Viscous Remanent Research Articles

Intensity and direction of magnetization in oceanic basalts

The magnetic properties of oceanic basalt samples from 55 DSDP sites (301 samples) and 112 dredge haul sites (311 samples) show wide variations, indicative of heterogeneity in the basaltic layer. Remanent intensity and Königsberger ratio are log-normally distributed and range over three orders of magnitude for each collection. For DSDP samples between-site variation is greater than within-site variation, suggesting horizontal variation of magnetization within the source layer of magnetic anomalies. Crustal magnetization estimates from inversion of short-wavelength oceanic magnetic anomalies may therefore be in error. By using arithmetic means rather than the geometric means appropriate for log-normal distributions, estimates of the magnetization of Layer 2 of around 4 × 10 −3 Gauss are obtained for the Indian, Pacific and north Atlantic oceans and Caribbean Sea, while lower averages are found in the south Atlantic and Antarctic oceans. Using this as a representative value for abyssal areas, the thickness of the magnetic source layer is greater than commonly assumed, and probably more than a kilometre. This interpretation is supported by DSDP data from some individual sites, notably the deep holes of Leg 37. The DSDP median destructive fields average only 123 Oe. Although many sites have much higher stabilities than this, there is evidence for unstable NRM at many sites where samples exhibit a strong tendency to acquire viscous remanence components. The low coercivity secondary magnetizations confuse identification of the stable direction during magnetic cleaning. Acquisition of high coercivity secondary remanence (CRM) as a result of maghemitization, which is extensive in the DSDP collection, may also have altered the original NRM direction. Remanent inclination data from four Tertiary DSDP sites (ages 38–40 Ma) have been combined to give a tentative Eocene VGP for east Antarctica at 68°S 64°E. It has not been possible to compare DSDP data in this way at other sites. Palaeolatitudes deduced from DSDP basalt remanent inclinations show large discrepancies from values expected from plate reconstructions at many sites. Incompletely removed secondary magnetizations, tectonic effects and palaeo-reconstruction errors, alone or in combinations, may be responsible for these discrepancies. A major source of deviation from the expected direction may be the rapid cooling of the basalt which does not allow adequate averaging of secular variation.

Nature of magnetic grains and their effect on the remanent magnetization of basalts

Abstract The nature of magnetic grains in basalts obtained from different parts of the world has been investigated. Results indicate that magnetic behaviour attributable to cation-deficient magnetite is common in basalts younger than Cretaceous, while that due to multidomain magnetite is widespread in much older rocks. Superparamagnetic grains occur in basalts more abundantly than originally presumed, which seem to be mainly responsible for the viscous remanent magnetization of such samples. Basalts which are inferred to contain predominantly optimum single-domain grains are found to be most suitable for palaeomagnetic work. However, many samples generally contain a wide range of grain sizes and this can account for the observed variation in their magnetic stability. One possible mechanism for the formation of such magnetic grains in basalts and its implications to palaeomagnetism is presented. Basalts whose magnetic behaviour is completely reversible on heating and cooling are very rare and because of this fact the reliability of palaeointensity determinations, involving heating of the samples even for one time, may be reduced to a considerable extent.

Magnetic properties of the oceanic crust: Considerations from the results of DSDP leg 34

