Geomagnetic Polarity Reversals Research Articles

A Coupled Map Lattice model for geomagnetic polarity reversals that exhibits realistic scaling

Seki and Ito (1993) showed that the geomagnetic polarity reversals had a power-law distribution and presented a simple model in which the geodynamo was assumed to be a system of magnetic spins in a critical phase-transition state. We present an improved, more realistic model, and obtain a power exponent in agreement with the observed value, which is about −1.5. The revised model is a Coupled Map Lattice (CML). A CML is a dynamical system with discrete time and space, but continuous state. In the present model, elementary dynamo evolves autonomously according to the Lorenz map obtained from Rikitake dynamo dynamics. We examine the behavior of the system and the distribution of polarity reversal intervals for various values of parameters. We find some sets of the parameters which yield a power exponent close to −1.5.

Preferred VGP paths during geomagnetic polarity reversals: Symmetry considerations

The reversing geomagnetic field has been said to have virtual geomagnetic poles (VGPs) confined to a pair of longitudes close to where magnetic flux is concentrated at the core surface today, at approximately 90°W and 110°E. This can be explained if flux remains concentrated on the same longitudes throughout the transition. The VGP path then depends on site position, reversal sense (R→N or N→R), and sense of flux migration (pole‐ or equator‐ward). For transitional fields which remain antisymmetric about the equator, N→R transitions with poleward flux migration give western longitude VGP paths for sites in the NE and SW quadrants and eastern longitude VGP paths for sites in the NW and SE quadrants. Sites on quadrant boundaries record rapid, variable VGP positions. Data from the last (Matuyama‐Brunhes) reversal are broadly consistent with poleward flux migration.

Geomagnetic polarity reversal model of deep‐tow profiles from the Pacific Jurassic Quiet Zone

The Jurassic magnetic “Quiet Zone” (JQZ) contains magnetic lineations, but their low amplitudes make correlation and interpretation difficult. Part of the problem is the separation of source and sensor for old, deep ocean crust. We increased anomaly amplitudes by collecting magnetic data along two deep‐tow profiles over western Pacific JQZ lineations. A magnetic polarity reversal timescale was constructed by matching deep‐tow anomalies with a simple, rectangular block magnetization model for oceanic crust. The polarity sequence covers ∼11 m.y. (156–167.5 Ma) and contains 88 pre‐M29 polarity chrons extending to Chron M41. A limitation of this model is its poor representation of the oldest anomalies upward continued to sea level. On deep‐tow profiles these anomalies have both long‐ and short‐wavelength components, but only the latter are easily modeled on a datum close to the source. An alternative polarity model was constructed to match the anomalies upward continued to sea level. This model retains only 44% of the deep‐tow model polarity chrons because of short‐wavelength attenuation by upward continuation. Because of the inferred periods and magnetization contrasts, we think many of the short‐wavelength anomalies represent paleofield intensity fluctuations. In contrast, polarity reversals have been documented by prior magnetostratigraphic work for the younger part of the timescale covered by our model. Thus our data may show a transition from a geomagnetic field behavior dominated by intensity fluctuations to one dominated by reversals.

Nonlinear quenching of current fluctuations in a self-exciting homopolar dynamo

Abstract. In the interpretation of geomagnetic polarity reversals with their highly variable frequency over geological time it is necessary, as with other irregularly fluctuating geophysical phenomena, to consider the relative importance of forced contributions associated with changing boundary conditions and of free contributions characteristic of the behaviour of nonlinear systems operating under fixed boundary conditions. New evidence -albeit indirect- in favour of the likely predominance of forced contributions is provided by the discovery reported here of the possibility of complete quenching by nonlineax effects of current fluctuations in a self-exciting homopolar dynamo with its single Faraday disk driven into rotation with angular speed y(τ) (where τ denotes time) by a steady applied couple. The armature of an electric motor connected in series with the coil of the dynamo is driven into rotation' with angular speed z(τ) by a torque xf (x) due to Lorentz forces associated with the electric current x(τ) in the system (just as certain parts of the spectrum of eddies within the liquid outer core are generated largely by Lorentz forces associated with currents generated by the self-exciting magnetohydrodynamic (MHD) geodynamo). The discovery is based on bifurcation analysis supported by computational studies of the following (mathematically novel) autonomous set of nonlinear ordinary differential equations: dx/dt = x(y - 1) - βzf(x), dy/dt = α(1 - x²) - κy, dz/dt = xf (x) -λz, where f (x) = 1 - ε + εσx, in cases when the dimensionless parameters (α, β, κ, λ, σ) are all positive and 0 ≤ ε ≤ 1. Within those regions of (α, β, κ, λ, σ) parameter space where the applied couple, as measured by α, is strong enough for persistent dynamo action (i.e. x ≠ 0) to occur at all, there are in general extensive regions where x(τ) exhibits large amplitude regular or irregular (chaotic) fluctuations. But these fluctuating régimes shrink in size as increases from zero, and they disappear altogether when ε = 1, leaving only steady régimes of dynamo action.

