Evidence for geomagnetic field excursions and secular variation from the Wrangell Volcanics of Alaska

1976 ◽  
Vol 13 (4) ◽  
pp. 547-554 ◽  
Author(s):  
D. K. Bingham ◽  
D. B. Stone
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Lava Flows ◽  
Dipole Field ◽  
Common Occurrence ◽  
Late Tertiary ◽  
Field Activity ◽  
Field Excursion ◽  
Geocentric Axial Dipole ◽  
Final Flow

Paleomagnetic studies have been made on 36 late Tertiary lava flows (3–4 m.y.) from the Wrangell Volcanics. Final flow mean remanent magnetization directions show excursions of the geomagnetic field away from a mean corresponding to a geocentric axial dipole field. They also point to the possibility that such excursions may have been a more common occurrence at the time of extrusion of these lavas than appears to have been the case in Quaternary times. These excursions may be due to increased non-dipole field activity. Calculation of the paleosecular variation including field excursion data leads to high values of PSV which do not agree with existing models. Exclusion of field excursion data gives a result that is consistent with current PSV models, but does not allow differentiation between them.

V. The remanent magnetization of unstable Keuper Marls

1957 ◽  
Vol 250 (974) ◽  
pp. 130-143 ◽  
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Laboratory Experiments ◽  
Natural Remanent Magnetization ◽  
Single Domain ◽  
Dipole Field ◽  
Main Field ◽  
Axial Dipole ◽  
Geocentric Axial Dipole ◽  
Three Components

Measurements of the directions and intensities of magnetization of Keuper Marls from Sidmouth are described. The natural remanent magnetization of these rocks is shown to be unstable in the geomagnetic field. Certain laboratory experiments are described which show the natural remanent magnetization to consist of three components, a primary component created on, or soon after, deposition, in the same direction as that of the natural remanent magnetization of Keuper Sandstones and Marls described by Clegg, Almond & Stubbs (1954); a secondary component in the direction of a geocentric axial dipole field in Britain acquired since the last reversal of the main field and a temporary component built up by the geomagnetic field between collection and measurement. The temporary and secondary components are believed to be isothermal remanent magnetizations and to be due to the red haematite cement. Application of Néel’s theory of the magnetization of small single-domain particles shows that haematite grains of less than 0·15 μ in diameter will be magnetically unstable. The temporary and secondary components of magnetization are explained in terms of Néel’s theory. A suggested test of stability is described.

A Palaeomagnetic Investigation of the Lundy Dyke Swarm

1957 ◽  
Vol 94 (3) ◽  
pp. 187-193 ◽  
Author(s):  
D. J. Blundell
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Lava Flows ◽  
Dyke Swarm ◽  
Upper Oligocene

AbstractThe directions of the natural remanent magnetization of samples collected from dykes on Lundy have been measured and related to those of Tertiary lava flows in Northern Britain. Evidence is given suggesting that the dykes are Tertiary and pre-Upper Oligocene in age, and that the geomagnetic field was reversed at the time of their intrusion.

The characteristics and significance of secondary magnetite in a profile through the dike component of the Troodos, Cyprus, ophiolite

1987 ◽  
Vol 24 (11) ◽  
pp. 2141-2159 ◽  
Author(s):  
J. M. Hall ◽  
B. E. Fisher
Keyword(s):  
Low Temperature ◽  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Common Occurrence ◽  
Thermal Metamorphism ◽  
Alteration Processes ◽  
Optical Evidence ◽  
Curie Temperatures ◽  
Different Parts

Secondary magnetite (SM) is the dominant Fe–Ti oxide over a 3–4 km thick vertical interval in a profile through the Troodos, Cyprus, ophiolite. SM is important in dikes and flows where dike density exceeds about 30% within the transition from extrusives to sheeted dikes and remains important until dike density falls below about 30% in the transition from sheeted dikes to plutonics. The association of SM with high dike density suggests that it is a product of the thermal metamorphism of hydrothermally metamorphosed dikes and flows. The most common occurrence of SM is at the site of primary titanomagnetites that had been altered earlier through deuteric, low-temperature, and regional hydrothermal alteration processes. SM often closely pseudomorphs the form of primary magnetite (PM) grains, but it can be distinguished from PM using a series of criteria. SM is low in TiO2 and other impurities. Estimated and observed Curie temperatures are undistinguishable, which, combined with optical evidence, indicates that SM is probably approximately stoichiometric.SM may also be present at depth in in situ oceanic crust, in other ophiolites, and in other hydrothermally altered volcanic sequences where the density of minor intrusions is sufficiently high.The formation of SM leads to the annealing of the features of altered PM that provide the ability to retain strong, stable remanent magnetization. In appropriate combinations of the timing of formation of SM and of geomagnetic field reversals, the different parts of a volcanic sequence may be magnetized with opposite polarities.

