volcanic bomb

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volcanic bomb A lump of ejected lava, more than 32 mm across, that has acquired one of several characteristic shapes during its trajectory to the ground. For example, bread-crust bombs have a glassy crust, criss-crossed with cracks; a spindle bomb has a tail formed by the twisting or spinning of the blob as it falls; rope or ribbon bombs are twisted strands of solidified lava; and cannonball bombs are formed from solid lumps of lava that have been rounded by abrasion as they bounced to rest.