In general, detailed geomagnetic field records have been used in investigations of hydrodynamic dynamo processes occurring within the Earth's outer core and as master curves for archeomagnetic dating of ancient artifacts and habitation sites. Virtual geomagnetic poles (VGPs) can be calculated from the directional data using the dipole formula but have only regional applicability due to the nondipolar contribution to the total field direction. Unfortunately, measurement of J requires time-consuming experiments, so most often only directional changes are determined. At any given locality, the SV process is characterized by changes in the magnetic field's inclination ( I), declination ( D), and intensity ( J). There are other more complex interpretations of the historical SV record that incorporate, for example, eastward and poleward drifting features of the nondipole field, standing foci with increasing and decreasing magnitudes, as well as random changes in the nondipole field. Such changes in the historical geomagnetic record have been primarily due to westward drift of nondipole sources that create isoporic foci (maxima and minima anomalies) in the surface field. Variation of the surface geomagnetic field due to changes in th Earth's internal field, over periods of a year to millions of years, is called secular variation (SV) or paleosecular variation (PSV) for prehistoric times. The new PSVL record provides a partial Holocene master curve for western North America and will be of particular value in dating geological and archeological materials using paleomagnetic directions. The PSV record from lava flows (PSVL), however, is perhaps more reliable in its general shape and chronology because of the higher fidelity of volcanic rocks as magnetic field recorders and because of the greater density of 14C dates. The younger segments of the curve derived from volcanic rocks are similar but less complex than other high-resolution PSV curves derived from lacustrine sediments, particularly the record at Fish Lake, Oregon. A curve defined by “spherical smoothed splines” is fitted to the VGPs, ranked by the quality of the age determinations, where the data density is highest between 3690 and −30 years before present (B.P.) (A.D. Approximately 60% of the paleomagnetic database has been previously published. A paleosecular variation (PSV) curve for western North America is presented on the basis of 94 virtual geomagnetic poles (VGPs) from dated volcanic rocks sampled at 446 sites.
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