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| The Mt. Fuji is located at the triple junction |
富士山はなぜ巨大なのか？
富士山は、フィリピン海、ユーラシア（アムール）、北米（オホーツク）プレートという３つのプレート境界に位置し、東西39㎞、南北37㎞で400ｦ3以上の体積を有する我が国最大の玄武岩質の成層火山である。富士山が巨大である理由として、マグマが長期間にわたり供給され続けていることが考えられる。
伊豆半島をのせたフィリピン海プレートは、駿河トラフを境界にしてユーラシア（アムール）プレートの下に、また相模トラフを境界にして北米（オホーツク）プレートのそれぞれ下に沈み込み、これら３つのプレートの境界が接する部分がほぼ富士山の直下にあたる。沈み込んだフィリピン海プレートは、厚い板状の形をしているため東海スラブと呼ばれる。
地震波のデータによれば、沈み込んだ東海スラブは、富士山の下で北西方向に進む関東スラブと西方向に進む東海スラブに引き裂かれているように見える。このため、地殻下部、更にはその下位のマントルからこの隙間を埋めるように多量のマグマが上昇している可能性があり、これが富士山に長期間にわたりマグマが供給されるシステムではないかとする考えが提唱されている
"The Triple Junction"
Mount Fuji is located at the triple junction where the Amurian Plate, the Okhotsk Plate, and the Philippine Sea Plate meet. Those plates form the western part of Japan, the eastern part of Japan, and the Izu Peninsula respectively.
Scientists have identified four distinct phases of volcanic activity in the formation of Mount Fuji. The first phase, called Sen-komitake, is composed of an andesite core recently discovered deep within the mountain. Sen-komitake was followed by the "Komitake Fuji," a basalt layer believed to be formed several hundred thousand years ago. Approximately 100,000 years ago, "Old Fuji" was formed over the top of Komitake Fuji. The modern, "New Fuji" is believed to have formed over the top of Old Fuji around 10,000 years ago.[37]
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| Hot Spring Resort, IZU-HAKONE area | | | |
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The volcano is currently classified as active with a low risk of eruption. The last recorded eruption was the HM ei eruption which started on December 16, 1707 (Hoei 4, 23rd day of the 11th month) and ended about January 1, 1708 (HM ei 4, 9th day of the 12th month) during the Edo period.
The eruption formed a new crater and a second peak (named HM ei-zan after the Hoei era) halfway down its side. Fuji spewed cinders and ash which fell like rain in Izu, Kai, Sagami, and Musashi.[39] Since then, there have been no signs of an eruption.
Scientists have identified four distinct phases of volcanic activity in the formation of Mount Fuji. The first phase, called Sen-komitake, is composed of an andesite core recently discovered deep within the mountain. Sen-komitake was followed by the "Komitake Fuji," a basalt layer believed to be formed several hundred thousand years ago. Approximately 100,000 years ago, "Old Fuji" was formed over the top of Komitake Fuji. The modern, "New Fuji" is believed to have formed over the top of Old Fuji around 10,000 years ago.[37]
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