Paleomagnetic samples of basalt and sediment from four Deep-Sea Drilling Project sites in the Nazca plate were studied in order to detect any possible paleolatitude changes and to evaluate the contribution of different portions of the igneous basement to the overlying magnetic anomalies. Penetration of the basement basalts yielded both coarse-grained massive flow units and fine-grained pillow basalts. The massive flow samples had titanomagnetite grains that were relatively unoxidized, while all the pillow basalts had been extensively subjected to low-temperature oxidation. Average intensity of magnetic remanence in the massive flow units (99.4×10−4 emu/cm3) was approximately an order of magnitude higher than that found in the pillow basalts (10.3×10−4 emu/cm3). All coarse-grained massive flow samples had large components of soft magnetization that was probably due to a viscous remanent magnetization (VRM) acquired both in situ and during the drilling process. In some cases this VRM component dominated the initial thermal remanence and could be responsible for the inability of some areas of the oceanic crust to record observable magnetic anomaly patterns. Large deviations in stable inclination from that expected from an axial centered dipole field were recorded by both basalt and sediment samples. Tightly grouped and anomalously high inclinations in the basalt samples from the younger sites indicate that the upper part of the oceanic igneous crust was formed in less than 102 yr, and this formation may be episodic. Paleomagnetic inclinations from the sediments, averaged over roughly 106 yr for each site, indicated no latitudinal change for the Nazca plate between 15 and 40 m.y. B.P. and are consistent with a southward paleolatitude movement of 3°–5° during the last 15 m.y.

Mesozoic magnetic lineations, the magnetic quiet zone, and sea floor spreading in the northwest Atlantic

The magnetic basement topography and associated magnetic anomaly lineation pattern in an area north of the New England seamounts lying between anomaly 31 and the continental margin were examined in light of the Mesozoic sea floor spreading history of the northwest Atlantic. Within the magnetic quiet zone the crust is typically oceanic as far landward as the slope anomaly and exhibits a normally polarized magnetism in the inner quiet zone. Magnetic lineations have been correlated with the Keathley sequence near Bermuda, and, based upon best estimates of the reversal time scale, cross-strike spreading rates are found to be about 1.7 cm yr−1 from −172 m.y. to −136 m.y. and between 0.9 and 1.0 cm yr−1 from −136 m.y. to −72 m.y. Landward of the Keathley sequence, magnetic anomalies decrease in amplitude, although the intensity of magnetization of the normally polarized crust exhibits average values. Magnetic observations within a large area of the quiet zone surveyed in detail can be fitted to the basement topography if a Jurassic pole position of normal polarity is assumed. However, three zones trending subparallel to the Keathley lineations require a weak reversely polarized magnetism. These low values of remanent magnetization are attributed to either viscous remanence or contamination of the original crust by widespread volcanic flows or intrusives during periods of predominantly normal magnetic polarity. The normal and reversed polarity sequence resulting from this study is correlated with the sequence obtained from paleomagnetic measurements on land. Trends of isochrons within this relatively limited oceanic area necessitate an alteration in the shape of the ridge axis. A difference in spreading rate and direction is obvious on either side of the New England seamount chain and is discussed with respect to the history of early plate motions in the northwest Atlantic.

Magnetic properties and paleomagnetism of some DSDP leg 33 basalts and sediment and their tectonic implications

Paleomagnetic and magnetic property studies have been made on sediments and basalts recovered from holes 315A (northern Line Island chain) and 317A (Manihiki Plateau) during leg 33 of the Deep-Sea Drilling Project (DSDP). The purpose of this investigation is twofold: (1) to investigate the magnetic properties of basalts from holes 315A and 317A and (2) to determine the origin of the inclination discrepancy between hole 317A carbonate and hole 317A basalt as reported by Cockerham and Jarrard [1976]. Basalts cored at hole 315A, DSDP leg 33, have irreversible Js-T curves (typical of submarine basalts) with Curie temperatures of approximately 300°C. When they are viewed with reflecting light, large (30–100 μ ) anhedral homogeneous titanomagnetite grains of class 1 oxidation are observed. These basalts have acquired a large viscous remanence. The six flow units cored yield an average inclination of −30.9° (σ = 14.4). Hole 317A basalts have reversible Js-T curves and Curie temperatures of about 570°C. Some 317A basalts have a secondary Curie temperature at about 360°C. The two Curie temperatures in a sample are believed to represent two compositionally different titanomagnetites. Under reflected light, class 3–4 deuterically oxidized titanomagnetite grains are observed with abundant ilmenite exsolution lamellae. These basalts are very resistant to AF demagnetization (median destructive field (MDF) values of ∼250 Oe). A mean inclination of −65.2° (σ = 6.1) has been calculated for these basalts, while the overlying carbonate sediments have a 20° shallower inclination (Ī = 46.7, σ = 9.0). Volcaniclastic sediments, 250 m in thickness, separate the basalts from the carbonates. The mean inclination of the volcaniclastics is −62.2° (σ = 2.4); however, the lowest samples of the carbonates together with the uppermost volcaniclastic samples have inclinations that systematically span the 20° difference. The difference in inclinations of the carbonates and the basalts is believed to be a result of tectonic tilting. The remains of shallow water molluscan fauna, volcaniclastics derived from hyaloclastite eruptions, and the magnetic properties of 317A basalts all suggest a subaerially or very shallow water (<200m)eruption of the 317A basalts.

