© Akira Baba
An instrument that measures geomagnetism is seen embedded in lava near Mount Fuji.
Mount Fuji may have erupted twice in close succession roughly a thousand years ago, according to a team of researchers at the Yamanashi Prefectural Government Mount Fuji Research Institute who used a geomagnetic dating technique.

Previous research using various methods has revealed that Mount Fuji has erupted 42 times over the last 2,200 years. It appears that there are stretches of time where the volcano erupted every few decades. However, even with estimates using data from sources such as ancient documents and geological surveys, there are still many eruptions that have yet to be dated.

Akira Baba from the Yamanashi research institute and his colleagues collected samples from lava flows at 380 locations around the foot of Mount Fuji, and analyzed the magnetic iron ore in the rock for the first time. When the ore is heated to high temperatures and then cooled, it records the geomagnetic strength and direction of the Earth's magnetic field at that particular time. Lava is heated to roughly 1,000 degrees Celsius during an eruption before cooling, so the scientists can examine the geomagnetic data documented in the ore to estimate when a given eruption occurred.

The analysis revealed that the data from lava on the north side of the volcano in Yamanashi Prefecture was a perfect match to that of a flow on the south side in Shizuoka Prefecture.This suggests that in approximately 1015, during the Heian Period, two large-scale eruptions occurred within a short period of time on both sides of the mountain.

The results have a variety of implications. Currently, the Yamanashi Prefectural Government's disaster management plans do not include cases of simultaneous or closely spaced eruptions. If one similar to the eruption around 1015 were to occur, the size of the effected areas would expand beyond previous estimates. It also means it is possible for areas rebuilding after recent eruptions to face yet another eruption soon after.

"If we can gather more evidence of a simultaneous eruption, then it will affect disaster countermeasures," Baba said. Geomagnetic dating still has a short history of use in volcanic research, but he expressed that the research group "would like to continue to increase the accuracy of the research."