Submicrometer- to micrometer-sized pits are dominant features on volcanic glass shards derived from explosive eruptions and deposited in many deep-sea sediments and subaerial tuffs. The morphology of these pits and experimentally simulated pits and calculations of pressure fluctuations during or just after fragmentation in a magma chamber suggest the pits were formed by collision of volcanic particles, probably in the conduit system. The range of velocities calculated for the particles from the fragmentation of a magma clot, from a few tens up to 120 m/s, is very similar to the range found from the experimental results, 25–140 m/s. A collision with excess energy may reduce the particle size in a secondary fragmentation.