Volcanic bomb most of the damage they cause is from impact, or subsequent fire damage. Bomb explosions are most often observed in "bread-crust" type bombs. Bombs are named according to their shape, which is determined by the fluidity of the magma from which they are formed. Volcanic bomb A volcanic bomb is a mass of molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. They cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they are extrusive igneous rocks.
Obsidian manufacture cutting and piercing tools and it has been used experimentally as surgical scalpel blades. The translation into English of "Natural History" written by Pliny the Elder of Rome shows a few sentences on the subject of a volcanic glass called obsidian ("lapis obsidianus"), discovered in Ethiopia by Obsidius, a Roman explorer. Obsidian is the rock formed as a result of quickly cooled lava, which is the parent material. Extrusive formation of obsidian may occur when felsic lava cools rapidly at the edges of a felsic lava flow or volcanic dome or when lava cools during sudden contact with water