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Hyaloclastite is a hydrated tuff-like breccia rich in black volcanic glass, formed during volcanic eruptions under water, under ice or where subaerial flows reach the sea or other bodies of water. It has the appearance of angular flat fragments sized between a millimeter to few centimeters. The fragmentation occurs by the force of the volcanic explosion, or by thermal shock during rapid cooling.
Several minerals are found in hyaloclastite masses. Sideromelane is a basalt glass rapidly quenched in water. It is transparent and pure, lacking the iron oxide crystals dispersed in the more commonly occurring tachylite. Fragments of these glasses are usually surrounded by a yellow waxy layer of palagonite, formed by reaction of sideromelane with water.
Hyaloclastite ridges, formed by subglacial eruptions during the last glacial period, are a prominent landscape feature of Iceland and British Columbia. Hyaloclastite is usually found at subglacial volcanoes, such as tuyas, which is type of distinctive, flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet.
In lava deltas, hyaloclastites form the main constituent of foresets formed ahead of the expanding delta. The foresets fill in the seabed topography, eventually building up to sea level, allowing the subaerial flow to move forwards until it reaches the sea again.