Paleomagnetic Secular Variation for the Last 2 million Years form Young Volcanic Rocks

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Precise dating of young volcanic rocks by K-Ar method was attempted to offer the basis of detailed paleosecular variation of the geomagnetic field. Samples for dating and paleomagnetic measurements were collected in the Zao Volcanic Group.Most of eighteen andesite lavas and dikes gave reliable radiometric ages, but exceptions are the dike "Roba no Mimi Rock" samples in which the ages of about 1 Ma were reported by the previous studies. Apparently some excess argon trapped in the dike samples is the cause of unsuccessful dating experiment. Review and summary were also given to unpublished age data for five volcano groups in Tohoku Japan obtained from the mass spectrometer of Yamagata University.Andesite lavas and dikes were collected for paleomagnetic study at sixteen sites in Zao Volcano. All samples are stable to AF and thermal demagnetizations, and 16 paleomagnetic directions were successfully determined in which the ages cover from 0.07-1 Ma. Paleointensity experiment by the Coe's version of the Thelliers' method was also made to the most stable 50 specimens selected from eight sites. However, the results were not as successful as the directional data. In spite of fairly high stability of remanence to both AF and thermal demagnetizations, half of the samples showed unreliable behavior in the paleointensity experiments. For one lava flow, Sainokawara lava of 0.21 Ma, two series of experiment were made in different inducing fields of 50 and 20 mT to five specimens in each experiment. Agreements were fairly good between the two sets except for one specimen.Statistical secular variation study was made for paleodirectional data set in Brunhes age including the newly obtained data in this study. The degree of the geomagnetic secular variation was about twice as large as the historical secular variation for the last 400 years.