Earth's Spin Axis Research Articles

Magnetic Field Reversals, Polar Wander, and Core-Mantle Coupling

True polar wander, the shifting of the entire mantle relative to the earth's spin axis, has been reanalyzed. Over the last 200 million years, true polar wander has been fast (approximately 5 centimeters per year) most of the time, except for a remarkable standstill from 170 to 110 million years ago. This standstill correlates with a decrease in the reversal frequency of the geomagnetic field and episodes of continental breakup. Conversely, true polar wander is high when reversal frequency increases. It is proposed that intermittent convection modulates the thickness of a thermal boundary layer at the base of the mantle and consequently the core-to-mantle heat flux. Emission of hot thermals from the boundary layer leads to increases in mantle convection and true polar wander. In conjunction, cold thermals released from a boundary layer at the top of the liquid core eventually lead to reversals. Changes in the locations of subduction zones may also affect true polar wander. Exceptional volcanism and mass extinctions at the Cretaceous-Tertiary and Permo-Triassic boundaries may be related to thermals released after two unusually long periods with no magnetic reversals. These environmental catastrophes may therefore be a consequence of thermal and chemical couplings in the earth's multilayer heat engine rather than have an extraterrestrial cause.

Paleomagnetic Euler poles and the apparent polar wander and absolute motion of North America since the Carboniferous

Paleomagnetic Euler poles and the apparent polar wander and absolute motion of North America since the Carboniferous

Do the earth's magnetic poles move?

Two distinct types of motion are commonly attributed to the geomagnetic poles — polar wander and dipole wobble. But two or three decades of intensive effort have failed to provide a sound understanding of either. Why? Polar wander is a very slow phenomenon associated with time scales of 107 to 108 years and thus intimately connected with the drift of the continents arising from plate tectonics. While there is no question of the validity of relative movements between the continents and the pole, the possibility of independent polar motion remains debatable. Dipole wobble is a more rapid phenomenon, most likely associated with time scales of 106 years or less. Currently the geomagnetic axis diverges from the Earth's spin axis by 11 1/2°, but paleomagnetic data indicate that, when averaged over a million years or so, the two coincide. Dipole wobble is the supposed mechanism responsible. The main difficulty here is one of isolating any dipole effects from perturbations of similar magnitude arising from the non-dipole fraction of the geomagnetic field — the so-called secular variation. Recent studies have significantly advanced our understanding of the secular variation, but the dipole wobble remains enigmatic.

Early Tertiary subduction zones and hot spots

Hot spots and subduction zones may control the position of the earth's rotation axis. Paleomagnetic reconstructions in the hot spot framework require a relative motion of about 10° between the hot spot reference frame and the earth's spin axis since the early Tertiary. If the viscosity of the mantle permits, a changing distribution of mass anomalies can control the position of the earth's rotation axis as it tracks the maximum principal axis of the perturbations. The present plate geometry should give the best indication of what features are associated with the controlling density anomalies. For the present, geoid anomalies indicate that subducting slabs are major mass contributions; removing these effects, a simpler ‘residual’ geoid remains with highs strongly correlated with hot spot locations. Although neither the hot spots alone nor subducting slabs alone have maximum principal axes near the present pole, adding the contributions of the two gives a combined axis within a degree of the pole, suggesting that these two effects control the location of the spin axis. To establish whether changes in plate geometry could account for the observed polar motion, the locations of subduction zones are reconstructed in the hot spot framework for the early Tertiary, a time of significantly different plate motions. For this reconstruction, the combined maximum principal axis of the subductionhot spot systems is not at the present pole but about 10° away in a location similar to the pole position required by paleomagnetic studies. This suggests that changes in subduction geometry may cause a shift in the position of the rotation pole and, furthermore, that inertia tensor effects may be the link between plate velocities and the spin axis.

Hotspot-spin axis motion or magnetic far-sided effect?

Palaeomagnetic data from central Pacific sediments and from the Hawaiian–Emperor volcanic chain indicate that, during the Tertiary, either the derived magnetic pole was located alternately on the far and near side of the geographical pole, or that the hotspot coordinate systems (thought to be fixed relative to the Earth's mantle) oscillated relative to the Earth's spin axis.

Subducted lithosphere, hotspots, and the geoid

The earth's largest positive geoid height anomalies are associated with subduction zones and hotspots. Although the correlation with subduction has been noted for many years, the correlation with hotspots is fully evident only when the subduction-related geoid highs are removed from the observed field. Using the assumption that subducted lithospheric slabs are uncompensated and are thermally re-equilibrated with the asthenosphere at the maximum depth of earthquakes, the expected geoid anomaly over subduction zones is calculated. This field provides a satis-factory fit to the observed circum-Pacific high. Subtraction of this predicted anomaly leaves a residual field which is correlated, at greater than the 99% confidence level, with the distribution of hotspots. Broad residual geoid highs occur over the central Pacific and the Africa/eastern Atlantic regions, the same areas where the hotspots are concentrated. The mass anomalies associated with hotspots and subducted slabs apparently control the location of the earth's spin axis.