The nonlinear differential equations governing a hierarchy of self-exciting coupled Faraday-disk homopolar dynamos

This paper discusses the derivation of the autonomous sets of dimensionless nonlinear ordinary differential equations (ODE's) that govern the behaviour of a hierarchy of related electro-mechanical self-exciting Faraday-disk homopolar dynamo systems driven by steady mechanical couples. Each system comprises N interacting units which could be arranged in a ring or lattice. Within each unit and connected in parallel or in series with the coil are electric motors driven into motion by the dynamo, all having linear characteristics, so that nonlinearity arises entirely through the coupling between components. By introducing simple extra terms into the equations it is possible to represent biasing effects arising from impressed electromotive forces due to thermoelectric or chemical processes and from the presence of ambient magnetic fields. Dissipation in the system is due not only to ohmic heating but also to mechanical friction in the disk and the motors, with the latter agency, no matter how weak, playing an unexpectedly crucial rôle in the production of régimes of chaotic behaviour. This has already been demonstrated in recent work on a case of a single unit incorporating just one series motor, which is governed by a novel autonomous set of nonlinear ODE's with three time-dependent variables and four control parameters. It will be of mathematical as well as geophysical and astrophysical interest to investigate systematically phase and amplitude locking and other types of behaviour in the more complicated cases that arise when N > 1, which can typically involve up to 6 N dependent variables and 19 N-5 control parameters. Even the simplest members of the hierarchy, with N as low as 1, 2 or 3, could prove useful as physically-realistic low-dimensional models in theoretical studies of fluctuating stellar and planetary magnetic fields. Geomagnetic polarity reversals could be affected by the presence of the Earth's solid metallic inner core, driven like an electric motor by currents generated by self-exciting magnetohydrodynamic (MHD) dynamo action involving motional induction associated with buoyancy-driven flow in the liquid metallic outer core. The study of biased disk dynamos could bear on the theory of the magnetic fields of natural systems where a significant background field is present (e.g., Galilean satellites of Jupiter) or when the action of motional induction is modified by electromotive forces produced by other mechanisms, such as thermoelectric processes, as in certain stars.

The last two geomagnetic polarity reversals recorded in high-deposition-rate sediment drifts

High-resolution records of geomagnetic polarity transitions are rare owing to the difficulty of preserving in the geological record details of a process with a duration of only a few thousand years. Lavas can record the instantaneous field on cooling, but lava records are themselves discontinuous and therefore polarity transition fields are seldom captured. Sedimentary records are more continuous, but typical deep-sea sedimentation rates of a few cm kyr−1are too low to record transition fields adequately. Here we report transition records from North Atlantic drift deposits with high sedimentation rates (∼12 cm kyr−1) at the Ocean Drilling Program sites 983 and 984. Virtual geomagnetic polar (VGP) paths are similar within and between sites for the last two geomagnetic reversals, attesting to a memory effect spanning back-to-back reversals. The complex VGP paths feature two VGP clusters (one located in northeastern Asia and the other in the South Atlantic Ocean off South America), and provide a link between the longitudinally constrained transitional VGPs often recorded in sediments1,2—which represent highly smoothed renditions of transition fields—and the scattered transitional VGPs3 and VGP clusters4 observed in lava records.

