Tenmei Great Eruption (Mount Asama) (1/2)The great eruption of Mount Asama that led to a great famine

Mount Asama, located on the border between Tsumagoi Village in Gunma Prefecture and Karuizawa Town and Miyota Town in Kitasaku District, Nagano Prefecture, is one of Japan's most famous active volcanoes. Mount Asama erupted violently between May and August 1783 (Gregorian calendar) during the Edo period. This eruption, known as the "Tenmei Great Eruption" or "Tenmei Asama Burn," caused many deaths, mainly in Tsumagoi Village, and was also one of the causes of the Tenmei Great Famine. In this article, we will provide an easy-to-understand explanation of the Tenmei Great Eruption.

Mount Asama, one of Japan's most active volcanoes

Mount Asama is an active volcano with an elevation of 2,568m, located on the border between Tsumagoi Village, Gunma Prefecture, and Karuizawa Town and Miyota Town, Kitasaku District, Nagano Prefecture. It is a cone-shaped stratovolcano formed by the accumulation of lava, pyroclastic flows, volcanic ash, and pumice, and has been worshiped as a sacred mountain since ancient times. It has been selected as one of the 100 most famous mountains in Japan, and is a popular mountain among climbers.

Mount Asama is a triple volcano consisting of "Kamayama" where the crater is located, "Maekakeyama" which is the second outer rim, and "Kurobanzan", "Kisayama", "Kengamine", and "Jabone-dake" which are the first outer rim. Kamayama is a dangerous area and is always off-limits, but Maekakeyama can be climbed or not depending on the eruption alert level. As of September 2024, eruption alert level 2 (restrictions around the crater) has been issued for Mount Asama, so it is currently not possible to climb it.

According to research, Mt. Asama's first outer rim, Mt. Kurofukuyama, became actively active about 100,000 years ago, growing into a stratovolcano. Mt. Kurofukuyama exploded, causing the eastern half of the summit to collapse. Next, the flat Hotokeiwayama (also known as Midagajoiwa) was formed, and activity began around 20,000 years ago. About 10,000 years ago, Mt. Maekakeyama, which formed in the crater of Mt. Kurofukuyama, became active, and a central crater cone was formed inside Mt. Maekakeyama. This grew into the present-day Kamayama due to the Great Tenmei Eruption.
　　
Busan is still active today, having erupted more than 50 times in the past 100 years, producing volcanic ash, cinders, and small pyroclastic flows. The most recent medium-sized eruption occurred in September 2004, producing cinders and volcanic gravel. Volcanic ash reached Chiba, Fukushima, and Yamagata prefectures, damaging agricultural crops mainly in Gunma prefecture. In August 2019, a small-scale phreatic eruption occurred.

The oldest record of an eruption is in the "Nihon Shoki"

Mount Asama is a volcano that has erupted many times since prehistoric times, but the oldest record of an eruption is in the "Nihon Shoki," which states that it occurred in March of the 14th year of Emperor Tenmu's reign (685) during the Asuka period. The passage that reads, "In this month, ash fell on the land of Shinano, and all vegetation withered" is believed to be an eruption of Mount Asama (there are various theories).

Subsequent large-scale eruptions occurred in 1108 and 1128. The eruption in 1128 caused volcanic debris to fall on the land of Kozuke (Gunma Prefecture), causing extensive damage to crops.

Mount Asama erupted repeatedly during the Sengoku period, and documents from that time show that the eruption in February 1582 could be seen from Nara and Kyoto. In fact, this eruption occurred right in the middle of Oda Nobunaga's Koshu campaign to destroy the Takeda clan. The last time it erupted was in 1534 (Tenbun 3), so the first eruption in about 50 years upset the Takeda army, to the point that some of the people fled.

In fact, the eruption of Mount Asama has long been said to be a sign of impending danger in the eastern provinces, and was a symbol of misfortune and disaster. In addition, the Tamon-in Diary, written by monks in Nara, states that the strange events in the eastern provinces were caused by the Emperor's prayers washing away the guardian deity of Takeda Katsuyori, an enemy of the imperial court.

Because these legends were believed, the eruption of Mount Asama led to a decline in morale among the Takeda troops, and some even defected to the Oda side. This means that the eruption of Mount Asama contributed to the downfall of the Takeda clan.

The Tenmei Eruption, the largest eruption in the Edo period

Mount Asama continued to erupt frequently even during the Edo period, but the largest of these was the "Great Tenmei Eruption," which lasted from May 9th (Gregorian calendar) to August 5th in 1783 (Tenmei 3). After about 90 days of high activity, Mount Maekake was turned into a sea of fire, and pyroclastic flows and debris avalanches occurred. More than 1,000 people were killed, and the accompanying river flooding and ashfall caused extensive damage, especially in the Kanto region.

