Mount Asama

A triple stratovolcano, 2,568 meters above sea level, located on the border between Nagano and Gunma prefectures. It is composed mainly of andesite. It is one of Japan's most active volcanoes, having erupted frequently since recorded history (since 685) until recent times. It is located where the Nasu and Fuji volcanic belts join, but it is generally considered to belong to the former. At the end of the Pleistocene (Pleistocene) tens of thousands of years ago, a stratovolcano was formed, and then a large steam explosion blew off the eastern half, forming a caldera, but the western side of the mountain still remains as the first outer rim (Mt. Kurofu, etc.). Later, a parasitic volcano, Mt. Sekisonzan (lava dome), was formed on the southern flank. About 20,000 years ago, the Hotokeiwa lava flow, a sticky dacite (dacite) rock, erupted from within the caldera, forming a flat, shield-like volcano, and to the east, a parasitic dacite volcano, Ko-Asama (lava dome). About 11,000 years ago, a large explosion occurred, depositing thick layers of dacite volcanic debris across northern Kanto, and pumice flows of the same material widely covered the foothills on both the north and south sides of the volcano. In the present day, eruptive activity resumed within the caldera about 5,000 years ago, forming the second outer rim, Maekakeyama.

The two biggest eruptions in recorded history occurred in 1281 (Kōan 4) and 1783 (Tenmei 3), both of which produced large explosions, pyroclastic flows, and lava flows, with the former producing a total of about 3 billion tons of ejecta material, and the latter about 1 billion tons. The latter killed 1,152 people. The Kambaramurai ruins date back to that time. After the Tenmei eruption, Kamayama, a central crater cone, was formed within the Maekakeyama crater. The Kamayama crater is almost circular with a diameter of about 350 meters, but its depth constantly fluctuates (0-250 meters), and becomes shallower during active periods. It is characterized by explosive eruptions, ejecting a total of hundreds of thousands of cubic meters of ejecta material, sending volcanic smoke several thousand meters above the summit, and often causing devastating damage through volcanic rocks, ashfall, and blast waves.

In 1911 (Meiji 44), the Earthquake Disaster Prevention Research Committee established Japan's first volcano observatory. The Karuizawa Weather Observatory (later reorganized as the Karuizawa Special Area Meteorological Observatory and the Asama Volcano Disaster Prevention Liaison Office) was established in 1922 (Taisho 11), and the Asama Volcano Observatory of the Earthquake Research Institute of the University of Tokyo, established in 1933 (Showa 8), continued the observation and research. The Volcano Disaster Prevention Liaison Office collects data on volcanic activity. The summit is bare land, but the mountainside is covered with forests and shrub grasslands of larch, red pine, and white birch, and the foot of the mountain has been developed, forming the highland villa areas of Karuizawa and Kitakaruizawa. There is a constant stream of visitors, whether they are here to escape the summer heat, enjoy outings in spring and autumn, or skate in winter. Blessed with beautiful scenery and abundant wild birds, the area is part of Joshinetsu Kogen National Park. There are hiking trails from Nakakaruizawa, Shinano Oiwake, and Komoro stations on the JR Shinetsu Main Line (now the Shinano Railway), and from Naganohara-Kusatsuguchi on the JR Agatsuma Line, and it takes about four hours to reach the summit, but in 1973 (Showa 48), access to the mountain was restricted due to volcanic activity. In recent years, restrictions have been relaxed on some of the trails starting from Komoro, but the summit remains off-limits at all times. The lava tree molds are a nationally designated Special Natural Monument. There is a volcano museum in Komoro City (Nagano Prefecture) that has a volcano shelter to protect against eruptions, and the Asama Volcano Museum in Naganohara Town (Gunma Prefecture). The Komoro City Asama Mountain Range Nature Observation Center at Kurumazaka Pass displays materials about the history of Mount Asama and its eruptions.

[Akira Suwa]

"Mount Asama" by Teisuke Yagi (1936, Shinano Mainichi Shimbun)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

長野・群馬県境にそびえる三重式の成層・円錐火山(えんすいかざん)。標高2568メートル。おもに安山岩からなる。有史以後（685年以後）も近年までしきりに噴火を繰り返してきた日本の代表的活火山。那須(なす)、富士両火山帯が合する位置にあるが、普通、前者に属させる。更新世（洪積世）末期、数万年前にまず成層・円錐火山が形成されたのち、大規模な水蒸気爆発でその東半が吹き飛ばされ、カルデラを生じたが、同山の西側はいまも第一外輪山（黒斑山(くろふやま)など）として残存する。その後、南側山腹に寄生火山の石尊山(せきそんざん)（溶岩円頂丘）を生じた。約2万年前、そのカルデラ内に粘り強い石英安山岩（デイサイト）質の仏岩溶岩流(ほとけいわようがんりゅう)が噴出して扁平な楯状(たてじょう)火山ができ、その東方には石英安山岩質の寄生火山、小浅間（溶岩円頂丘）を生じた。約1万1000年前、大爆発して石英安山岩質の火山砕屑(さいせつ)物を関東北部一帯に厚く降り積もらせ、かつ、同質の軽石流が火山の南北両側の麓(ふもと)を広く覆った。現世に入り、約5000年前にカルデラ内で噴火活動が再開され、第二外輪山、前掛山(まえかけやま)が形成されてきた。

　有史以後の二大噴火は1281年（弘安4）と1783年（天明3）におき、ともに大爆発、火砕流、溶岩流が発生し、噴出物総量は前者は約30億トン、後者は約10億トン。後者では死者1152人を出した。鎌原村遺跡(かんばらむらいせき)はそのときのものである。天明大噴火後、前掛山火口内に中央火口丘の釜山(かまやま)が生まれた。釜山火口はほぼ円形で直径約350メートルであるが、深さは絶えず変動し（0～250メートル）、活動期には浅くなる。爆発型噴火が特徴で、噴出物総量数十万立方メートル、噴煙を山頂上数千メートル以上にあげ、噴石、降灰、爆風などでしばしば惨害を出す。

　1911年（明治44）日本最初の火山観測所を震災予防調査会が設け、1922年（大正11）創設の軽井沢測候所（その後、軽井沢特別地域気象観測所と浅間山火山防災連絡事務所に改組）と、1933年（昭和8）創設の東京大学地震研究所浅間火山観測所がその観測研究を受け継いだ。火山防災連絡事務所は火山の活動データを収集している。山頂部は裸地であるが、山腹にはカラマツ、アカマツ、シラカンバなどの森林や低木草原が広がり、裾野(すその)は開拓され、軽井沢、北軽井沢の高原別荘地帯がある。夏の避暑、春秋の行楽、冬のスケートと来遊者が絶えない。風光に恵まれ、野鳥に富み、上信越(じょうしんえつ)高原国立公園に属する。JR信越本線（現、しなの鉄道）中軽井沢、信濃追分(しなのおいわけ)、小諸(こもろ)と、JR吾妻(あがつま)線長野原草津口の各駅から登山路が通じ、約4時間で登頂できたが、1973年（昭和48）火山活動のため入山が規制された。近年、小諸を登山口とする一部コースの規制が緩和されたが、頂部は常時立入り禁止になっている。熔岩樹型は国指定特別天然記念物。噴火対策用の火山シェルターをもつ小諸市（長野県）の火山館、長野原町（群馬県）の浅間火山博物館がある。車坂峠にある小諸市浅間連峰自然観察センターには浅間山の歴史や噴火についての資料が展示されている。

［諏訪　彰］

『八木貞助著『浅間山』（1936・信濃毎日新聞）』

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