Thermal fluctuation analysis: A new technique in rock magnetism

Thermal fluctuation analysis is a new method of determining ‘magnetic grain size’ v and microscopic coercive force HK in particles too small for direct domain observations. When it is used with single-domain particles, thermal fluctuation analysis is a method of magnetic granulometry, since v is the physical grain volume and HK is usually closely related to grain shape. For multidomain particles, v corresponds to one Barkhausen jump of a domain wall, and HK describes the opposition to wall motion. The technique requires three steps: (1) measurement of coercive force Hc and saturation magnetization over a broad temperature range, (2) separation of the contributions of HK and the ‘fluctuation field’ Hq to HC, in a manner suggested by the Neel (1949) theory, and (3) calculation of average values of v and room temperature HK from Hq(T) and HK(T). Thermal fluctuation analysis is the only simple means of calculating average v; and HK separately. If thermal fluctuations are important, as is always the case for the fine particles or small wall displacements involved in low-field thermoremanence or viscous remanence, thermal and alternating field demagnetization do not resolve v and HK unambiguously, because observed coercivities and blocking temperatures TB depend on both v and HK. The method is tested by applying it to single-domain materials of known grain size. Fluctuation analysis is then used to infer the possible domain structure of 500- to 2200-A magnetite particles and to point up a deficiency in the Stacey and Banerjee (1974) procedure for detecting ordering of lattice defects impeding domain wall motion. Potential broader applications include determining mean grain sizes of single, and under favorable conditions, double populations of magnetic carriers in rocks, defining average elongation for single-domain particles in rocks and synthetic dispersions (e.g., magnetic recording tapes), and deducing the mechanism of anisotropy (shape, crystalline or magnetoelastic) or of opposition to wall motion (strain, inclusion or magnetostatic). The main limitations to the method are that the averages of v and HK become nonstationary if Hc measurements are made within the TB range and that step 2 (separation of HK(T) and Hq(T)) is difficult unless HK is predominantly due to a single anisotropy or wall opposition mechanism.

Magnetic properties of oceanic pillow basalts from Macquarie Island

WE summarise here the results of a combined palaeomagnetic, hysteresis, thermomagnetic, and electron microprobe analysis of late Tertiary oceanic pillow basalts from Macquarie Island, which indicate that pillow basalts from a significant depth in the oceanic lithosphere have suffered burial metamorphism during the crustal accretion process. This metamorphism results in an intensity of natural remanent magnetisation (NRM) of ∼10−3 gauss, a substantial component of viscous remanent magnetisation (VRM), and an order of magnitude decrease in the Koenigsberger ratio relative to unaltered pillow basalts. Since a pillow lava layer with such a low NRM intensity is insufficient to account for the observed amplitude of marine magnetic anomalies, we conclude that in the case of Macquarie Island the underlying basaltic and doleritic dyke complex must also contribute to the observed anomalies.

