The textural classification of volcaniclastic deposits (Fig. 3.12) is a modification of the Wentworth scheme. Coarse material (over 64mm) is divided into volcanic blocks, which were solid when erupted, and volcanic bombs, which were partly molten and have cooled in the air; consolidated into a rock these are referred to as volcanic breccia and agglomerate respectively. Granule to pebble-sized particles (2–64mm) are called lapilli and form a lapillistone. Accretionary lapilli are spherical aggregates of fine ash formed during air fall. Sand-, silt- and clay-grade tephra is ash when unconsolidated and tuff upon lithification. Coarse ash/tuff is sand-sized and fine ash/tuff is siltand clay-grade material. Compositional descriptions hinge on the relative proportions of crystals, lithic fragments and vitric material, which is fragments of volcanic glass formed when the molten rock cools very rapidly, sometimes forming pumice.