Bombs are glowing clots of liquid
lava larger than 64 mm (2.5 in)
in average diameter. Because the clots are soft when ejected
from the vent, their shapes can
be modified during flight. The bombs may start out as long irregular strings
or as discrete clots of liquid. The strings commonly break up into short
segments during flight. Cylindrical or ribbon bombs are segments that
fall to the ground intact. Ribbon bombs are often twisted during flight.
The thicker portions of the ribbons may separate from the rest during
flight be pulled into spheres, forming ball-shaped (spherical) bombs.
Other bombs may be spindle-shaped or irregular in shape. Many have a crust
a few centimeters thick that looks much like that of a loaf of French
bread. Some are flattened on one end, apparently on impact, indicating
the bomb was not completely hardened. Because volcanic bombs cool very
rapidly, usually during flight, they are often glassy in texture.
In the 1992 eruption of Crater Peak on Mt. Spurr, large volcanic bombs
traveled up to 4 km (2.4 mi) from the crater.
The size of these blocks and
bombs ranged from 10 cm (4 in) up to 2 m (6.6 ft). Bombs littered the
crater area and sides of the volcano.