Basaltic lava bombs erupted by Mauna Kea Volcano, Hawai'i. Credit: J. P. Lockwood / U.S. Geological Survey.
A volcanic bomb is a lava fragment that is ejected while viscous (partially molten) and larger than 64 millimeters (2.6 inches) in diameter. Many volcanic bombs acquire rounded aerodynamic shapes during their travel through the air. Volcanic bombs include breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs, and "cow-dung" bombs.