Onuma and Mt. Komagatake
Mt. Komagatake sedately stands tall over Onuma.
The history of Mt. Komagatake is the history of repeated major eruptions.
［Contrast between Onuma and Mt. Komagatake］
The beauty of Lake Onuma, Lake Konuma and Lake Junsai in Onuma Quasi-National Park is enough to enchant visitors by itself, but combined with the contrasting beauty of Mt. Komagatake, the beauty becomes even more eye-opening. If you want to look back at the history of Onuma Quasi-National Park, you need to learn how Mt. Komagatake was formed.
The oldest record of Mt. Komagatake concerns the major eruption in 1640 (Kanei eruption, Matsumae History, date of writing unknown). Based on such records and geographical research results, we know that the Kanei eruption began with the rumblings in the mountain that lasted for several hours. After that, a part of the area around the summit collapsed, and a debris avalanche occurred. The debris avalanche flowed into Funka Bay, which caused a major tsunami that killed more than 700 people on the coast. After three days of volcanic activity, the eruption rapidly abated and things calmed down in around 70 days. One theory has it that the debris avalanche that hit the southern foot of the mountain blocked the Orito River and other streams to form Lake Onuma and Lake Konuma.
［History of Mt. Komagatake’s eruptions］
Following the Geological Features of Nanae Town published by Nanae Town in 1964, the formation of Mt. Komagatake and the process of eruptions are assumed here. With repeating eruptions, Mt. Komagatake grew to a large cone-shaped volcano at an altitude of 1,700 meters around 500,000 years ago. A major eruption 50,000 to 300,000 years ago collapsed the upper third of the mountain to form almost the same shape as the current mountain (refer to Eruptions and Mudflow Hills). It is considered that mudflows from the eruption stemmed the flow of the rivers at the foot of the mountain to form Former Lake Onuma. Then, several eruption activities divided Former Lake Onuma into Lake Onuma, Lake Konuma and Lake Junsai. After a dormant period of around 25,000 years, the volcano spewed pyroclastic materials and flows 6,000 years ago and then entered a dormant period of more than 500 years again. After spewing pyroclastic materials and flows 5,500 years ago and then entering a long dormant period of around 5,000 years, the volcano resumed its activity in the Edo era. Today, Mt. Komagatake is in its fourth active period.
The first eruption of this fourth active period is the abovementioned Kanei eruption, which formed the current appearance of the mountain. Over the last several dozen volcanic activities, the summit area collapsed, and the highest peak, Kengamine (1,131 m), Sawaradake (1,113 m) and Sumidamori (892 m) on the outer rim of the crater floor as well as Komanose (approx. 900 m), Umanose (approx. 850 m) on the ridge line were formed. Mt. Komagatake has erupted a dozen times over around 350 years since the Kanei eruption, with the latest one a small eruption on October 25, 1998. The number of major eruptions between 1640 and 1998 is four including the Kanei eruption.
The second major eruption occurred on September 25, 1856 (Ansei eruption). On the day, a violent eruption started at around 9:00 in the morning, and sent pyroclastic materials on to the eastern foot of the mountain, burning out 17 houses and resulting in two deaths and numerous injuries. On the southeastern foot of the mountain, pyroclastic flows followed pyroclastic materials, resulting in more than 20 deaths. This eruption created a crater (Ansei crater) 200 meters in diameter at the summit, in which a lava dome was formed.
The third major eruption occurred in June 1929, 73 years after the Ansei eruption, causing a catastrophic situation with pyroclastic flows. The amount of volcanic ash and pumice that spewed from the mountain was twice as much as when Mt. Unzen Fugendake in Nagasaki erupted, and it all spewed within a day. The scale of the explosion must have been beyond imagination. The mountain rumbled on June 15, two earthquakes occurred on June 16, and a small eruption started at around 0:30 before dawn on June 17. The small eruption turned into a violent pumice eruption at around 10, which sent an ash column up to an altitude of 14,000 meters. The eruption activity further intensified at around 12:30 and caused pyroclastic flows that lasted until around 24:00 on the same day. After that, the intensity rapidly decreased and the eruption activity ended at 3:00 on June 18.
On the following day, rain sent mudflows to the foot of the mountain, increasing the damage. Shikabe Village (today’s Shikabe Town) was covered with more than one meter of ash and pumice. Although the number of deaths was only two, the eruption demolished 365 houses, partially destroyed 1,500 houses and wreaked enormous harm on livestock animals, cultivated fields and fishing grounds. It also filled the Ansei crater and created a new large crater (230 m in diameter).
Along with a rumbling, an eruption started at around 8:00 in the morning on November 16, 1942, 13 years after the major eruption in 1929. Smoke from the eruption reached a height of 8,000 meters and pillars of fire were observed around the crater. A pyroclastic surge also occurred, but fortunately it did not reach the foot of the mountain, and did not cause serious damage although a few centimeters of volcanic ash covered the area. However, an around 1,600 meter-long chasm was formed from north-northwest to south-southeast on the crater floor at the summit.
