Volcanic black sand grain. Coloured scanning electron micrograph (SEM) of an Icelandic black sand particle. When lava contacts water, it cools rapidly and shatters into sand and fragmented debris of various size. Much of the debris is small enough to be considered sand. A large lava flow entering an ocean may produce enough basalt fragments to build a new black sand beach almost overnight. Magnification: x120 when printed 10 centimetres wide.