Photograph by D.W. Wieprecht in March 1997

This dacite breadcrust bomb (about 15 cm in diameter) was erupted from the lava dome at Mount St. Helens, Washington.

Breadcrust bomb

A breadcrust bomb is a volcanic bomb with a cracked and checkered surface, sometimes resembling the surface of a loaf of bread. The cracks develop when the outer surface of a partially molten lava fragment cools to form a brittle surface and then subsequently cracks as the hot interior expands due to the continued growth of gas bubbles.

Related photo glossary terms:

• Volcanic bomb
• Block