MAGNETIC ORIENTATION OF BOREHOLE CORES

Geophysics ◽  
1969 ◽  
Vol 34 (5) ◽  
pp. 772-774 ◽  
Author(s):  
M. Fuller
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Standard Procedure ◽  
Natural Remanent Magnetization ◽  
Core Sample ◽  
The Core ◽  
Viscous Component ◽  
Late Tertiary ◽  
Geomagnetic Pole ◽  
Orientation Method

The use of natural remanent magnetization (NRM) to orient boreholes was reported at least thirty years ago (e.g., Lynton, 1938). The method depends upon determining the direction of remanent magnetization of the sample and relating it to the geomagnetic field in which the rock was presumed to have been magnetized. If the NRM faithfully records the relevant geomagnetic field and the field is known, the orientation of the core sample is available. Unfortunately, the ancient geomagnetic pole positions are not, in general, sufficiently well known to make this method particularly successful for rocks that are older than late Tertiary. Moreover, the presence of a weak, viscous component of magnetization parallel to the present geomagnetic field at a given site may produce erroneous results unless it is recognized and eliminated. However, the existence of this component provides another means of orienting the core sample. Isolation of this component might initially appear to be difficult, but it is actually a standard procedure of paleomagnetism; and, in fact, many studies have implicitly demonstrated that the direction of the present geomagnetic field at the sample site is recoverable (e.g., As and Zijderveld, 1958; Zijderveld, 1967). Indeed a number of people have recognized the possibility of using this component to orient borehole samples (e.g., Hargraves, 1969—private communication). The use of this soft viscous component has been advocated recently to distinguish between normal and reversely magnetized rocks in connection with tests of the sea floor spreading hypothesis (Irving and Roy, 1968). Nevertheless, no explicit demonstration of the technique of orienting borehole cores has been published. In the course of paleomagnetic surveys, our demagnetization studies have revealed a number of examples of behavior which makes the orientation method possible. This note describes such behavior and explains how the orientation might be recovered.

Challenging the dipolar paradigm for Proterozoic Earth

2022 ◽  
Author(s):  
James W. Sears
Keyword(s):  
Magnetic Field ◽  
Geomagnetic Field ◽  
Inner Core ◽  
Ultraviolet B ◽  
Late Permian ◽  
Early Paleozoic ◽  
Thin Spherical Shell ◽  
Late Neoproterozoic ◽  
Geocentric Axial Dipole ◽  
Drift Path

ABSTRACT A robust, geology-based Proterozoic continental assembly places the northern and eastern margins of the Siberian craton against the southwestern margins of Laurentia in a tight, spoon-in-spoon conjugate fit. The proposed assembly began to break apart in late Neoproterozoic and early Paleozoic time. Siberia then drifted clockwise along the Laurussian margin on coast-parallel transforms until suturing with Europe in late Permian time. The proposed drift path is permitted by a geocentric axial dipole (GAD) magnetic field from Silurian to Permian time. However, the Proterozoic reconstruction itself is not permitted by GAD. Rather, site-mean paleomagnetic data plot ted on the reconstruction suggest a multipolar Proterozoic dynamo dominated by a quadrupole. The field may have resembled that of present-day Neptune, where, in the absence of a large solid inner core, a quadrupolar magnetic field may be generated within a thin spherical shell near the core-mantle boundary. The quadrupole may have dominated Earth’s geomagnetic field until early Paleozoic time, when the field became erratic and transitioned to a dipole, which overwhelmed the weaker quadrupole. The dipole then established a strong magnetosphere, effectively shielding Earth from ultraviolet-B (UV-B) radiation and making the planet habitable for Cambrian fauna.

Manifestation of the Gothenburg geomagnetic field excursion in the Barents Sea bottom sediments

2007 ◽  
Vol 47 (6) ◽  
pp. 781-786 ◽  
Author(s):  
E. G. Guskova ◽  
O. M. Raspopov ◽  
A. L. Piskarev ◽  
V. A. Dergachev
Keyword(s):  
Bottom Sediments ◽  
Geomagnetic Field ◽  
Barents Sea ◽  
The Barents Sea ◽  
Sea Bottom ◽  
Field Excursion

scholarly journals Possible consequences of a dwindling geomagnetic field

1971 ◽  
Vol 2 (3) ◽  
pp. 46
Author(s):  
I.K. Crain
Keyword(s):  
Geomagnetic Field ◽  
Dipole Field

Recently, Facer (1971) has discussed the problem of the dwindling of the geomagnetic field, and estimated that in 810 years (AD 2781) the dipole and non-dipole fields will be roughly equal and that in 1931 years (AD 3902) the dipole field will be essentially zero. Various authors (Crain and Crain, 1970; Cox, 1968; Parker, 1969) have suggested that these conditions are highly favourable for the production of geomagnetic reversals.

Sign in / Sign up

Export Citation Format

Share Document