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