Volcanic bombs usually have the composition of the lava erupted by a particular volcano. The lava clots have a high silica content, with a high proportion of quartz. Clots from intermediate composition lavas have a silica content of 55 to 65 percent. Mafic volcanoes are mainly nonexplosive, and bombs are less likely to form.

TEXTURE Breadcrust volcanic bombs have a fine-grained crust and may show coarser crystals within. The crust is marked and cracked because of the force of impact with the ground. They may contain small fragments of country rock torn from around the volcanic pipe.

ORIGIN Volcanic bombs are small to large molten lava clots that have been ejected from a volcano by violent eruption and have landed on the Earth. The lava clots are usually made of viscous lava, which cools on the outside during flight, forming a skin that cracks on impact of landing to produce the “breadcrust” surface.
The bombs may sometimes measure over 3 ft (1 m) in diameter. When they land in volcanic ash, these bombs will often form a crater.

Group: IGNEOUS

Origin: Extrusive

Grain size: Fine

Crystal shape: Anhedral

Classification: Felsic to mafic

Occurrence: Volcano

Color: Dark