Field-strength dependence of the viscous remanent magnetization in lunar samples

Measurements of the viscous remanent magnetization (VRM) in lunar soil breccia 14313 have been made. It is shown: (a) that the decay of VRM can be described best by a curve of the form log(t/t+ t 0) where t 0 is the acquisition time, (b) that in fields less than 4 Oe, the slope of the decay curve for t ≪ t 0 is linear, and the intercept of the straight-line portion of the decay curve is equal to the application time, (c) that in fields above 4 Oe, the slope of the decay curve tends to saturate following a Langevin-like function and the intercept time derived from the straight-line portion of the decay curve is greater than the acquisition time. By fitting experimental data for a range of fields including the non-linear portion it is possible to determine values for both the total moment taking part in the viscous process and for the effective volume of iron grains participating in the VRM process.

The magnetization and densities of precambrian rocks and iron-ores of northern Sweden

In an attempt to improve the interpretation of the geophysical results of an exploration programme in Norrbotten country, northern Sweden, a systematic investigation has been made of the magnetization and densities of the main rock and ore types. The rocks have low porosities and their densities vary predictably according to their composition between 2.6 and 3.1 g cm −3 with an average of 2.73 g cm −3 for the Precambrian bedrock of the area. The high grade magnetite ores have susceptibilities in the 0.5 –1.2 emu cm −3 range but for the normal rock types there is a well defined upper limit at about 2·10−2 emu cm −3 and there is no general systematic variation with the composition. The remanent magnetization in both the rocks and magnetite ores is generally weak with Q-values between 0.1 and 1.0 and shows many features characteristic of viscous remanent magnetization (VRM) including a coincidence of the directions of magnetization with that of the geomagnetic field. In the rocks the field direction is undisturbed but due to the strong demagnetization fields within the magnetite ores the direction of the incucing field is determined by the form and position of the ore deposit. The remanent magnetization generally influences only the amplitudes of the magnetic anomalies observed but not their forms. There is some evidence from the results of this survey that there is a significant difference between the remanence of samples collected from outcrops and from drillcores, suggesting that the weak VRM can be disturbed by drilling processes.

Origin of magnetic instability in sediment cores from the central North Pacific

Previous paleomagnetic studies on deep-sea sediment cores from the central North Pacific have shown that the natural remanent magnetization (NRM) of these 'red clay' sediments was unstable below several meters depth in each core. It was also noted that the magnetic instability was related to the presence of a relatively large 1ow-coercivity component of magnetization. The purpose of this investigation was to characterize the rock magnetic properties in three select cores from this region to determine the physical .origin of the unstable magnetization. The principal findings of our investigation were as follows. (1) The ability to acquire a viscous remanent magnetization increased with depth in each core, particularly at about the level where unstable magnetization became evident. (2) The magnitude and stability of the observed NRM of the magnetically unstable section of each core can be explained by a viscous remanence acquired in the presence of the earth's magnetic field over a period of time ranging from only several weeks to several thousands of years. (3) The unstable magnetization, believed to be of viscous origin, was attributed to the presence of a magnetic mineral similar in structure and composition to maghemite. This mineral may have resulted from the low-temperature oxidation of very fine grained magnetite at about the time of deposition of these sediments. The extrapolated ages of the levels at which unstable magnetization becomes evident in the cores from this region suggest a close correspondence with the times of established upper Cenozoic climatic changes. Considerations of the alteration of the sedimentary regime resulting from the changes in climate can provide a satisfactory explanation for the observed change in magnetic properties. In a paleomagnetic study of a large suite of sediment cores from the North Pacific Opdyke and Foster [1970] reported a marked instability of magnetization that occurred with depth in most cores of 'red clay' lithology. The sediment cores in which the instability of magnetization was noted were located within a region of the central North Pacific characterized by a generally low sediment thickness and by Brunhes epoch sedimentation rates of the order of only several millimeters per thousand years (Figure 1). Both the low sedimentation rate and the small sediment thickness probably are related to the low biogenic productivity in the area [Fleming, 1957] and to the large distances from continental sediment sources. The marginal trenches surrounding much of the North Pacific basin also act as sediment traps and inhibit bottom transport of sediment to the center of the basin. The magnetic instability in these cores was characterized by ! /

