Paleomagnetic directions from Fuji volcano, Japan: For uncovering the eruption history

Abstract:

Fuji volcano is one of the largest active volcanos in Japan, having erupted mostly basalt in chemistry. Some of the eruptions have been correlated with historical documents. Koyama (2007) identified the geological units of Hoei pyroclastic products, Kenmarui 1 lava flow, Aokigahara lava flow and Takamarubi lava flow to the historical eruptions at CE 1707, CE 937, CE 864-866 and CE 800-802, respectively. Takada et al.(2016) have been revised the geological map of Fuji volcano, and categorized the rock-stratigraphic units into 185, which includes units upto BCE 17,000. However, for more than half of those units, the eruption ages were not precisely by geological methods, so that we have carried out the paleomagnetic direction measurements from volcanic products on Fuji volcano. It enables us to reassign the ages of some eruptions by paleomagnetic method.

The samples were collected from 170 sites (65 units) of lava flows, pyroclastic flows and pyroclastic cones of mostly known eruption ages (Takada et al., 2016). As a result, the paleomagnetic directions mostly agree with archaeomagnetic secular variation in Japan during the last 1600 years. One of the historical eruptions, Aokigahara lava flow shows paleomagnetic directions shallower than archaeomagnetic secular variation curve of corresponding age. It is, however, fit with the recent archaeomagnetic results of old kilns in the 9^th century within a95 range (Hatakeyama and Kitahara, 2019). The paleomagnetic directions of Takamarubi lava flow, which has been assigned CE 800-802, fit with several other lava flows erupted between CE 580 to 700 from NE-SW eruptive fissures. There are possibilities that the 5 units, showing similar petrological features, were simultaneous eruptions, and thus older than the previous thoughts. The paleomagnetic directions of Kenmarui 1 lava flow, which has been assigned CE 937, show age range of CE 1000-1030 indicating simultaneous eruption with Fudosawa lava flow. Both of those age coincidences are seen between the lava flows of northern and southern flanks. They may imply that Fuji volcano has a tendency of simultaneous eruption in the opposite sides.

Our findings suggest that paleomagnetic method can improve eruption history of Fuji volcano.