Eocene paleolatitude, climate, and mammals of Ellesmere island

During the early and middle Eocene Ellesmere Island was situated at almost the same latitude as at present. Marine paleotemperatures locally reached 15°C. Mean paleo-obliquity of Earth's spin axis was substantial and there is no physical model for any other value than the present mean value. In agreement with geochemical and additional biological data, the Ellesmere mammalian fauna indicates a warm climate in the Arctic about 50 m.y. ago. How this can be compatible with significant periods of winter darkness remains a problem, but decreased solar energy production after the middle Eocene is an uninvestigated possibility. The Donn and Shaw (1977) model of Eocene paleoclimate is in conflict with several lines of evidence presented here.

Neogene relative motion between the Pacific plate, the mantle, and the Earth's spin axis

The northward rates of motion for the central equatorial Pacific during the past 24 Myr vary between ∼20–30 mm yr−1 based on the migration of sedimentary facies, ∼30 mm yr−1 based on the age and orientation of the Hawaiian ridge, and ∼50–60 mm yr−1 based on palaeomagnetically determined sediment and basalt palaeolatitudes. To resolve the differences between hot spot and palaeomagnetic results we may need to invoke a long-term drift in the orientation of the geomagnetic field, or a northerly component of motion of the Hawaiian hot spot relative to the Earth's spin axis.

Plate tectonics in the Phanerozoic

Paleomagnetic observations, oceanic magnetic lineations, and the present continental margins are combined in an interactive computer program to generate maps for seven periods in the Phanerozoic Era. The results are presented in the familiar Mercator projection and with less distortion on other projections. On an azimuthal-equidistant map projection with origin on the centroid of the continental lithosphere the boundaries display a high degree of symmetry and order. By using geological evidence on the position of geosynclines, volcanic assemblages, and zones of diastrophism an attempt is made to model a reconstruction of major plate boundaries in the past. During the Cambrian and Ordovician Periods the continental segments were as widely dispersed as at present and formed a ring of plates on the paleoequator but with North Africa and South America contiguous and close to the south pole. In these, as in other periods, there is symmetry about the spin axis. Extensive plate motion occurred at about the time of the Caledonian Orogeny when the continental segments evolved toward the formation of a single large group called Pangaea by Wegener. This evolution occupied much of the late Paleozoic and Mesozoic Eras. Toward the end of the Cretaceous Period a more dispersed form began to develop yielding the present pattern of two antipodal quasi-circular plates separated by a ring of more irregular quasi-elliptical plates. The results suggest the presence of a slowly evolving mantle-wide convection system that is symmetric about the Earth's spin axis. The dominant pattern during the late Paleozoic and the Cenozoic was of plates with dimensions of 60–120° in which the convective pattern may be described in terms of spherical harmonics of the third order. In the Mesozoic and late Paleozoic the plate sizes were of the order of 90–180° and physical parameters describing convection were presumably dominated by second order terms.

Test of the dipolar nature of the geomagnetic field throughout Phanerozoic time

THE morphology of the geomagnetic field throughout the Earth's history is of interest first, in a general way, to geomagnetists in terms of long term implications concerning the Earth's core, its thermal regime and the motions in it, and, second, in a very particular way, to palaeomagnetists since the geocentric dipole model is universally used in mapping observed field directions into palaeomagnetic poles. Runcorn1,2 has given strong reasons for supposing the mean geomagnetic field to be symmetrical about the Earth's spin axis. He has also argued3, on the basis of early palaeomagnetic data, that a dipolar field coupled with continental motion is preferable to higher order axial multipole fields. Here I discuss an observational test, based on the much larger body of palaeomagnetic data now available, to discriminate between a dipole field and those of higher order multipoles.

The influence of the direction of the geomagnetic dipole on the position of the neutral sheet

An analysis of neutral sheet crossings detected by the NASA-Goddard Space Flight Center magnetometer on the Explorer 34 satellite has shown that the diurnal wobble of the geomagnetic dipole measurably influences the position of the neutral sheet at 30 RE. Viewed in the solar-ecliptic coordinate system, the dipole wobble originates from the 11° angle between the earth's spin axis and the dipole axis, has a period of 24 hours, and can be resolved into components of motion perpendicular and parallel to the earth-sun line. The perpendicular component exerts a torque on the magnetotail and causes the neutral sheet to pivot about an axis approximately parallel to the solar wind direction. The parallel component encourages the neutral sheet to move above and below the solar-magnetospheric equatorial plane with an amplitude of 1 RE. This latter motion is consistent with the neutral sheet's being hinged at a geocentric distance of 5.25 RE. Although hinging is not as dominant a feature of neutral sheet motion as pivoting at 30 RE, both are in phase with the 24-hour precession of the dipole axis about the earth's spin axis.

