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 do not have
grains making them
extrusive igneous rocks. Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight. Bombs can be extremely large; the 1935 eruption of
Mount Asama in Japan expelled bombs measuring 5–6 m in diameter up to 600 m from the vent. Volcanic bombs are a significant volcanic hazard, and can cause severe injuries and death to people in an eruption zone. One such incident occurred at
Galeras volcano in
Colombia in 1993; six people near the summit were killed and several seriously injured by lava bombs when the volcano erupted unexpectedly.