Mount Amagi - Amagisan

A volcanic group stretching from east to west in Shizuoka Prefecture in the central-eastern part of the Izu Peninsula. The highest peak is Banzaburo-dake (1,406 meters), followed by Banjiro-dake (1,300 meters) and Houki-san (1,024 meters), and the mountain range stretches from east to west. It is considered to be a double volcano formed by activity from the middle Pleistocene (Pleistocene) onwards, and is a stratovolcano with a shield-like shape. It is also called the Amagi Volcanic Group because it has many parasitic volcanoes. The base of the Amagi volcano is the Neogene Yugashima Group, which is widely distributed across the peninsula, and early activity formed Mt. Asama, Mt. Misuji, and Mt. Noborio in the south, and is made of andesitic lava. The next activity that formed the main body of Amagi Volcano is thought to have been around the source of the Shiratagawa River, which flows southeast, and erupted andesitic lava. The rocks were altered so violently that the mountain body broke up, forming an eroded caldera-like topography. Hacchoike Pond in the west along the ridgeline is a lateral crater, and Togasayama Mountain to the east emitted large amounts of lava, forming the Amagi Plateau. Side volcanoes formed after the Holocene (Alluvial Epoch, the last era of the geological age) include Hachiyama and Hachikuboyama on the southwest side, from which basaltic lava also flowed, filling the valleys of the Kano River and Kawazu River, forming Jorennotaki Falls and Kawazu Nanadaru Falls. The rocky mountain on the northeast side, Yahazuyama and other blocky volcanoes are made of rhyolitic rock. Kawagodaira, located north of Shirata Pass (1,197 meters), which was active about 2,800 years ago, is characterized by the eruptions of volcanic ash and the outflow of pumice mudflows, and its surface layer is collected as Amagi anti-fire stone.

Mount Amagi was formerly known as Mount Kano and Mount Amagi, and has a long history of logging and transportation, as a forest directly under the control of the shogunate. Pine, cedar, cypress, zelkova, camphor, sawara, oak, fir and hemlock trees were known as the "Nine Amagi Trees" and were protected as imperial trees. In the past, the area was famous for producing charcoal, but now it is famous for its wasabi and shiitake mushrooms. The area around the volcano is also blessed with hot springs, including Yugashima, Yugano, Mine, Atagawa and Okawa. The vegetation is also distinctive, with forests of andromeda and himeshara trees, and flowers of rhododendrons and Amagi azalea creating a plant landscape unique to Amagi.

In addition, the traverse course that runs from the Amagi Plateau along the ridge past Haccho Pond to Amagi Pass is a typical hiking course in Izu.

[Kitagawa Mitsuo]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

静岡県、伊豆半島中東部に連なる火山群。最高峰は万三郎岳(ばんざぶろうだけ)（1406メートル）で、万二郎(ばんじろう)岳（1300メートル）、箒木(ほうき)山（1024メートル）がそれに次ぎ、連山は東西に延びる。更新世（洪積世）中期以降の活動によって形成された二重火山とも考えられ、成層火山で楯状(たてじょう)の形態をもつ。側火山も多いので天城火山群ともいう。天城火山の基盤は、半島に広く分布する新第三系湯ヶ島層群であり、初期の活動は南方の浅間(せんげん)山、三筋(みすじ)山、登尾(のぼりお)山などを形成し、安山岩質溶岩からなる。次の天城火山本体を形成した活動は、南東流する白田川(しらたがわ)源流部あたりが火口と考えられ、安山岩質溶岩を噴出させた。岩石の変質作用が激しく、山体の解体が進み、侵食カルデラ状の地形を呈する。稜線(りょうせん)に沿う西部の八丁池(はっちょういけ)は側火口であり、東方の遠笠山(とおがさやま)からは多量の溶岩を流出し、天城高原となった。完新世（沖積世。地質時代最後の世）以降にできた側火山には、南西側の鉢山や鉢窪(はちくぼ)山があり、そこからは玄武岩質溶岩も流出し、狩野川(かのがわ)、河津川の谷を埋めて浄蓮ノ滝(じょうれんのたき)や河津七滝(かわづななだる)をつくった。北東側の岩山、矢筈(やはず)山、などの塊状火山は流紋(りゅうもん)岩からなる。約2800年前に活動した白田峠（1197メートル）北側のカワゴ平は、火山灰の噴出と軽石質泥流の流出を特色とし、表層は天城抗火(こうか)石として採取されている。

　天城山は古くは狩野山、甘木山ともよばれ、伐木、搬出の歴史も古く、幕府直轄の御料林であった。マツ、スギ、ヒノキ、ケヤキ、クスノキ、サワラ、カシ、モミ、ツガは天城九木とよばれ、御制木として保護されていた。かつては木炭の生産、現在はワサビやシイタケが名産となっている。火山周辺は温泉にも恵まれ、湯ヶ島、湯ヶ野、峰、熱川(あたがわ)、大川などがある。植生にも特色をもち、アセビ、ヒメシャラの樹林、シャクナゲ、アマギツツジの花は天城特有の植物景観となっている。

　また、天城高原から尾根伝いに八丁池を経て天城峠に至る縦走コースは、伊豆の代表的なハイキングコースである。

［北川光雄］

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