Hyaloclastite is a volcanic rock formed by the rapid cooling and fracturing of lava upon contact with water, typically during submarine or subglacial eruptions. This rock is composed of glassy fragments and is important in understanding volcanic processes in underwater or glaciated environments and the interactions between lava and water.
Source Fisher, R. V., & Schmincke, H.-U. (1984). Pyroclastic Rocks. Springer.