The name volcanic "bomb" is applied to fragments thrown out by an active volcano in a liquid or partially liquid state. Their form is wholly or partly determined during flight through the air, while in a liquid state.

Bombs naturally fall into two classes. Those which start as liquid masses, acquire some symetrical form during flight and retain this form upon landing. These forms are very characteristic, usually symmetrical and unless broken after falling, show fracture surface.

A second class of these bombs starts as a solid angular fragment which has either been reheated in the volcano hearth until the surface has melted or has been dipped in molten lava in the course of its upward movement through the crater. Such a bomb may have any form, depending upon the shape of the original fragment, and the coating may have any thickness.

The bombs of either of the classes may be breadcrusted. The first type presumably by the quick cooling during flight which produces a solid shell around the liquid nucleus. The sudden contraction causes the solid surface to crack while the central liquid portion remains intact, giving an appearance of "breadcrust".

A solid fragment which becomes molten on the surface because of the intense heat to which it is subjected.

The size of volcanic bombs (both classes) varies from 10 or 12 feet in diameter down to indefinitely small fragments. The very small ones are commonly called lapilli. Very large ones are rare, for the obvious reason that the large sizes do not fly and if they did the crust acquired in flight would not be strong enough to hold them together after landing. Most of the bombs found on Wizard Island are under two feet in diameter, while the largest ones found on Mt. Mazama are about four feet in diameter.