more cosmic rays reaching the Earth, causing greater production of the cosmogenic isotopes beryllium 10 and carbon 14. The Laschamp event was the first known geomagnetic excursion and remains the most thoroughly studied among the known geomagnetic excursions.List of geomagnetic reversals
List of geomagnetic the beginning and end of geomagnetic reversals. The ages of the beginning and end of each period of normal (same as present) polarity are listed.
Source for the last 83 million years: Cande and Kent, 1995 Ages in million years before present (Ma).Magnetic field reversal
Magnetic field reversal may refer to:
Brunhes–Matuyama reversal, approximately 780,000 years ago
Gauss-Matuyama reversal, approximately 2.588 million years ago
Jaramillo reversal, approximately one million years ago
Laschamp event, a short reversal that occurred 41,000 years ago
Reversal of the solar magnetic field
Magnetization reversal, a process leading to a 180° reorientation of the magnetization vector with respect to its initial direction
Polarity reversal (seismology), a local amplitude seismic anomalyNorth Magnetic Pole
The North Magnetic Pole is the wandering point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downwards (in other words, if a magnetic compass needle is allowed to rotate about a horizontal axis, it will point straight down). There is only one location where this occurs, near (but distinct from) the Geographic North Pole and the Geomagnetic North Pole.
The North Magnetic Pole moves over time due to magnetic changes in the Earth's core. In 2001, it was determined by the Geological Survey of Canada to lie west of Ellesmere Island in northern Canada at 81.3°N 110.8°W / 81.3; -110.8 (Magnetic North Pole 2001). It was situated at 83.1°N 117.8°W / 83.1; -117.8 (Magnetic North Pole 2005 est) in 2005. In 2009, while still situated within the Canadian Arctic territorial claim at 84.9°N 131.0°W / 84.9; -131.0 (Magnetic North Pole 2009), it was moving toward Russia at between 55 and 60 kilometres (34 and 37 mi) per year. As of 2017, the pole is projected to have moved beyond the Canadian Arctic territorial claim to 86.5°N 172.6°W / 86.5; -172.6 (Magnetic North Pole 2017 est).Its southern hemisphere counterpart is the South Magnetic Pole. Since the Earth's magnetic field is not exactly symmetrical, the North and South Magnetic Poles are not antipodal, meaning that a straight line drawn from one to the other does not pass through the geometric centre of the Earth.
The Earth's North and South Magnetic Poles are also known as Magnetic Dip Poles, with reference to the vertical "dip" of the magnetic field lines at those points.Outline of geophysics
The following outline is provided as an overview of and topical guide to geophysics:
Geophysics – the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation. However, modern geophysics organizations have a broader definition that includes the hydrological cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial relations; and analogous problems associated with the Moon and other planets.Polarity chron
A polarity chron, or chron, is the time interval between polarity reversals of Earth's magnetic field. It is a term used in magnetostratigraphy (a branch of geology) to name the time interval represented by a magnetostratigraphic polarity unit. It represents a certain time period in geologic history where the Earth's magnetic field was in predominantly a "normal" or "reversed" position. Chrons are numbered in order starting from today and increasing in number into the past. As well as a number, each chron is divided into two parts, labelled "n" and "r", thereby showing the position of the field's polarity. A chron is the time equivalent to a chronozone or a polarity zone.
It is called a "polarity subchron" when the interval is less than 200,000 years long.South Atlantic Anomaly
The South Atlantic Anomaly (SAA) is an area where the Earth's inner Van Allen radiation belt comes closest to the Earth's surface, dipping down to an altitude of 200 kilometres (120 mi). This leads to an increased flux of energetic particles in this region and exposes orbiting satellites to higher-than-usual levels of radiation.
The effect is caused by the non-concentricity of the Earth and its magnetic dipole. The SAA is the near-Earth region where the Earth's magnetic field is weakest relative to an idealized Earth-centered dipole field.South Iwo Jima
South Iwo Jima (南硫黄島, officially Minami-Iōtō, also frequently Minami-Iwō-jima, Minami-Iōjima: “South Sulfur Island”) is the southernmost island of the Volcano Islands group of the Ogasawara Islands, 60 km south of Iwo Jima. It is 1300 km south of Tokyo, 330 km SSW of Chichi-jima. Its area is 3.4 km² and the shore length, 7.5 km. Along the shoreline there are few bays and inlets, and it is covered with mostly rocks and little sand. To the rear are sea cliffs that rise to 200 m in height. The peak on South Iwo Jima is the largest in the Ogasawara Islands at 913 m and its average slope angle is 45 degrees. The northwest side of this volcano, which has a relatively stable shape and is not eroding much, rises at a gentler incline of 30 degrees. Another defining feature of the island is that it does not have rivers, lakes, marshes or a freshwater system of any kind.
South Iwo Jima is part of a volcanic front which has been active for 2,588,000 years beginning in the Quaternary period and started to form as a result of volcanic activity in that period. Though it is not clear when it emerged as an island, rocks which have been gathered and analyzed show no signs of geomagnetic reversal, so it is believed that the island is no more than a few hundred thousands years old. It was discovered in October 1543 by Spanish sailor Bernardo de la Torre on board of carrack San Juan de Letrán when trying to return from Sarangani to New Spain.The island is now administered as part of the Ogasawara-mura of Tokyo and is unpopulated. Its pronounced Japanese name was changed on June 18, 2007 to Minami-Iōtō, the kanji unchanged. The next southern island (geologically) is Farallon de Pajaros of the Northern Mariana Islands while Iwo Jima is roughly 60 km to the north.Vine–Matthews–Morley 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.