Breadcrust bomb erupted by Mount St. Helens, Washington

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.

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