Structure of Paleosecular Variation : Relation to the Core Dynamics

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Experimental studies with a newly introduced thermal demagnetizer were made on many volcanic rocks from the Unzen Volcano and the Ontake Volcano, Japan. Angular standard deviation(ASD) of the paleosecular variation(PSV) in the Brunhes Chron obtained from Unzen was in the same level as those from other areas of Japan, but it was larger for Older Unzen than Younger Unzen. Paleomagnetic methods were also applied to some volcanostratigrphic problems ; estimation of emplacement temperature of pyroclastic flows and lava identifications. One of the most important results obtained from the paleomagnetic studies of the Ontake Volcano is the establishment of the geomagnetic excursions in the latest Pleistocene in Japan. Reliability of the excursions is very high because they were found from two andesite lava flows, in which stabilities of the remanences are much higher than those from sediments. Paleointensities obtained from one of the two lava flows are abut 1/10 of the present-day value, and the associated virtual geomagnetic poles(VGPs) locate near the central Pacific Ocean. As the VGP positions in the central Pacific were also reported from the Auckland excursion in New Zealand, it was proposed that the equatorial dipole moment becomes dominant during the excursion. To explain the structure of PSV, a new statistical model was developed, in which Gaussian fluctuation was given to an eccentric dipole. This simple model explains most of the structures of PSV ; latitude dependent ASD and the shape of the histograms of the paleointensities.