Climatically Controlled Sediments, the Geomagnetic Field, and Trade Wind Belts in Phanerozoic Time

On five Lambert equal-area maps and three Lambert equal-area "composites" of the world in Phanerozoic time are plotted distributions of evaporites, tillites, coal, eolian sands, and radiolarian cherts. The paleolatitude distributions of tillites and evaporites confirm the plausibility of the view that the Earth's magnetic field has approximated to a geocentric dipole whose axis paralleled the Earth's spin axis throughout Phanerozoic time. Coal formed in a wide range of latitudes. Paleolatitude distributions of oceanic cherts support those dynamic models of the atmosphere that require the later Mesozoic and Tertiary high-pressure subtropical belts to have been more or less stable features of atmospheric circulation. Paleolatitude distributions of eolian sands provide inconclusive evidence for the extent of these belts. Though rainfall and temperature have varied at a particular paleolatitude, marked equatorward shrinkage of the subtropical high-pressure belts during ice-sheet glaciations and marked poleward expansion during ice-free epochs is not evident from geological observations. There is a need for quantitative work on these problems and for an integration of the continental with the oceanic data.

Does the Mantle Roll?

Comparison of plume traces with palaeomagnetic data from lithospheric plates for the past 50 m.y. suggests that the mantle has rolled about its own independent axis within an outer lithospheric shell which as a whole is fixed in respect to the Earth's spin axis.

Sedimentary Facies and Plate Tectonics of Equatorial Pacific

Facies patterns of post-Eocene equatorial Pacific pelagic sediments indicate northward motion of the Pacific plate of about 3 cm/year with respect to the earth's spin axis, whereas the age gradient of volcanoes in the Hawaiian chain indicates a northward component of motion of the plate with respect to a hot spot in the subjacent asthenosphere of about 6 cm/year. The discrepancy is ascribed to southward motion of the upper part of the asthenosphere with respect to the spin axis. To examine this possibility more closely, a theoretical isopach map is generated for post-Eocene equatorial sediments, to compare with thicknesses known from drilling and seismic studies. In the model, sediment accumulation rates at all depths are several times faster close to the equator than farther away. The sedimentation model is combined with a plate-motion model with the following elements: (a) Pacific plate moves westward away from the rise crest at 10 cm/year; (b) sea floor deepens gradually as it moves away from the rise, with constant crustal-age versus depth relation; (c) plate moves northward with respect to the earth's spin axis at 3 cm/year; and (d) hot spots in the asthenosphere, which give rise to chains of volcanoes, move southward with respect to the spin axis, with ame speed. The model isopachs correspond fairly well to the known sediment thicknesses. Details of pre-Oligocene motions of the plate versus the earth's spin axis are uncertain, but mid-Cretaceous volcanoes have been shifted about 30° north. Northwest motion of the plate over hot spots in the asthenosphere at about 5 cm/year generated seamount chains parallel with the Line Islands in the period between about 30 and 100 m.y. ago.

Astronomically Determined Dates and Alignments

This is a tutorial article concerned with problems related to dates and, geometrical alignments determined from astronomical considerations. The mathematical problems often arise in connection with attempts to specifiy days of the year in the past or in connection with ancient edifices. In this way the solutions are of interest to anthropologists, archeologists, art historians, folklorists, historians, and astronomy enthusiasts. While the methods of solution presented are general, specific problems are also employed for illustrative purposes. Some problems involve the famous ancient edifices of Abu Simbel, Cheops' Pyramid, and Stonehenge, and bright celestial objects such as the Sun, Sirius, and the Pleiades. Solutions depend on a knowledge of spherical trigonometry and basic astronomy, and often involve the phenomenon of the precession of the Earth's spin axis. Effects of precession are described geometrically and algebraically.

Palaeolatitudes from JOIDES Deep Sea Sediment Cores

IN the framework of plate tectonics the Earth is thought to be composed of a mosaic of rigid plates riding on top of a viscous asthenosphere. The motion of these plates with respect to the Earth's spin axis can be determined using the methods of palaeomagnetism. Measurement of continental rocks1, oceanic islands2 and on seamounts3,4 all yield data which can be used to determine these motions.

Paleomagnetism of the southern continents and plate tectonics

A new method, utilizing paleomagnetic determinations on continents and ocean basins, is proposed to produce paleomagnetic reconstructions in terms of continued rotations of some of the larger plates of the lithosphere about axes which have remained fixed in the present geographic coordinates for times of the order of 100 to 150 million years. The solution is not unique but it is obtained with only one arbitrary assumption applied to all such continents and therefore is preferable to reconstructions which assign arbitrary motions to each continent. This method is applied to determine the motion of the southern continents since the Jurassic. The paleomagnetic reconstruction of Gondwanaland obtained is similar to that suggested on geological evidence alone and the proposed movements of the various plates do account for many of the observed features of the floor of the Indian Ocean. The success of this model suggests that the pivot points about which continental drift of the southern continents took place since the Jurassic are fixed in the mantle with respect to each other and the Earth's spin axis.