Magnetic properties of deep-sea drilling project basalts from the North Pacific Ocean

Magnetic properties of oceanic basalts from Deep-Sea Drilling Project (DSDP) sites 36, 54, 57, 77, 83, and 84 are described. Secondary components of magnetization were found to be significant at many of these sites. The basalts were placed in deuteric oxidation class 1 on the basis of opaque mineralogy observations. These, together with the analysis of irreversible high-field thermomagnetic curves measured in air, indicated that the dominant magnetic minerals were nonstoichiometric titanomagnetites. The remanent magnetizations were stable against af demagnetization, having median destructive fields (mdf) of 250–550 oe and Königsberger ratios of 10–50. These magnetic properties are compatible with some of the basic assumptions of sea floor spreading. However, the remanent inclinations, although stable, did not give interpretable paleolatitudes for the sites, as has also been observed in other studies of partially oriented submarine basalts. Class 1 basalt from hole 57 had a coarse grain size and lower Curie temperatures than the other basalts. Its remanence was very unstable (mdf of 40 oe) and could be accounted for in direction, intensity, and stability by a viscous remanent magnetization (VRM) acquired in situ during the Brunhes normal polarity epoch. Similar unstable magnetizations have been observed in several other DSDP basalts. The VRM of the oceanic crust is proposed as an explanation of some magnetic ‘quiet zones’.

Theory of the magnetic viscosity of lunar and terrestrial rocks

Lunar materials exhibit two distinct types of viscous or time‐dependent magnetic behavior. Igneous rocks and largely recrystallized breccias, whose magnetic properties are due to multidomain iron, typically have weak magnetic viscosity, but decay persists for very long times following even a brief exposure to a field. Lunar soils and low metamorphic grade breccias, which contain an important fraction of metallic iron of single‐domain and superparamagnetic size, generally acquire an anomalously strong viscous remanence, whose decay time is about equal to the time of exposure to the field. Four theories of magnetic viscosity are reviewed in this paper in an attempt to interpret the very different viscous properties of these two types of rocks. The Richter (1937) and Néel (1949) theories are appropriate to soils and low‐grade breccias, whereas igneous and recrystallized rocks are better described by the multidomain theories of Néel (1950) and Stacey (1963). Both the Stacey and Néel theories correctly predict logarithmic magnetic viscosity, in spite of the fact that the central role played by the internal demagnetizing field in multidomain grains is ignored in Néel's formulation. This apparent paradox has been resolved. Analysis of the particularly simple case of a two‐domain particle shows that the distribution of asymmetrical nonidentical potential barriers required by Néel is automatically generated from the simpler distribution of symmetrical identical barriers proposed by Stacey through the action of the demagnetizing field. Experimental evidence on many facets of viscous magnetization, from terrestrial as well as lunar materials, is reviewed in detail before a final evaluation of the various theories is made. One interesting conclusion is that the magnetic viscosity of multidomain particles, although relatively weak, is still too strong to be explained by displacements of entire domain walls. Either displacements of small wall segments or rotation of pseudo‐single‐domain moments could account for the enhancement of magnetic viscosity in these particles.

Palaeomagnetism of the quaternary sediments of the locality Červený Kopec (red hill)

As the result of the palaeomagnetic research into the loesses and soils of Quaternary age from the locality of Cervený Kopec (Czechoslovakia) it was found that the viscous remanent magnetism (RM) of these rocks exceeds the stable component of the RM. Figures 1–6 show polar graphs and the demagnetization curves corresponding to cases when the rocks sedimented in a magnetic field, the direction of which corresponded to the direction of the present geomagnetic field, or when the X and Z components of the field were of the opposite sense. The positions of the magnetic N pole were computed for the interval of transition from Matuyama's epoch of negative polarity to Brunhes's epoch of positive polarity. At the time of the reversal it was found that the pole moved along an oval at time intervals of about 10 000 years and that their was an apparent west-ward drift of the pole. The reversal found in the Quaternary is compared with the reversal in the Pliocene. The agreement in the pole movements in these two cases seems to indicate constant causes of these large changes in the magnetic field and a possible relation between the pole movement and the boundaries of the lithospheric blocks.

