Nishinoshima, a submarine volcano in the Ogasawara Arc, approximately 1 000 km south of Tokyo, Japan, suddenly erupted in November 2013, after 40 years of dormancy. Olivine‐bearing phenocryst‐poor andesites found in older submarine lavas from the flanks of the volcano have been used to develop a model for the genesis of andesitic lavas from Nishinoshima. In this model, primary andesite magmas originate directly from the mantle as a result of shallow and hydrous melting of plagioclase peridotites. Thus, it only operates beneath Nishinoshima and submarine volcanoes in the Ogasawara Arc and other oceanic arcs, where the crust is thin. The primary magma compositions have changed from basalt, produced at considerable depth, to andesite, produced beneath the existing thinner crust at this location in the arc. This reflects the thermal and mechanical evolution of the mantle wedge and the overlying lithosphere. It is suggested that continental crust‐like andesitic magma builds up beneath submarine volcanoes on thin arc lithosphere today, and has built up beneath such volcanoes in the past. Andesites produced by this shallow and hydrous melting of the mantle could accumulate through collisions of plates to generate continental crust. Nishinoshima, a submarine volcano in the Ogasawara Arc, ~ 1 000 km south of Tokyo, Japan, suddenly erupted in November 2013, after 40 years of dormancy. Primary andesite magmas originate directly from the mantle as a result of shallow and hydrous melting of plagioclase peridotites. It is suggested that continental crust‐like andesitic magma builds up beneath submarine volcanoes on thin arc lithosphere today, and has built up beneath such volcanoes in the past. 西之島は東京から 1 000 km 南に位置する,  小笠原弧の海底火山である.  2013 年 11 月,  西之島は 40 年ぶりに噴火した.  陸上と海底の調査により,  西之島海底火山の本体が安山岩であることが明らかとなり,  海底から採取された溶岩 (斑晶に乏しく,  かんらん石を含有する安山岩) の分析からは,  安山岩マグマの新しい成因モデルが提示された.  マントルの浅い部分 (30 km 以浅) は斜長石かんらん岩で構成され,  この低圧マントルの含水下の部分融解により,  初生安山岩マグマが生成した.  このマグマから,  主にかんらん石が結晶分別することによって,  西之島の安山岩マグマがつくられた.  一方,  西之島付近の古い時代の小海丘は玄武岩でできており,  初生マグマの組成が,  玄武岩質から安山岩質へと,  時間とともに変化したことも示された.  すなわち,  マントルの融解深度が地殻直下へと浅くなってきたことが示された.  地殻の薄い海洋島弧において生成された安山岩は,  プレートの衝突帯で集積することにより,  大陸へと成長していく, と考えられる.