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 . 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 ( 16-20 ft ) in diameter up to 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 . On July 16 , 2018 , 23 people were injured on a tour boat near the Kilauea volcano as a result of a basketball-sized lava bomb from the 2018 lower Puna eruption
2018 lower Puna eruption
2018 lower Puna eruption
Volcanic bomb