Hyaloclastite is a hydrated tuff-like breccia rich in black volcanic glass, formed during volcanic eruptions under water or under ice. 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 oxidecrystals 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 Ice age, 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.
