When surface lava quenches quickly in seawater, it forms volcanic glass called pillow basalt (pillow lavas) and hyaloclastite (fragmented volcanic glass). Pillow basalts are bulbous, sack-like formations of rapidly chilled basaltic glass with fine-grained or glassy rims; hyaloclastite is an accumulation of glassy angular fragments produced by thermal fragmentation during quenching.