A numerical modelling study on the processes of uplift and planation of the Tibetan Plateau

The Tibetan Plateau has experienced a number of processes of uplift and planation alternately since about 45 Ma B. P. when it began to raise. A differential equation model for describing the Plateau altitude variation with time is formulated on the basis of previous field studies and a theoretical hypothesis: if palaeomagnetic polarity is positive, the convective activity in the earth is strong; orogenic movement is violent; and the raising velocity of the Plateau is high andvice versa. The analytical solution of the equation is obtained. The altitude variation from the beginning of the Plateau uplift to present is computed through using the geomagnetic polarity reversals timing series and interstellar atomic hydrogen concentration data. A comparison between the model results and the field studies indicates that the former is quite similar to the latter. The model results are able to basically reproduce the alternating processes of uplift and planation of the plate geological history. In the present model, the influences of the denudation and the geomagnetic polarity and interstellar atomic hydrogen concentration on the raising velocity of the Plateau altitude are mainly considered.

A numerical modelling study on the processes of uplift and planation of the Tibetan Plateau

The Tibetan Plateau has experienced a number of processes of uplift and planation alternately since about 45 Ma B. P. when it began to raise. A differential equation model for describing the Plateau altitude variation with time is formulated on the basis of previous field studies and a theoretical hypothesis: if palaeomagnetic polarity is positive, the convective activity in the earth is strong; orogenic movement is violent; and the raising velocity of the Plateau is high and vice versa . The analytical solution of the equation is obtained. The altitude variation from the beginning of the Plateau uplift to present is computed through using the geomagnetic polarity reversals timing series and interstellar atomic hydrogen concentration data. A comparison between the model results and the field studies indicates that the former is quite similar to the latter. The model results are able to basically reproduce the alternating processes of uplift and planation of the plate geological history. In the present model, the influences of the denudation and the geomagnetic polarity and interstellar atomic hydrogen concentration on the raising velocity of the Plateau altitude are mainlv considered.

Conference on the magnetization of the oceanic crust steers future research

Because marine magnetic anomalies arise from the combination of seafloor spreading and geomagnetic polarity reversals, they delineate a history of global plate motions and geomagnetic field behavior. Thirty years ago, interpretation of sea surface magnetometer profiles led to the plate tectonics revolution. Recent developments in high resolution magnetic studies are similarly changing our view of the structure and evolution of oceanic crust and beginning to answer basic questions concerning geomagnetic field behavior.In response to these developments, the Conference on the Magnetization of Oceanic Crust was held September 21–24,1996, on Orcas Island in Washington State. Forty‐seven scientists representing 20 institutions in seven countries attended the conference, which was funded by the National Science Foundation, the Ridge Interdisciplinary Global Experiment (RIDGE), and the United States Science Advisory Committee (USSAC).

Fission track dating of ancient man site in Baise, China, and its significances in space research, paleomagnetism and stratigraphy

A large number of artifacts (stone tools) have been discovered in Baise (Bose), Guangxi, China in recent years. They show that ancient man were living in the south of China during very ancient times. During excavation, tektites were discovered in the same layer of deposits as the stone tools. The structure of the layers of deposits in this site was never disturbed, which is the evidence that the stone tools were left behind by the ancient man at the time when the tektites fell on the earth, which were slowly covered over by layers of deposits. Fission track dating has been carried out on tektites. The age of the tektites is 0.732 ± 0.039 Ma, which is also the age of the ancient man in Baise, Guangxi, China. The annealing degree of spontaneous fission tracks in the tektite was investigated by measuring track diameters. A correction of age for track fading has been made by track diameter technique. According to current understanding, tektites were formed by the impact of falling asteroids or comets on the earth's surface. The dating shows that a big impact occurred on the earth 0.732 Ma ago. The coincidence of the age of the tektites with the age of geomagnetic polarity reversal (∼ 0.73 Ma) from the Matuyama Epoch to the Brunhes Epoch proposes a plausible explanation that the possible cause of the geomagnetic polarity reversal is due to the big impact of space objects falling onto the earth. The result of the dating also set up a standard for inferring the ages of the deposits in South China as well as in South-East Asia where laterite (red soil) deposits exist in the entire region.

