Pillow lava - pillow lava

Japanese: 枕状溶岩 - まくらじょうようがん（英語表記）pillow lava

Lava in the form of piles of rounded lumps (pillows) resembling pillows or rice bales. Also called pilorava or bale-shaped lava. It is formed when basaltic lava erupts on the seafloor and rapidly cools and solidifies underwater. When the surface of the lava comes into contact with water, it becomes a glassy shell, but the unconsolidated lava inside breaks the shell and continues to form new pillows, forming small mountains several tens of meters high on the seafloor. The largest pillows are several meters long, and their circular cross-sections are about 1 meter in diameter. The cross-section also shows a concentric structure, with radial cracks from the center, giving it the appearance of a rice bale seen from the side. The gaps between the pillows are filled with hyaloclastite (water-cooled clastic rock) and limestone. It is mainly found in the central axis of mid-ocean ridges and undersea volcanoes on hotspots.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

枕や米俵のような丸みを帯びた塊（枕）が積み重なった形状の溶岩。ピローラバ，俵状溶岩ともいう。海底で噴出する玄武岩質の溶岩が水中で急冷，固結することにより生じる。溶岩の表面は水と接触するとガラス質の殻となるが，中にある未固結の溶岩が殻を破って次々と新しい枕をつくり続けることで，高さ数十mの小山が海底で形成される。個々の枕は，大きいもので長さ数m，円形をなす断面は直径約 1mに及ぶ。断面には同心円構造も見られ，中心部からは放射状の割れ目が入り，米俵を横から見たような観を呈する。枕と枕のすきまはハイアロクラスタイト（水冷破砕岩）や石灰岩などが埋めている。おもに中央海嶺の中軸部やホットスポット上の海底火山で見られる。

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