hyaloclastite

thumb|Hyaloclastite between Pillow lava|pillows of lava in [[Montana]] thumb|Pahoehoe|Pahoehoe lava enters the Pacific at [[Hawaii Volcanoes National Park, the Big Island of Hawaii]]

Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek hyalus) fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin margins on the lava flow surfaces and between pillow lavas as well as in thicker deposits, more commonly associated with explosive, volatile-rich eruptions as well as steeper topography. Hyaloclastites form during volcanic eruptions under water, under ice or where subaerial flows reach the sea or other bodies of water. It commonly 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 and spallation during rapid cooling.