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Paleointensity of the geomagnetic field was studied with precisely dated lava flows and pyroclastic flows in Japan older than 3, 000 years B. P. by the Thellier's stepwise heating method. The errors inherent in the ages of the rocks were fully evaluated in this work. Samples collected from five lava flows and three pyroclastic flows provided successful results. Two examples were found indicating that the apparent linear NRM-TRM relation in the Arai diagram is probably due to the effect of TCRM produced during heat treatment. This phenomenon was detected by comparing the result with those from other specimens taken from the same flow and treated with the same procedure in the Thellier's experiment simultaneously with two samples mentioned above. This result strongly suggests that the Thellier's experiment should be done by using multiple at least three specimens. Japanese data including the present results are consistent to already proposed trend of secular variation in which geomagnetic field intensity gradually increased from about 6, 000 years B. P. to 2, 000 years B. P. and then gradually decreased to the present-day value. Secular variation of shorter periods also seems to exist superposed on that of a longer period.

Geomagnetic paleointensities for the period 6,000 to 3,000 years B.P. determined from lavas and pyroclastic flows in Japan.