According to records from that time, the first eruption occurred on May 9th, and after a brief period of dormancy, on June 25th a column of black smoke rose accompanied by sounds and rumbling, and volcanic ash began to rain down. There was another brief period of dormancy, and on July 17th rumbling and ashfall were observed. Full-scale activity began on July 21st, with intermittent eruptions, with volcanic ash even flying as far as the Tohoku region. In particular, there are records of the eruption on July 28th causing doors and paper screens to shake in Edo and causing ash to rain down, which shows the shock of the common people of Edo.

A series of violent eruptions occurred from the night of August 2nd to the early morning of the 3rd. The sun was pouring down on Edo, rumblings were felt as far as Nagoya, and volcanic thunder and cinders turned Maekakeyama into a sea of fire. There were further eruptions from the afternoon of August 3rd to the morning of the following day, causing fires to fall. Ash is said to have flown as far as Choshi (present-day Choshi City, Chiba Prefecture).

The violent eruption on August 4th sent pyroclastic flows along the northern foot of the mountain, and in the central Kanto region, the sky was darkened by the heavy rain of ash. There is a record that says, "It became dark, so we lit lanterns and paper lanterns," which shows that a considerable amount of ash was falling.

Volcanic activity reaches its peak from the nights and mornings of the 4th to the 5th of August. Extremely violent eruptions cause large explosions, pillars of fire rise, the sky turns red, volcanic thunder rumbles, and pyroclastic debris falls. These scenes have been observed in various locations. Earthquake-like rumblings have also been recorded in the Kansai region.

According to drawings of the scene, pyroclastic material like fireballs rained down, and according to documents, it was like a heavy rain shower. The people of Karuizawa-shuku (Karuizawa, Karuizawa-machi, Kitasaku-gun, Nagano Prefecture) fell into a state of panic, and evacuations began.

Meanwhile, at around 10:00 on August 5th, a huge explosion occurred that was heard as far as Kyoto and Hiroshima, causing the northern foot of Mt. Asama to collapse and lava to fall. This caused the Kambara pyroclastic flow and debris avalanche. The Kambara pyroclastic flow and debris avalanche flowed down the northern foot of the mountain, burying the village of Kambara (Kambara, Tsumagoi Village, Gunma Prefecture) that was located there. It then entered the Agatsuma River and turned into the Tenmei mudflow, which broke a natural dam (blockade) near Yamba (Naganohara Town, Gunma Prefecture) in the Agatsuma River valley, causing a major flood. The mudflow then flowed into the Tone River, washing away many villages in the Tone River basin and reaching Choshi and Edo. The mudflow washed away the Shin-Ohashi and Eitai-bashi bridges on the Sumida River.

In addition, the pyroclastic flow that flowed out in another direction at this time destroyed the Ashiuda settlement (Ashiuda, Tsumagoi Village, Agatsuma District, Gunma Prefecture). According to the Ministry of Land, Infrastructure, Transport and Tourism, the damage caused by the Tenmei eruption included 1,151 deaths, 1,061 houses washed away, 51 houses lost, and over 130 houses collapsed.

Damage caused by the Tenmei eruption: Kamahara Village became Japan's "Pompeii"

Kamahara Village suffered the most damage from the Tenmei eruption. The village, buried by mudflows, has been likened to Pompeii, which was buried by the eruption of Mount Vesuvius in Italy, and has been dubbed "Japan's Pompeii." According to records, about 100 houses in Kamahara Village were engulfed by pyroclastic flows, and 477 people died. At the time, there were about 570 people living in around 100 houses in the village, meaning that more than 80% of the villagers died. Only 93 people survived, having fled to Kamahara Kannon-do on higher ground.

Excavation surveys were carried out in Tsumagoi Village several times between 1980 and 1991, and a new survey is scheduled to begin in 2021 and continue for six years until 2026. These surveys have revealed a variety of things. For example, it was initially thought that the cause of the burial of Kamahara Village was a hot, sludgy "thermal mudflow," but the survey results revealed that it was a debris avalanche made up of "dry material at room temperature," in other words, rocks and stones. Recent excavation surveys have provided more detailed information about the flow of the debris avalanche and confirmed the contents of the literature, and future surveys are expected to be of help.

Damage caused by the Tenmei eruption 2. Damage to crops that contributed to the Tenmei famine

The fall of pumice and volcanic ash from the Tenmei eruption caused great damage to crops and buildings. The smoke was carried by the westerly winds to the east-southeast, so volcanic ash fell all over the Kanto region, especially in the east-southeast direction from Mt. Asama.