Abstract
Vulcanian activity is one of the most common eruption styles of arc andesitic volcanism on Earth. It ejects and deposits volcanic bombs around the source crater. Although paleomagnetic studies of volcanic bombs are limited, such studies can potentially provide more opportunities for high-resolution paleomagnetic dating of volcanic activity. In this study, paleomagnetic dating was applied to large (> 1 m) volcanic bombs around active craters in the Azuma volcano group, NE Japan. Oriented samples were collected from the interior parts of five large volcanic bombs situated on gentle slopes, a few hundred meters from the source crater. More than six core samples were collected from each bomb and all samples were subjected to a range of rock magnetic experiments, including anisotropy of magnetic susceptibility (AMS) and thermal/alternating field demagnetization (THD/AFD) analyses. The Characteristic Remanent Magnetization (ChRM) directions for specimens from all bombs were well-defined, have small α95 (< 2.5º), and are in close agreement with each other. Comparing our measured overall mean direction (Dm = 355.5º, Im = 49.8º, α95 = 1.6º) with modeled geomagnetic field estimates and a reference secular variation curve for this area (using MATLAB-based archaeomagnetic dating tool), we suggest that the volcanic bombs were produced in the historical Meiji period (1893–1895 CE) eruption. In addition, a combination of the data of ChRM, AMS, thermomagnetic analyses, hysteresis measurement, and XRF analysis indicates that the volcanic bombs were derived from a plug of lava in the conduit under the solidification point (ca. 800 °C), but above the Curie point of the titanomagnetite remanence carrier (around 300 °C). We show that volcanic bombs can be powerful for paleomagnetic dating if certain sampling conditions, such as quantity, situation, size and portion are satisfied.Graphical
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