Lengths of geomagnetic polarity intervals

The paleomagnetism of the Deccan Trap lavas in the western Ghats were studied in detail. Several stability tests were performed but the most effective one seemed to be progressive alternating field demagnetization. The direction of stable remanence was determined in 28 Mahabaleshwar and 5 Amboli lavas. It was noted that there is a good correlation between the stability of remanence and certain properties of magnetic minerals.Most rocks with evidence of high temperature oxidation are magnetically more stable than others.As there is no significant serial correlation nor systematic large movement in directions of NRM in Mahabaleshwar lavas, the directions of the NRM may well represent the secular variation in the geomagnetic field in Eary Tertiary.The present data show a scatter of directions larger than the Quaternary paleomagnetic results, suggesting a bigger secular variation at that time. A statistical method to determine the upper limit of the time interval of volcanism was applied to the Mahabaleshwar section with the result that their activity had not lasted more than 5 m.y. at 5 % significance level. The mean pole position for all the lavas in this study shows a good agreement with the hypothesis that India moved northward in the Cenozoic era with great speed.

Section: Estimation Of the Time Interval Of The Deccan Volcanismmentioning

confidence: 99%

Paleomagnetism of the Deccan Trap Basalts in India

Kono

Kinoshita

Aoki

1972

“…The probability density implied by Cox's (1968) model is compared with the observed data and it is shown that the data do not support this model. A statistical model is then developed in which the TDMs have a truncated Gaussian distribution, the non-dipole field strength is linearly proportional to the TDM and the palaeointensity determination itself introduces a random error with standard deviation of about 10%.…”

Section: Introductionmentioning

confidence: 99%

Variations in the geomagnetic dipole 2: Statistical analysis of VDMs for the past 5 million years.

McFadden

McElhinny

1982

107

Virtual Dipole Moments (VDMs) are analysed to determine the distribution of True Dipole Moments (TDMs) from which they were derived. The data set used is all the available VDMs for the past 5 million years which have associated Virtual Geomagnetic Poles (VGPs) with latitudes greater than 45°. These data do not support a model which predicts a cyclic variation of the dipole moment during stable polarity periods. Neither do they support a model in which the field strength of the non-dipole components is a constant ratio of the mean dipole field strength. Instead they support a model in which the TDMs have a truncated Gaussian distribution and the field strength of the non-dipole components is linearly proportional to the TDM. These components introduce a Gaussian distributed scatter to the observed VDMs with standard deviation about 18.6 of the dipole moment, and the palaeointensity determinations themselves introduce a further Gaussian distributed scatter with standard deviation about 10%. The data give no reason to reject the hypothesis of a common mean and variance for the (untruncated) Gaussian distribution of the normal and reverse polarity TDMs but the hypothesis of a common truncation point can be rejected. This indicates a difference in the overall properties of the two polarity states.The term Palaeomagnetic Dipole Moment (PDM) is introduced for the value at which the truncated Gaussian distribution of TDMs peaks. As determined from the present data the PDM for the past 5 million years is 8.67+0.65x1022Am2 with 95c confidence. The estimated standard deviation of the untruncated distribution is 3.63+0.75x1022Am2 with 95c confidence, this variation (41.9c) relating solely to variation in the dipole moment itself. This reinforces the view that fluctuations in the dipole moment are much greater than are observed in data for the past 104 years, suggesting that they must occur over time scales of 105 or 106 years.

“…Our reason for reconsidering the question of the conservation of total energy over long periods of time is its relevance to current attempts to explain geomagnetic reversals on the basis of statistical models (Cox, 1968(Cox, , 1969Nagata, 1969;Parker, 1969). In these models, cyclonic convection cells in the core produce reversals by a two-step mechanism.…”

Section: Introductionmentioning

confidence: 99%

“…In the models of Nagata (1969) and Parker (1969) the occurrence of a reversal depends only on the spatial distribution of cyclones and not on the intensity of the dipole field. In the model of Cox (1968), the occurrence of reversals depends both on the distribution of cyclones and on the field strength of the cyclonic disturbances (i.e. , nondipole field) relative to the dipole field.…”

Section: Introductionmentioning

confidence: 99%

Changes in the Total Magnetic Energy External to the Earh's Core

Verosub

Cox

1971

Self Cite

The loss of magnetic energy accompanying the decrease in the earth's dipole field during the past 50 years has been shown by McDonald and Gunst to have been balanced by an equal increase in the energy of the nondipole field. It is shown that over time intervals of the order of 2000 years, conservation of the total magnetic energy stored outside the earth's core is not indicated by paleomagnetic measurements of ancient directions and intensities of the field. However during the in the total magnetic energy during the past 120 years at an annual rate of 0.01 percent, which is much less than the average rate during the past 1550 years inferred from paleomagnetic measurements.