Marine magnetic anomalies as recorders of geomagnetic intensity variations

In addition to providing a robust record of past geomagnetic polarity reversals, marine magnetic anomalies often show shorter wavelength variations, which may provide information on geomagnetic intensity variations within intervals of constant polarity. To evaluate this possible geomagnetic signal, we compare sea surface profiles of the Central Anomaly with synthetic profiles based on Brunhes age (0-0.78 Ma) paleointensity records derived from deep sea sediments. The similarity of the synthetic profiles and observed profiles from the ultra-fast spreading southern East Pacific Rise suggests that geomagnetic intensity variations play an important role in the magnetization of the oceanic crust. This interpretation is further supported by systematic variations in the pattern of the Central Anomaly at slower spreading ridges, which are entirely consistent with a progressively smoother record of the sediment-derived paleointensity. If the sedimentary records, as calibrated to available absolute paleointensity data, accurately record variations in dipole intensity over the Brunhes, it follows that much of the Brunhes was characterized by geomagnetic intensities lower than either the mean dipole moment for the past 10 ka or the average for the period from 0.05 to 5.0 Ma. Furthermore, the sediment paleointensity records reflect the significant increase in geomagnetic intensity, from a low of ∼ 2 × 1022 Am2 near 40 ka to a peak value (11 × 1022 Am2) at ∼ 3 ka, that has been well documented from absolute paleointensity determinations. We suggest that geomagnetic intensity variations may be the most important cause of the rapid changes in the source layer magnetization near the ridge crest and the resultant Central Anomaly Magnetic High.

Waves and stationary patterns in the core and the interpretation of transition fields in geomagnetic polarity reversals

Waves and stationary patterns in the core and the interpretation of transition fields in geomagnetic polarity reversals

Age and duration of the Matuyama-Brunhes geomagnetic polarity reversal from [formula omitted] incremental heating analyses of lavas

Constraints on the timing of geomagnetic polarity reversals have come mainly from KAr, or more recently 40Ar39Ar, age determinations of lavas or their K-rich phenocrysts that erupted prior or subsequent to particular geomagnetic events. We have obtained 40Ar39Ar isochron ages using incremental heating techniques on groundmass separates, phenocryst-poor whole rock samples, or plagioclase, from eight basaltic to andesitic lavas that erupted during the Matuyama-Brunhes (M-B) polarity transition at four geographically dispersed sites. These eight lavas range from 784.6 ± 7.1 ka to 770.8 ± 5.2 ka (1 σ errors); the weighted mean, 778.7 ± 1.9 ka, gives a high-precision age that is remarkably consistent with revised astronomical age estimates for the M-B polarity transition [6,12,13]. Despite uncertainties in absolute calibration of 40Ar39Ar ages relative to mineral standards used as neutron fluence monitors, our age determinations are consistent with five other 40Ar39Ar studies focused on the M-B transition. These results confirm that the earlier KAr based geomagnetic polarity time scale underestimated the age of the M-B reversal by about 6%. None of the eight isochron ages are distinguishable from one another at the 95% confidence level. However, we are tantalizingly close to testing for the duration of this reversal. One lava at the base of a sequence of transitionally magnetized flows in Chile and the uppermost lava in a similar sequence on Maui are only just indistinguishable in age at the 95% confidence level and preserve different magnetic orientations. We suggest that the ∼ 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the M-B reversal from deep sea sediment cores. Together with the short duration (∼ 2 kyr) of the directional reversal observed in several different marine sediment sections, our data suggest that reversal of the field's direction could have occurred at slightly different times depending on the position of the recording site.

Age of Etendeka flood volcanism and associated intrusions in southwestern Africa

Research Article| July 01, 1996 Age of Etendeka flood volcanism and associated intrusions in southwestern Africa Paul R. Renne; Paul R. Renne 1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 Search for other works by this author on: GSW Google Scholar Jonathan M. Glen; Jonathan M. Glen 1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 Search for other works by this author on: GSW Google Scholar Simon C. Milner; Simon C. Milner 2Geological Survey of Namibia, P.O. Box 2168, Windhoek, Namibia Search for other works by this author on: GSW Google Scholar Andrew R. Duncan Andrew R. Duncan 3Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa Search for other works by this author on: GSW Google Scholar Author and Article Information Paul R. Renne 1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 Jonathan M. Glen 1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 Simon C. Milner 2Geological Survey of Namibia, P.O. Box 2168, Windhoek, Namibia Andrew R. Duncan 3Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (7): 659–662. https://doi.org/10.1130/0091-7613(1996)024<0659:AOEFVA>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paul R. Renne, Jonathan M. Glen, Simon C. Milner, Andrew R. Duncan; Age of Etendeka flood volcanism and associated intrusions in southwestern Africa. Geology 1996;; 24 (7): 659–662. doi: https://doi.org/10.1130/0091-7613(1996)024<0659:AOEFVA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Detailed 40Ar/39Ar laser step-heating analyses of mineral separates from five volcanic units in Namibia and Angola and five intrusions in Namibia yield important geochronological data for the Etendeka igneous province. Ten plateau dates on plagioclase, hornblende, and biotite between 131.7 ± 0.7 and 132.3 ± 0.7 Ma were obtained, and a late syenite from the Messum intrusive complex yielded a slightly younger hornblende plateau date of 129.3 ± 0.7 Ma. Magnetostratigraphy of the volcanic rocks in three sections up to 700 m thick, laterally spanning more than 100 km, suggests that the flows record only two geomagnetic polarity reversals. Precise temporal coincidence with the Paraná flood volcanic province in South America indicates that Etendeka volcanism does not represent a significantly younger phase of magmatism that migrated from northwest to southeast over 10 m.y., as has recently been proposed. The duration of intrusive activity was at least 2–3 m.y. longer than recorded by volcanism, and its total duration awaits further constraints. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural instability of the Rikitake Disk Dynamo

