*The contents may be subject to change without prior notice.

 

K. Z. Nanjo¹⁾ and M. Yoshimoto²⁾

1) NaDiR, Global Center for Asian and Regional Research, University of Shizuoka

2) Mount Fuji Research Institute, Yamanashi Prefectural Government

 

Eruptions of Mt. Fuji can be divided into two types. One is an explosive eruption that spews volcanic ash into the air, which eventually falls down and covers the ground. The other is a non-explosive eruption that produces lava flow on the ground. The mechanism of these eruptions can be explained by likening Mt. Fuji to a beer bottle (Fig. 2-3).

 

Fig. 2-3: The process of a volcanic eruption. An explosive eruption on the left and a non-explosive eruption on the right.

 

Immediately after a beer bottle is opened, a lot of carbon dioxide gas remains trapped in the beer. If you shake the bottle, the beer will come shooting out of the top. On the other hand, if you open a beer bottle and leave it for a while, carbon dioxide gas will escape from the beer, so even if you shake the bottle, the beer will not bubble. An explosive eruption can be likened to the shaking of a beer bottle that contains a lot of carbon dioxide gas, and a non-explosive eruption to the shaking of a beer bottle after carbon dioxide gas is released. If gas is released as the magma is rising, the eruption will be non-explosive, producing lava flow only. On the other hand, if gas remains trapped in magma, some impact, like the opening of a beer bottle, can cause the magma to erupt explosively. If we know how gas is escaping from magma before an eruption occurs, we can mitigate the damage, but this is not yet possible at the current level of science.

Because it is almost impossible to predict whether an eruption will be explosive or non-explosive, we have to develop disaster risk mitigation plans to prepare for both eruption types. The speed of lava that would flow from Mt. Fuji is roughly equal to walking speed, so you might think you could safely flee on foot if the eruption is non-explosive. But even if lava flows slowly, it may not be possible to easily evacuate, for there is the risk of being surrounded by lava flow in all directions, making it impossible to move.

According to documents dating to 864 to 866, the Jogan Eruption formed long cracks on the slope of Mt. Fuji, which lava then flowed down (Fig. 2-4). The lava filled a lake called Senoumi and created a lava plateau on which the Aokigahara Forest grew.

 

Fig. 2-4: The Jogan Eruption formed long cracks on the slope of Mt. Fuji (red dashed line), causing lava to flow down along the cracks. The lava filled a lake named Senoumi (black dotted line), where Lake Saiko and Lake Shojiko were formed later. Lava also reached Lake Motosuko, and created a basis on which the Aokigahara Forest grew.

The 1707 Hoei Eruption is a typical example of an explosive eruption that spewed volcanic ash into the air (Fig. 2-5). Volcanic ash released by this eruption was deposited to a depth of 2 centimeters in Tokyo, dozen centimeters in Yokoyama, and 2 meters at the foot of Mt. Fuji. The eruption also created a new crater on the slope of Mt. Fuji, which was larger than the summit crater of the mountain. Some researchers have studied the day-by-day change in the height of ash plumes by analyzing various old records and the characteristics of ash deposited on the ground (Fig. 2-6). Their results showed that the eruption that began on December 16 sent ash plumes about 15 to 20 kilometers into the air.

 

Fig. 2-5: Distribution of ash deposition following the Hoei Eruption. The numbers on the curved lines indicate the thickness of ash deposited on the ground in centimeters. For example, line 8 connects the spots where ash is considered to have been deposited to a depth of 8 centimeters.

 

Fig. 2-6: The height of ash plumes in the Hoei Eruption.

Old documents also record various phenomena observed before the Hoei Eruption. For example, on October 28, 1707, 50 days before the Hoei Eruption, the Hoei Earthquake occurred at the Nankai Trough, and then noises began to be heard at Mt. Fuji, some 10 days before the Hoei Eruption. From December 15, the day before the eruption, an increasing number of earthquakes were felt, followed by some larger earthquakes, and then the Hoei Eruption occurred on December 16. According to records, these precursory phenomena were observed before the Hoei Eruption, though we can’t tell what will happen before a future eruption of Mt. Fuji.