When basalt enters water passively, it forms pillow basalt . These are bulbous bodies with quenched, glassy rinds. They are often spherical, with diameters of 30-100 cm, although many are lobate. During formation, the outer surface of each lobe chills instantaneously. However, the constant injection of lava may cause a single lobe to swell, thus cracking the glassy rind, allowing a new lobe bud forth.
Individual pillows can break off the budding lobe and cool as a single mass.
As the interior of a pillow cools, it forms crude cooling joints that radiate inward, perpendicular to the cooling surface. Each pillow will settle into gaps between the pillows below it. In continental lakes, pillow accumulations can be several tens of meters thick, whereas those that develop on the ocean floors can be hundreds of meters thick. Basalt fragmentation can occur by the explosive eruption process, or by an essentially nonexplosive process associated with the spalling of pillow basalt rinds by thermal shock or chill shattering . The hyaloclastite fragments are equant, angular shards of black glass, generally between .25 and 2.0 cm in diameter .