The Rikitake‐type disk dynamo, consisting of two identical single Faraday‐disk dynamos of the Bullard type coupled together, has been analyzed intensively for more than thirty years by: (a) mathematicians interested in its ability to behave chaotically, and (b) geophysicists needing low‐dimensional electromechanical analogues of the geodynamo in their efforts to elucidate the complex time series of geomagnetic polarity reversals over geological time. Implicit in the theory is what now appears to be an unwarranted assumption, namely that mechanical friction is unimportant both qualitatively and quantitatively as a dissipative agency in comparison with ohmic effects, and can therefore safely be neglected. The assumption is inconsistent with the findings of the present study, which shows that mechanical friction can render the Rikitake dynamo “structurally unstable” and consequently incapable of producing chaotic oscillations. Research on low‐dimensional analogues of the geodynamo should concentrate attention on structurally stable systems, with properties less dependent on the starting conditions.

New evidence for extraordinarily rapid change of the geomagnetic field during a reversal

Palaeomagnetic results from lava flows recording a geomagnetic polarity reversal at Steens Mountain, Oregon suggest the occurrence of brief episodes of astonishingly rapid field change of six degrees per day. The evidence is large, systematic variations in the direction of remanent magnetization as a function of the temperature of thermal demagnetization and of vertical position within a single flow, which are most simply explained by the hypothesis that the field was changing direction as the flow cooled.

Extending the climatic precession curve back into the Late Miocene by signature template comparison

The rhythm of sedimentary cycles reflecting the climatic precession signal (19 and 23 kyr components) from the upper Miocene of the Bou Regreg section at Ain el Beida near Rabat, Morocco, had been analyzed to determine the age of the geomagnetic polarity reversals of Subchron 5N1 (C3An.1n). A new method of analysis of cycle “signatures” uses adjusted data series as represented by a signal obtained from grey‐level traces through image‐enhanced photographs of the sediment cycles. The measured, “timeless” signal is reduced and geometrically transformed to a signature template in order to compare its pattern with similar time constrained segments of the target precession signal (retrodicted for 35° N). Conventional statistical and spectral analysis tests are used to regress to the best possible fit. A date estimate of 5.94 Ma for the Chron 5/Gilbert boundary (C3An.1n/C3r), which occurs near the times of the closure of the Rifian Corridor and the beginning of the “drawdown” phase of the Messinian Salinity Crisis, is found to be in close agreement with independent approximations derived from extrapolation of sea floor spreading, radiometric dating, and younger Earth‐orbital tuning ages.

Chaos in geophysical fluids I. General introduction

Irregular buoyancy-driven flows occur in the atmospheres and fluid interiors of the Earth and other planets, and of the Sun and other stars, where they influence and often control the transfer of heat. Their presence is manifest in or implied by a wide variety of observed phenomena, including external magnetic fields generated by self-exciting magnetohydrodynamic (MHD ) dynamo action. Based on the laws of classical mechanics, thermodynamics and, in the case of electrically conducting fluids, electrodynamics, the governing mathematical equations are well known, but they are generally intractable owing to their essential nonlinearity. Computers play a key role in modern theoretical research in geophysical and astrophysical fluid dynamics, where ideas based on chaos theory are being applied in the analysis of models and the assessment of predictability. The aim of this paper is to provide a largely qualitative survey for non-specialists. The survey comprises two parts, namely a general introduction (Part I) followed by a discussion of two representative areas of research, both concerned with phenomena attributable to symmetry-breaking bifurcations caused by gyroscopic (Coriolis) forces (Part II), namely (a) large-scale waves and eddies in the atmospheres of the Earth, Jupiter and other planets (where, exceptionally, laboratory experiments have been influential), and (b) MHD dynamos. Various combinations of Faraday disc dynamos have been studied numerically as low-dimensional nonlinear electromechanical analogues of MHD dynamos, particularly in efforts to elucidate the complex time series of geomagnetic polarity reversals over geological time. The ability of the intensively studied Rikitake coupled disc dynamo system to behave chaotically appears to be a consequence of the neglect of mechanical friction, the inclusion of which renders the system structurally unstable.

