A volcanic bomb at Mt. Dagmálafjall in S-Iceland. A volcanic bomb is a mass of molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects fragments of lava during an eruption. They cool into solid fragments before they reach the ground. Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight - like this one.