Abstract
Abstract A paleomagnetic study was carried out on volcanic rocks from Unzen volcano: samples were collected from a total of 69 sites with 19 sites in pyroclastic flows and 50 sites in lava flows. Ages for the flows were determined either by K-Ar methods or detailed field surveys, and indicate that all of the flows were deposited during the Brunhes chron. After demagnetization 10 pyroclastic and 48 lava flows had stable site-mean directions. One lava flow in the Senbongi area with a K-Ar age of 197±17 ka had an intermediate virtual geomagnetic pole (VGP) at 8.3°N, 21.6°E. The age and VGP position apparently correlate with the Iceland Basin event, suggesting that it is a record of the event on volcanic rocks. Our study added 53 Brunhes-aged VGPs to the overall collection for Japan, increasing it by 40% to a total of 175 VGPs. After excluding all site-mean directions having α95 > 10° and VGP latitude < 50°, the remaining 148 VGPs have a mean pole at 89.7°N, 40.9°E (A 95 = 2.2°) showing no significant deviation from the geographic pole. The angular standard deviation (ASD) was calculated as 15.2° ± 1.2° (N = 148), which is compatible with paleosecular variation models from the literature. However, the data set was found to deviate from a Fisher distribution. The actual meaning of the ASD value after removing the intermediate VGPs needs to be reconsidered.
Highlights
Unzen volcano, which is situated on the western side of Kyushu Island, Japan, was active during the period from 1991 to 1995 (Nakada et al, 1999; Hoshizumi et al, 1999)
As expected from the lava flow ages, all but one of the site-mean directions was of normal polarity, i.e., the corresponding virtual geomagnetic pole (VGP) latitudes were greater than 50◦N
The exception was the direction for site UZ09 which has a VGP latitude as low as 8.3◦N
Summary
Unzen volcano, which is situated on the western side of Kyushu Island, Japan, was active during the period from 1991 to 1995 (Nakada et al, 1999; Hoshizumi et al, 1999). We have found a reliable record of a geomagnetic excursion in a lava flow from Unzen volcano.
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