The Age of Viscous Remanent Magnetization of Hadrian's Wall (Northern England)

Summary Following Neel's considerations on magnetic viscosity, an experimental approach is made to determine the age of viscous remanent magnetization (VRM) of certain rocks. The time-dependent critical internal fluctuation field is measured directly using AC demagnetization techniques. There are still some unsolved theoretical problems such as the ambiguous significance of the logarithmic time constant Q and the irreversible susceptibility χirr in the formulas developed by Neel. To prove the proposed method we have used blocks of quartz dolerite (Whin Sill) which were built into the Roman Hadrian's Wall in northern England. The historical age of the wall–constructed during the first centuries after Christ–and the age of VRM calculated from the magnetic measurements coincide fairly well. The method may therefore be applicable to certain dating problems in geology, e.g. to correlate Quaternary rock deposits, and in archaeology, if well-calibrated apparatus and statistical methods are used. Clearly the method cannot be used for ages greater than that of the last main reversal of the geomagnetic field.

Magnetic viscosity, quadrature susceptibility, and frequency dependence of susceptibility in single-domain assemblies of magnetite and maghemite

Neel has derived an expression relating the decay of viscous remanent magnetization and the variation of susceptibility with frequency in noninteracting single-domain assemblies of ferrimagnetic oxides, such as magnetite. This theory is extended to obtain a relationship for the quadrature susceptibility in such a sample. Measurements of these three parameters for soil samples and artificially prepared samples that contain single-domain magnetite or maghemite grains are described; the results obtained are in good agreement with theory.

Preisach Diagrams and Magnetic Viscosity Phenomena for Soils and Synthetic Assemblies of Iron Oxide Grains

Preisach diagrams for soils derived from sedimentary geological strata and for synthetic samples containing dispersed magnetite grains are presented and are used as the basis for distinguishing between superparamagnetic, stable single domain and multidomain grains. In particular it is established that between 10 and 20 percent of the total magnetic constituents in a typical soil are well-dispersed stable single domain grains with diameters of about 250A, the majority of the remainder being superparamagnetic grains with slightly smaller diameters.The Preisach-Neel theory for interacting single domain grains is extended so that the associated magnetic viscosity phenomena can be represented on the Preisach diagram in terms of a fluctuating viscosity field and experimental results justifying the introduction of this concept are included. This presentation provides a clear indication of those areas of the Preisach diagram, and hence the components of the overall magnetic grain distribution in the sample, that are associated with any particular viscosity process. It is thus shown that the value for the coefficients for the decay of viscous remanent magnetism can be used for comparing the magnetic grain size distributions in different samples.

On the applicability of lunar breccias for paleomagnetic interpretations

Many of the breccias returned by the Apollo missions are capable of acquiring a substantial viscous remanent magnetization (VRM) which is of two forms. The first one has an upper limit to the relaxation times of about 100 to 1000min which corresponds to a grain diameter of about 145 A. This suggests that the maximum relaxation time is determined by the transition from superparamagnetic to stable single domain particles. The second form of VRM follows the classical logt dependence typical for multidomain grains with a wide distribution of relaxation times. Hysteresis loop measurements yield the same kind of grain size distributions. In addition the analysis shows a fivefold enrichment of native iron in the breccias and soils as compared to the igneous rocks. In spite of a large VRM some breccias contain a stable remanent magnetization. Its intensity is typically 10−6emu/gm, the same value found for igneous rocks. It is possible, therefore, to use some of the breccias to reconstruct the history of the lunar magnetic field.