Evidence for seafloor spreading in the Laxmi Basin, northeastern Arabian Sea

Marine magnetic anomaly data from the Laxmi Basin for the first time reveal the presence of fairly correlatable NNW-trending magnetic lineations. These magnetic lineations are symmetric about a central negative magnetic anomaly and the axis of symmetry coincides with a characteristic short-wavelength free-air gravity low. The anomalies are interpreted as representing a two-limbed seafloor spreading sequence which can be equated to the A28–A33 interval of the geomagnetic polarity reversal timescale. These results suggest that the Laxmi Basin is underlain by an oceanic crust. Evidence of seafloor spreading in this basin possibly implies a pre-A27 spreading episode in the evolutionary history of the Arabian Sea.

Geomagnetic polarity reversals: A connection with secular variation and core‐mantle interaction?

Geomagnetic polarity reversal remains one of Nature's most enigmatic phenomena. Dynamo theory admits solutions in pairs with reversed magnetic fields B and −B, but detailed calculations are required to understand how the field can change sign. Theory also admits separate solutions with different symmetry across the equatorial plane, the symmetric (ES) “quadrupole” and the antisymmetric (EA) “dipole” solutions, which may be important in the reversal process and which offer a simple framework for interpreting small paleomagnetic data sets. Ordinary secular variation leads to very large changes in the magnetic field over several centuries and could easily develop into full reversal; the theory of secular variation, which is relatively well developed, may therefore help in understanding reversals. Other clues to geomagnetic reversals come from the Sun, whose magnetic field reverses every 11 years. Paleomagnetic data show the Earth’s magnetic field reverses every million years or so, with each transition taking about a thousand years, during which the intensity may fall by as much as 1 order of magnitude. Reversal frequency undergoes a modulation on the long timescale (107 years) of mantle convection, and there have been two long intervals in the past with no reversals. Such behavior is typical of a highly nonlinear dynamical system, but the very long timescale of changes in reversal frequency, and its close proximity to the overturn time of mantle convection, suggests some control of the dynamo by the mantle. Short‐term phenomena, such as change in the length of day and secular variation, have been studied extensively for evidence of core‐mantle interactions, and we may draw on this body of evidence in order to understand long‐term effects. Three physical mechanisms have been proposed: topographic, electromagnetic, and thermal, with the last two being most significant for long‐term effects. Symmetries allow the dynamo to generate an EA field, with the major component a dipole, but lateral variations on the core‐mantle boundary may lead to magnetic fields with no symmetry, reflecting the structure of the boundary anomalies. Changes in reversal frequency on the mantle convection timescale could arise either from changes in total heat flux from the core to the mantle or from instabilities associated with lateral variations at the core‐mantle boundary. Neither mechanism is well understood, but the former involves significant changes to the Earth's overall heat budget, whereas the latter must always arise as a natural consequence of deep‐mantle convection. Recent measurements of transition fields show pole paths that lie close to two preferred longitudes near 90°W and 90°E; if substantiated, the result would provide the first definitive evidence of long‐term mantle control of the geomagnetic field. Further evidence suggests that the geomagnetic pole during stable polarity also lies along these two longitudes and that magnetic flux at the core surface tends to concentrate along the same longitudes, as does the present field. A simple theory is proposed relating changes in the core field to apparent transition paths measured at the Earth's surface. The model shows that longitude confinement of the transition paths can occur for quite complicated core fields and that surface intensities can drop by 1 order of magnitude, on average, simply because of the reduction in length scale of the transitional field. Simple transition paths may be an indication of some organization of flux at the core surface but not of large‐scale or small‐amplitude core fields.