Idaho Bureau of Mines and Geology Bulletin Craters of the Moon National Monument, Idaho

Probably the most curious of all the lava features that are found in the Monument are the lava bombs that lie scattered on the cinder and spatter cones. These bombs range in length from half an inch to more than 13 feet and in diameter from a quarter of an inch to 3 feet. There are three classes of bombs—spindle bombs, ribbon bombs, and bread-crust bombs. Spindle bombs are those that have tapering projections on the ends of a football shaped body. These bombs owe their origin to a clot of lava being hurled through the air with a rotational or spiral motion. Most of the best-formed bombs found by the writer were very close to their source, indicating that their flight was of short duration. Some of these bombs had long, tapering twisted ends that projected 3 to 10 inches from the main part. These long, thin strips of lava were broken off by the fall of the projectile and lie scattered on the surface. They are called ribbon bombs because of their shape. Some of the ribbon bombs owe their origin to the pulling apart of two clots of lava which traveled at slightly different speeds or in slightly different directions. The longer ribbon bombs, such as those 2 to 13 feet in length and only a few inches wide, seem to have originated in one of two different ways. A few of them are simply lava clots that landed on the side of a cone and ran down the slope while still plastic. Weathering and the removal of the cinders adjacent to some of these long lava trickles have caused them to become detached and to lie loose on the surface like bombs. Other ribbon bombs are formed by the throwing out from a spatter or cinder cone of a large clot of very liquid lava that stretches out during its flight and falls on cinders sufficiently loose to prevent its breaking into pieces.

PLATE XII—A few lava stalactites from the caves. They were formed by the dripping of the hot lava from the roofs of the tunnels. Natural size.

Bombs of a third kind are light and porous and covered with a crust or skin that resembles the crust of bread. These are known as bread-crust bombs. They are formed by a clot of lava filled with gas being hurled through the air. During its flight a skin or crust forms on the clot, and as the bomb swells owing to the expansion of the included gases the skin is cracked open in many places, producing a number of small checks on the surface. In baking bread the same process can be observed. First a thin crust bakes, and then, as the gases from the yeast expand, the loaf swells and the crust is broken, showing numerous small cracks.