Palaeomagnetic Results from Palaeozoic Sediments of Northern Australia

Summary A palaeomagnetic study was made of 52 samples from an Upper Devonian formation and 97 samples from three Cambrian formations. In most cases, the application of demagnetization procedures failed to reveal other than a recent magnetization, presumably acquired by chemical means during weathering processes. Seventeen samples, four of which had been collected a decade earlier, appeared to yield the direction of original magnetization of the Middle Cambrian Hudson Formation. Assigning unit weight to sample mean results, the palaeomagnetic pole position for the period of deposition is calculated to be 19E, 18 N with A,, = 13. The data for the Jinduckin Formation have been re-assessed using sample results, and these give a slightly revised pole position for the Lower Ordovician at 25 E, 13 S with Ags = 11. The first results from a recent palaeomagnetic survey of the Palaeozoic of northern Australia indicated the widespread occurrence of a sometimes dominant secondary magnetization (Luck 1970; McElhinny & Luck 1970). The importance of this effect was revealed during measurements on 149 samples from four other sedimentary formations collected in the same general region (Table 1). The natural remanent magnetization (NRM) of most of these samples was diiected along the Earth's present field. A similarly directed secondary component, presumably representing a viscous remanent magnetization (VRM), had been readily removed from most samples of the Jinduckin Formation by the application of partial demagnetization techniques (Luck 1970). In contrast, the present paper reports that recently acquired magnetization could not be cleaned from the Blatchford Formation, the Hart Spring Sandstone, the Cockatoo Formation or from most samples of the Hudson Formation. This hard, secondary component was taken to represent a chemical remanent magnetization (CRM) imposed by weathering processes. In favourable circumstances, the effect might be neutralized by a chemical method such as devised by Collinson (1965) for separating the red pigment from the specularite of red beds. This technique may be regarded as a means of partial demagnetization (Collinson 1967) if the secondary magnetic component resides in the pigment. Disc-shaped (3.5 cm x 1.0 cm) and cylindrical (2.8 cm x 2-8 cm) specimens were cut from most samples. The discs were investigated using an astatic magnetometer

Palaeomagnetic Studies of Icelandic Lava Flows

Summary This paper describes a palaeomagnetic investigation of 107 igneous bodies, mostly basaltic lavas, of Pliocene and Pleistocene age in Iceland. A variety of conclusions has resulted from the study. Palaeomagnetic directions in lavas erupted in 1729, 1783 and 1875 do not support the postulated ‘cycloid’ path for the Icelandic geomagnetic pole. Data are given for an application of geomagnetic reversal stratigraphy showing that Bering Strait opened more than 3 My ago, allowing Pacific boreal mollusks to enter Icelandic waters; these data also suggest that there were at least nine extensive glaciations since a time over 2 My ago. Lava flows erupted on Snaefellsnes Peninsula, western Iceland, contain large amounts of viscous remanent magnetization; partial demagnetization studies reveal a predominant reversed polarity for most of these lavas with one pronounced ‘excursion’ of virtual geomagnetic poles to very low latitudes. Detailed studies of a dike cutting a lava flow show that the lava's remanence is unaffected beyond one dike width from the dike contacts. Brunhes epoch paleosecular variation is consistent with a model combining a changing nondipole field like the present one with a dipole wobble of about 11°.

Single domain grain distributions I. A method for the determination of single domain grain distributions

An extension of Néel's single domain theory to distributions of single domain grains is given. The method of deducing the form of the distribution involves the measurement of the temperature dependence of initial susceptibility and of the thermal demagnetization curves of isothermal and thermoremanent magnetizations between 77 and 300–400 °K. In addition it is shown that whatever the shape of the distribution, the slope of the acquisition of isothermal remanence can be related to the initial slopes of the variation of isothermal remanence with time and temperature, the thermoremanence with temperature, and the viscous remanence with time. The theory applies to any single domain grain distribution for which the spontaneous magnetization and the coercive force in the absence of thermal fluctuations can be regarded as constant over the above temperature range.